Dams trigger exponential population declines of migratory fish

Zhenli Huang; Haiying Li

doi:10.1126/sciadv.adi6580

. 2024 May 10;10(19):eadi6580. doi: 10.1126/sciadv.adi6580

Dams trigger exponential population declines of migratory fish

Zhenli Huang ^1,^*, Haiying Li ¹

PMCID: PMC11086605 PMID: 38728390

Abstract

The impact of dams on global migratory fish stocks is a major challenge and remains seriously underestimated. China has initiated a dozen fish rescue programs for the dams on the Yangtze River, focusing on five flagship species―Chinese sturgeon, Chinese paddlefish, Yangtze sturgeon, Chinese sucker, and Coreius guichenoti. Despite 40 years of effort, these five fishes are on the verge of extinction. Here, we propose an analytical tool that includes a framework of fish migration taxonomy and six life cycle models, the concepts of invalid stock and the dam impact coefficient, and a simplified population model. We then clarify the migration patterns and life cycles of these fishes and show that the Yangtze dams have severely disrupted the life cycle integrity of these species, causing seven types of invalid stocks and their exponential population declines. Last, we discuss six scientific misjudgments underpinning the fish rescue programs and recommend reforms to China’s fish rescue strategy.

China’s dams on the Yangtze River have driven five flagship fish species to extinction due to a lack of targeted mitigation.

INTRODUCTION

Dams have engendered major challenges for migratory fish globally (1–4). More than 58,000 dams higher than 15 m have been built in most of the world’s rivers (5) because most countries have overwhelmingly focused on building dams for the multi-objective utilization of freshwater resources themselves while ignoring or overlooking the huge economic and ecological value of migratory fish. In February 2021, 16 conservation organizations released a report, The World’s Forgotten Fishes, declaring that migratory fish stocks have declined by 76% since 1970, and worldwide populations of “mega fish” have declined by an astounding 94%. Globally, threats to migratory fish have been linked to factors other than dams, including overfishing, water pollution, and sand mining (6). Because of a lack of applicable tools and available data, it has been impossible to distinguish the relative contributions of dam and non-dam factors, leading to large discrepancies between expected results and reality and, crucially, a pervasive underestimation of dam impacts (7). Today, the recovery of flagship migratory species from population collapse or extinction in large dammed rivers is becoming an intractable global problem. This includes the various sturgeon species affected by the Iron Gates I/II on the Danube River in Europe, the Volgograd Dam on the Volga River in Russia, and the Three Gorges Dam (TGD) on the Yangtze River in China (8–10).

The Yangtze River, which is the longest river in China and the third longest in the world, spans 6387 km with a drainage area of 1.8 million km² (11). Originating from the snow-covered mountain Geladandong on the Qinghai-Tibet Plateau, it is divided into three reaches: the upper (4504 km) above Yichang, the middle (955 km) from Yichang to Jiujiang, and the lower (938 km) from Jiujiang to the river mouth. The stretch of the river from Zhimenda to Yibin is known as the Jinsha River (3464 km). It is divided into three sections: the upper from Zhimenda to Shigu, the middle from Shigu to Panzhihua, and the lower from Panzhihua to Yibin. The Yangtze River is China’s most abundant water resource, with the highest freshwater fish biodiversity and the largest number of migratory fish species. It is also home to more than 400 species of fish, 22 of which are listed as nationally protected species (12). Hydropower development on the mainstem of the Yangtze River began in the 1970s with the Gezhouba Dam (GD). By the end of 2021, a dozen large dams had been successfully constructed on the upper Yangtze and middle/lower Jinsha rivers (Fig. 1, text S1, and table S1), with more under construction on the upper Jinsha. These dams form China’s largest hydropower base and contribute to improving flood control, navigation, and power supply.

Fig. 1. — By the end of 2021, a dozen large-scale cascade dams had been completed in the upper reaches of the Yangtze River and the middle and lower sections of the Jinsha River. The lowermost of these is the Gezhouba Dam, preceded upriver by the TGD, Xiangjiaba Dam, Xiluodu Dam, Baihetan Dam, Wudongde Dam, Guanyinyan Dam, Ludila Dam, Longkaikou Dam, Jin’anqiao Dam, Ahai Dam, and Liyuan Dam. The inner information presented in the images of the 12 dams includes the dam’s name, river closure time (year–month), impoundment time (year–month), installed capacity [megawatt (MW)], and backwater length (km), arranged from top to bottom. All photo credits: Zhenli Huang and Haiying Li.

When the GD, the first dam across the mainstream of the Yangtze River, was built in the 1970s, the Chinese government explicitly demanded that the dam consider the conservation of fish. To this end, a heated debate over conservation targets and fish passages raged from 1981 to 1982 (13). In late 1982, the Chinese government adopted the suggestions of the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences that the Chinese sturgeon (Acipenser sinensis) should be the sole focus of conservation efforts (14). Subsequently, the IHB conducted nearly all of the studies on fish conservation in the 12 cascade dams (15, 16), identifying five fish species as key targets for conservation: the Chinese sturgeon, the Chinese paddlefish (Psephurus gladius), the Yangtze sturgeon (Dabry’s sturgeon, Acipenser dabryanus), the Chinese sucker (Myxocyprinus asiaticus), and Coreius guichenoti. The primary conservation measures for each dam—including restocking, fishing bans, and fish natural reserves—did not include targeted interventions, such as the construction of fishways to promote upstream and downstream connectivity or environmental flows to mitigate hydrological shifts. After four decades of implementation, the conservation efforts for each dam, collectively known as the Fish Rescue Programs (FRPs) of the Yangtze dams, have failed to achieve their goal of protecting the five flagship fishes (17–20). However, the failure of the FRPs provides a rare opportunity to examine the true impact of dams on migratory fish.

Dams can harm migratory fish by disrupting their life cycles and then causing population extinctions. However, the quantitative effects of dams on these species are poorly understood, causing a substantial underestimation of their overall impact. This is due to the lack of a clear classification system for fish migration, particularly for potamodromous fish, which results in a skewed understanding of the various migration patterns and corresponding life cycles. Moreover, the lack of standardized methodologies for identifying complete life cycles has resulted in a notable scarcity of global-scale life cycle datasets (21). In addition, the inability to conduct a quantitative evaluation of the majority of migratory fish due to data limitations exacerbates this issue. Hence, we propose an analytical tool aimed at quantitatively evaluating and contrasting the effects of the Yangtze cascade dams on the five fish species, using them as a case study. This tool includes a three-tiered migration classification system, six descriptive life cycle models, the concepts of invalid stocks and the dam impact coefficient (DIC), and a simplified population model. Here, we first determine the migration pattern and life cycle of each of the five fish species using the migration classification system and the life cycle model. Then, we analyze the barrier effect of dams on the life cycle of each species and reveal the underlying mechanisms contributing to the decline of fish populations by estimating the invalid stocks and DICs. Consequently, we use the simplified model to reconstruct the population decline processes of these five fishes. Last, we discuss scientific misjudgments and recommend reforms to China’s dam-related fish rescue strategy.

RESULTS

Identifying migration patterns and life cycles of the five fish species

When designing dams, accurately understanding the migratory behavior and life cycle of fish is the foundation of fish conservation. Without a scientific framework of migration classification and a life cycle analysis tool, incorrect assumptions and misjudgments may be made about migratory fish traits and behaviors. To date, Chinese ichthyologists have typically used an outdated classification system of fish migration, dividing freshwater fish into “migratory,” “semi-migratory,” and “sedentary” (22). As a result, the migration patterns and life cycles of migratory fish in China are typically unclear or incorrect (23–27), leading to the FRPs having a weak scientific foundation (11–16). For example, the Chinese paddlefish has been described as “one of the world’s largest freshwater fishes” (17, 23), which overlooks its diadromous nature, and the migratory Yangtze sturgeon has been described as a “resident” freshwater fish (24) that does not migrate long distances (25), although its migratory distance is up to 2200 km. There is an urgent need to reclassify the migratory fish of the Yangtze River and clarify their life cycles. Here, we propose a three-tiered and three-category framework for fish migration taxonomy, including definitions of various migration patterns, accompanied by six descriptive life cycle models of dam-related migration patterns (Fig. 2 and Methods). Within this framework, we further distinguish potamodromy into two types—riverine potamodromy and lacustrine potamodromy—with definitions and corresponding life cycle models. Using a procedure for determining migratory fish life cycle (Methods), we can access the full life cycles of data-limited fish by identifying their migration patterns.

Fig. 2. — Here, we present the fish migration classification system (A), which introduces two types of potamodromy. On the basis of this system, we propose six descriptive life cycle models (B) that assist in acquiring the entire life cycles of data-limited fish species in four steps. See Methods and text S2 for definitions of migration types or subtypes and their corresponding life cycle models.

Our findings indicated that the migration patterns of the five fishes belonged to one of two categories―diadromy and potamodromy (Fig. 3 and text S2). The Chinese sturgeon exhibits anadromy (type) of diadromy (category), which inhabits a marine environment for most of its life, swimming the 2850 km up to breed in the upper Yangtze River (Fig. 3A). The Chinese paddlefish exhibits the freshwater amphidromous subtype of the amphidromous type in the diadromous category, and it breeds in the upper Yangtze River; their juveniles need to migrate downriver to the sea, a distance of 2750 km. The juveniles then undergo osmoregulation in saltwater in their early growth stage and then return to the Yangtze River (Fig. 3B). Because of this portion of its life cycle, the Chinese paddlefish cannot be considered a strictly freshwater (17, 23), anadromous (18), or potamodromous fish (28). The remaining three species (the Yangtze sturgeon, the Chinese sucker, and C. guichenoti) exhibit the riverine type of potamodromy (category), breeding in the upper Yangtze River and completing their life cycles within freshwater, and their farthest migration distances are up to 2200, 2800, and 1700 km, respectively (Fig. 3, C to E). We found no evidence of partial migration behavior (that is, populations composed of resident and migratory contingents) in any of the five fish species.

We found that the life cycles of migratory fish in rivers usually include at least three bidirectional migrations for varying purposes at specific growth stages: the downriver migration of larvae and juveniles for feeding and refuge, the upriver migration of subadults and adults for reproduction, and the downriver migration of post-spawners for feeding. While we revealed that migration triggers are endogenous and linked to gonadal development (fig. S1), they may also be exogenous and depend on environmental factors, such as water temperature and fluctuations in water level. Next, we present the life cycle findings for each of the five fish species before the construction of the Yangtze dams.

Chinese sturgeon

The Chinese sturgeon belongs to the order Acipenseriformes, the family Acipenseridae, and the genus Acipenser in the binomial system. It was granted first-class national protected animal status in China in 1988, listed as critically endangered (CR) on the International Union for Conservation of Nature’s Red List of Threatened Species (IUCN Red List) in 2010, and listed in CITES Appendix II in 1998 (29). There are 19 spawning sites along an 800-km stretch of the Maoshui-Wanzhou section, with the major sites between Yibin and Hejiang, such as Sankuaishi (Fig. 3A), Pianyanzi, Jinduizi, Tielutan, and Wanglongqi (25). Breeding occurs during the last flood in October, commonly referred to as the “autumn water descending” phenomenon. Concurrently, water temperature decreases to 18° to 20°C, accompanied by an increase in flow velocity and sediment content. Females reproduce between 13 and 34 years old and males between 8 and 27 years old, and the sex ratio (females to males) is 1:1. The species migrate between Yibin and the sea, spending most of its life on the East China Sea’s continental shelf.

The anadromous nature of the Chinese sturgeon has consistently remained undisputed. Nonetheless, the preceding depiction of its life cycle was overly simplistic and devoid of comprehensive migratory details. In the case of this extensively examined species, we have acquired an intricate portrayal of its full life cycle within the Yangtze River using a migration dynamics model (text S4.1) (30, 31). As shown in Fig. 4A, adults at the gonadal stage III migrate to the Yangtze estuary every summer from June to August. They then move up to the Hubei section in September, Yichang in December, and Yibin in July of the following year. Once they reach Yibin Maoshui, a series of upstream rapids prevent them from moving further, so they congregate between Maoshui and Hejiang from July to October. Approximately 70% of the spawners complete their first breeding batch by mid-October, while the remaining 30% complete their second breeding batch before mid-November. After spawning, the post-spawners at gonadal stage II migrate downstream and reach the estuary within 16–28 days. After spawning at the Sankuaishi spawning site (an example), fertilized eggs sink and adhere to the gravel substrate and hatch after 5 days. The number of days post-hatching (dph) was used to characterize the age of individual fish. The downstream migration of juveniles can be divided into three phases: drift stage (1 to 8 dph), cover stage (9 to 18 dph), and self-migration stage (19 to 270 dph). Juveniles arrive in the estuary at 7 months, undergo osmoregulation, and grow rapidly. At 9 months, they depart the estuary and enter the sea.

Fig. 4. — (A) Chinese sturgeon (anadromy). (B) Chinese paddlefish (freshwater amphidromy). (C to E) Yangtze sturgeon, Chinese sucker, and *C. guichenoti*, respectively (riverine potamodromy). See text S4 for details. GD, Gezhouba Dam; TGD, Three Gorges Dam; XJB, Xiangjiaba Dam; XLD, Xiluodu Dam.

Chinese paddlefish

The Chinese paddlefish belongs to the order Acipenseriformes, the family Polyodontidae, and the genus Psephurus in the binomial system. It was granted first-class national protected animal status in China in 1988, listed as extinct (EX) on the IUCN Red List in 2022, and listed in CITES Appendix II in 1998 (18). The Chinese paddlefish, the longest fish in the Yangtze River, measures up to 7 m in length and weighs up to 908 kg. It mainly lives in the freshwater environment of the Yangtze River and can be found in various locations such as the mainstream, tributaries, river-connected lakes, estuary, and East China Sea (32). Its spawning sites are located in the upper reaches of the Yangtze River, specifically in the lower section of the Jinsha River. In Yibin City, there are at least two spawning sites: Jiangan and Baishuxi (33). Baishuxi (Fig. 3B) is approximately 10 km downstream of Sankuaishi, a Chinese sturgeon spawning site. Females lay sticky eggs on a pebbly substrate, and the breeding season occurs between March and April. Females typically breed between 6 and 16 years old, while males breed between 5 and 10 years old. The sex ratio is 1:2 (34).

The existing understanding of the migration pattern and life cycle of Chinese paddlefish is erroneous, resulting in a severe underestimation of the influence of the GD on the fish (17, 18, 23, 28). To rectify this, we re-evaluated survey data to validate the migration pattern as freshwater amphidromy, and we incorporated these findings into its life cycle model (Methods and text S4.2). The resulting life cycle of the Chinese paddlefish is illustrated in Fig. 4B. When males aged 5 years and females aged 6 years reach sexual maturity, they breed at Baishuxi (an example) from late March to early April. The fertilized eggs sink and adhere to pebbles and hatch into juveniles that migrate downstream and feed along nearshore waters. By July and August, they reach the estuary at 4 to 5 months old. They continue to feed and grow in estuarine and coastal areas until they become subadults. The subadults reach gonadal stage II at 1.5 to 2 years old and migrate from saltwater to freshwater in the Yangtze River. They then begin the upriver migration to the original spawning site, occasionally entering tributaries and lakes for feeding before returning to the mainstream and moving toward the upper reaches of the river. The migration time for subadults from the estuary to the spawning ground can range from 1 to 4 years. When subadults reach gonadal stage III and are ready to breed, they migrate to the Yibin-Chongqing section during the fall and winter seasons. After breeding, post-spawners at gonadal stage II migrate downstream and disperse throughout the habitat from Yibin to the estuary for feeding.

Yangtze sturgeon

The Yangtze sturgeon belongs to the order Acipenseriformes, the family Acipenseridae, and the genus Acipenser in the binomial system. It was granted first-class national protected animal status in China in 1988, listed as EX in the wild (EW) on the IUCN Red List in 2022, and listed in CITES Appendix II in 1998 (19). The Yangtze sturgeon is mainly distributed in the upper reaches of the Yangtze River and its tributaries, including the Minjiang, Tuojiang, and Jialing rivers, with a small portion in the middle and lower reaches of the Yangtze River. Spawning sites are located between the Yibin-Hejiang section, such as Anbian (Fig. 3C), Nanguang, Majiaheishibao, and Guanyintuo. Females lay sticky eggs in spring and late fall, with most breeding occurring in spring. Spawners consist of males aged 4 to 7 years and females aged 6 to 8 years, with a sex ratio of 1:2.5 (25).

Upon conducting a thorough analysis of our proposed definition of riverine potamodromy and carefully examining the existing data, we determined that the Yangtze sturgeon can be accurately classified as a riverine potamodromous fish. In addition, we collected survey data to incorporate into its life cycle model (Methods and text S4.3). The resulting life cycle of the Yangtze sturgeon is shown in Fig. 4C. In March-April and November-December, 4- to 7-year-old males and 6- to 8-year-old females breed in a dispersed manner along the 300-km Yibin-Hejiang section of the Yangtze River. The fertilized eggs sink and adhere to gravel and hatch after 4 to 5 days. At 13 dph, the larvae start feeding at the spawning sites. At 30 dph, the juveniles migrate downstream along the nearshore waters to search for habitats with abundant food and to disperse and mitigate competition and predation risks. The majority of juvenile individuals reside and forage within the mainstream and tributaries of the Luzhou-Chongqing section of the Yangtze River, while a minority migrate with the current to the middle and lower reaches of the Yangtze River, extending as far as Tongling City in Anhui Province. When 3-year-old males and 5-year-old females reach gonadal stage III, they begin to migrate upstream to the spawning sites in the Yibin-Hejiang section for breeding. During this period, which lasts at least 1 year, they occasionally enter tributaries or lakes to feed and return to the mainstream to continue their upstream journey. As gonadal maturity progresses, 4-year-old males and 6-year-old females arrive at spawning sites. After breeding, the post-spawners disperse and stay in the upper reaches for feeding. Once their gonads reach stage III, they migrate to the spawning sites to reproduce again. Female fish usually reproduce every 2 years.

Chinese sucker

The Chinese sucker belongs to the order Cypriniformes, the family Catostomidae, and the genus Myxocyprinus in the binomial system. It was granted second-class national protected animal status in China in 1988 and listed as vulnerable in the China Red Data Book of Endangered Animals in 2003 (35). The Chinese sucker is mainly distributed in the Yibin-Chongqing section of the Yangtze River and the lower reaches of the Minjiang River, a major tributary of the upper Yangtze. It has also been observed in the middle and lower reaches of the Yangtze River (36). Spawners are primarily located in the lower Minjiang River and the Yibin-Chongqing section, while juveniles and subadults are in the middle and lower Yangtze River. Breeding occurs on gravel beaches from March to April at spawning sites such as Longxuanzi (Fig. 3D), Menkantan, Ganlongzi, Wangyemiao, Sipo, and Nanzhuatan in the lower Minjiang River. The sex ratio is 1:1.39, and the average fecundity is 286,700 eggs. Historically, there were two other populations in the Yalong River and the Jialing River (37), but they were scarce in the 1970s.

The current understanding of the migration pattern and life cycle of the Chinese sucker remains uncertain (26). To clarify this matter, we have undertaken a reanalysis of survey data to validate the riverine potamodromous nature of this fish’s migration. Subsequently, we integrated the acquired data into its life cycle model (Methods and text S4.4). The resulting life cycle of the Chinese sucker is illustrated in Fig. 4D. Spawners (females aged 7 to 10 years; males aged 7 to 11 years) reach the Yibin section before February each year. From early February to early March, some spawners (mostly males) migrate up the Minjiang River and arrive at the Longxuanzi spawning site (an example) on the lower Minjiang River. In March and April, many spawners move upstream, and more females appear. Breeding is finished by then. The fertilized eggs sink, adhere to the gravel substrate, and hatch after 10 days. The larvae stay in the riverbed at the spawning site for 1 to 3 dph, and at 4 dph, they start feeding. At 19 dph, they start migrating in groups. Some stay in plentiful feeding areas, while others continue dispersing with the water current to suitable downstream feeding areas. A few reach the estuary by the current year’s fall. Every spring, subadults aged 3 to 6 years with stage II gonads start their homing migration from the middle and lower reaches of the Yangtze River to the spawning sites in the lower section of the Minjiang River for at least 4 years. After breeding, post-spawners at gonadal stage II migrate to the mainstream and tributaries in the Yibin-Wanzhou section for feeding. When their gonads reach stage III, they migrate upstream to the spawning sites to reproduce again.

C. guichenoti

C. guichenoti belongs to the order Cypriniformes, the family Cyprinidae, the subfamily Gobioninae, and the genus Coreius in the binomial system. It was granted second-class national protected animal status in China in 2021. The fish is mainly distributed in the upper and middle reaches of the Yangtze River, with a historical presence in tributaries such as the Yalong, Dadu, and Minjiang rivers (37, 38). The species breeds in the rapids of the river and produces floating eggs that drift and diffuse downstream with the current. The distribution of spawning grounds spans from Jin’anqiao in Yunnan Province to Yibin in Sichuan Province, with large spawning sites in the middle Jinsha River, such as Jin’anqiao, Duomei (Fig. 3E), and Xuzhou (Yibin). Notably, Jin’anqiao accounted for the majority, representing 84% of the total breeding size (38). The fish attain reproductive maturity within 3 to 7 years for males and 4 to 7 years for females, showing a minimal variance between sexes. The sex ratio of females to males is 1.37:1 (39, 40). The breeding season for this species is from late April to early July, with the peak spawning period in May to early June (40, 41). Breeding activity occurs only when the water temperature is 19° to 22°C, and the water level is rising or falling (42). Fertilized eggs require 48 to 62 hours to hatch at water temperatures of 20.5° to 21.9°C, and the larvae start feeding 4 days after hatching (40, 43, 44).

Previous studies on the migratory patterns of C. guichenoti have used a variety of terms and vague descriptions. In addition, these studies have primarily focused on the eggs and/or larvae (eggs/larvae), lacking a clear description of the entire life cycle, which includes various life stages (39–44). To address this, we reanalyzed survey data to validate the migratory pattern as riverine potamodromy. Subsequently, we integrated these findings into the life cycle model (Methods and text S4.5). The resulting life cycle is shown in Fig. 4E. From April to July each year, spawners (4- to 7-year-old females and 3- to 7-year-old males) congregate at spawning sites within an approximately 1000-km stretch between Jin’anqiao and Yibin. After breeding, the fertilized eggs drift with the current for approximately 1100 km to complete their embryonic development. This results in larvae being distributed along a 1700-km stretch between Yibin and Wuhan, which is the main feeding area for the recruitment stock (larvae, juveniles, and subadults). Larvae reside and feed there, occasionally entering into neighboring tributaries or lakes for feeding. Subadults over 1 year migrate upstream, taking at least 2 years from the Yibin-Wuhan section to the spawning sites between Jin’anqiao and Yibin. They feed and grow during the migration. After breeding, post-spawners mainly disperse to the Jin’anqiao-Yibin section to feed and reproduce again once their gonads are mature.

Estimating the invalid stocks and DICs for the five fish species

Dams may change the biophysical features of a river through impoundment, habitat fragmentation, blockage, alteration of the hydrological regime, hypolimnetic discharge, and nutrient retention (12, 45). These alterations may directly influence the growth, gonadal development, migration, and reproduction of fish in the river. We emphasize that river closure and dam impoundment have distinct impacts on migratory fish. River closure, typically for dam construction, involves diverting water flow into a tunnel or canal, increasing flow velocity, and preventing fish from migrating upstream. Dam impoundment involves the storage of water for power generation, cutting off upstream and downstream connectivity. As a result, the river closure allows eggs/larvae to drift downstream but prevents upstream migration of fish below the dam. After impoundment, fish below the dam need facilities to move upstream, while adults above the dam cannot easily move downstream, and eggs/larvae or juveniles depend on reservoir flows and turbine structures for successful downstream passage. In the daily-regulated reservoir, such as the GD, the mean flow velocity in the reservoir exceeds the floating velocity of the eggs/larvae (0.3 m/s), allowing the majority of individuals to pass through the turbines. Conversely, in the seasonally regulated reservoir, such as the TGD, the flow velocity near the dam site is less than the floating velocity, resulting in the retention of all eggs/larvae within the reservoir area and subsequent mortality of underdeveloped eggs/larvae.

We divide the species population into spawning stock (spawners), which are sexually mature adults participating in the current year’s breeding, and recruitment stock, which includes larvae, juveniles, and subadults that have not reached the reproductive age and sexually immature adults/post-spawners that do not participate in the current year’s breeding. Here, we define the “barrier effect” of the dam as a phenomenon where the dam cuts off upstream and downstream connectivity and changes the biophysical features of the river, preventing all or part of the spawning and recruitment stocks from completing their life cycle, especially reproduction, leading to generation of invalid stocks.

We introduce the term “invalid stock” to describe the impact of dams on the migratory fish population. This refers to individuals in the spawning and recruitment stocks that cannot reproduce due to the dam barrier effects. The individuals that can reproduce are referred to as “valid stock.” Dams can render the entire or a portion of the fish population’s spawning and recruitment stocks invalid. Seven distinct types of invalid stocks describe different combinations of spawning and recruitment stocks. These types include partial spawning stock (PSS; type 1), total spawning stock (TSS, type 2), partial recruitment stock (PRS, type 3), total recruitment stock (TRS, type 4), PPS combined with PRS (PSS + PRS, type 5), PPS combined with TRS (PSS + TRS, type 6), and TSS combined with TRS (TSS + TRS, type 7) (Fig. 5A). One combination (TSS + PRS) cannot exist alone and instead becomes either type 2 or type 7. This is because if all the spawners are invalid, then there will be no reproduction and the recruitment stock will disappear.

Fig. 5. — (A) Seven types of invalid stocks. (B to F) Schematic showing the barrier effects of dams on the five fishes. In the panel: GD; TGD; XJB; XLD; BHT, Baihetan Dam; WDD, Wudongde Dam; GYY, Guanyinyan Dam; LDL, Ludila Dam; LKK, Longkaikou Dam; JAQ, Jin’anqiao Dam; AH, Ahai Dam; LY, Liyuan Dam.

We developed a simplified model to quantify the impact of dams on data-limited migratory fish (Methods and text S3). This model includes a parameter called the DIC, denoted by β, which ranges from 0 to 1. A lower DIC indicates a greater barrier effect of dams on fish survival rates and the proportion of invalid stock and vice versa. We proposed four methods to estimate the DIC: the catch method, the abundance method, the valid stock method, and the analogy method (Methods).

We examined the barrier effects of the Yangtze dams on the five fishes and found that they produced all seven types of invalid stocks (movies S1 to S5). We also estimated the DICs for these fishes (text S4).

Chinese sturgeon

Since 1982, this fish has not been observed breeding above the GD, and only 24.2% of the spawning stock below the GD completed breeding between 1981 and 2002 due to the mismatch between the gonad mature period (GMP) and the spawning window period (SWP) (30). As a result, 75.8% of the spawning stock was invalid (type 1) (Fig. 5B). After the TGD began operating in 2003, the proportion of invalid stock reached 95.5 to 100% (almost type 2) because the dam increased the water temperature by 2.7°C below the GD during the breeding season, resulting in a reduction in the SWP. The operation of the Xiangjiaba and Xiluodu dams in 2013 further increased the water temperature, resulting in no overlap between the GMP and the SWP and thus entirely inhibiting breeding. Thus, the TSS became invalid (type 2). The cumulative impact of the cascade dams on the reproductive success of this anadromous fish has shifted the invalid stock from type 1 to type 2. This indicates that effective natural breeding of the Chinese sturgeon has ceased since 2013, rendering the species functionally EX. The “barrier effects” of the Yangtze dams on the Chinese sturgeon are manifested in delayed gonadal development by shortening the migration distance and inhibiting reproduction by elevated water temperatures (30).

We used the abundance method to estimate the Chinese sturgeon DICs corresponding to the cumulative impact of the cascade dams during different periods. This method relied on annual population size data (30). In the spawning stock, females were 13 to 34 years old and males were 8 to 27 years old. Therefore, the impact of the GD on males and females began in 1989 and 1994, respectively, while the impact of the TGD started after its impoundment in June 2003 and that of the Xiluodu Dam began in 2013. Table 1 shows the DICs for the spawners corresponding to the GD, TGD, and Xiluodu Dam. The β value of GD is 0.85 for males and 0.92 for females, indicating that the GD has more impact on males than females. The increasing effect of the Yangtze dams on the spawners, corresponding to β = 0.88 to 0.93, indicates that the Chinese sturgeon has high life history plasticity, with a recession rate of approximately 10% per year. Before 1981, the marine stock was above 30,000 individuals. Only approximately 5% of the marine stock enters the Yangtze River each year (30). As a result, the population has been declining for more than 40 years.

Table 1. DICs for the five fishes in the Yangtze River.

In the table, GD, Gezhouba Dam; TGD, Three Gorges Dam; XJB/XLD, Xiangjiaba Dam/Xiluodu Dam. Parentheses indicate the applicable DIC period. See text S4 for details. N/A, not applicable.

Fish species	Dam
Fish species	GD	TGP	XJB/XLD
Chinese sturgeon	0.93 (1989–2002) for spawners; 0.85 (1989–2002) for males; 0.92 (1994–2002) for females	0.882 (2003–2012) for spawners; 0.976 (2003–2012) for males; 0.842 (2003–2012) for females	0.881 (2013–) for spawners; 0.842 (2013–) for males; 0.911(2013–) for females
Chinese paddlefish	0.63 (1981–1984) for the old spawning stock; 0.78 (1985–) for the new spawning stock	N/A	N/A
Yangtze sturgeon	0.75 (1981–)	N/A	N/A
Chinese sucker	0.85 for spawners above the GD (1981–2009) and below the GD (1985–2002), for juveniles (1981–2002)	0.98 for spawners (2010–) and juveniles (2003–) above the TGD; 0.74 for spawners and juveniles (2003–) below the GD	N/A
*Coreius guichenoti*	0.83 for eggs/larvae (1981–2002) above the GD; 0.8 for subadults and adults below the GD (1981–2002)	0.98 for eggs/larvae (2003–2007) above the TGD; 0.32 for subadults and adults below the GD (2003–)	0.53 (2008–) for spawners and eggs/larvae above the TGD

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Chinese paddlefish

Before the river closure of the GD in January 1981, Chinese paddlefish spawners and partial subadults migrated to the upper Yangtze River during the fall and winter of 1980 (Fig. 5C). The spawners, known as the “old” spawning stock, were 6 to 16 years old for females and 5 to 10 years old for males. Partial subadults, 2 to 5 years old, known as the “new” spawning stock, represented a small proportion of the subadults. The majority of the subadults were blocked by the GD’s river closure and remained below the dam. Therefore, breeding activities of the Chinese paddlefish in the Yangtze River were conducted by the old stock until 1985, when they were gradually replaced by the new stock. After 1981, all juveniles that hatched in the upper Yangtze had to migrate downstream through the turbines of the GD. As a result, the TRS was invalid (type 4) because the GD prevented upriver migration, meaning successful breeding above the GD did not supplement the spawning stock. The barrier effect of the GD on the Chinese paddlefish is evidenced by the need for newly hatched juveniles to pass through the dam’s turbines and the obstruction of their return to their natal habitat. In addition, the inability of the fish to reproduce below the dam exacerbates the adverse effects. Thus, the Chinese paddlefish has sealed its fate of inevitable extinction from a life cycle perspective after 1981, relying solely on the new and old spawning stocks above the dam to survive for another decade.

We used the catch method to estimate the DICs of Chinese paddlefish. Wei et al. (46) documented the quantity of fish captured below the GD between 1981 and 1995. The captured fish had successfully navigated downstream through hydraulic turbines from their upstream spawning grounds, thus providing insight into fluctuations in the spawning stock size. As shown in fig. S2, the pre-1985 population decline was primarily due to the declining old stock, whereas the post-1985 decline was driven by the new stock. Using the simplified model, we performed a regression analysis on the pre- and post-1985 data, yielding the DICs of β = 0.63 (1981–1984) and β = 0.78 (1985–) (Table 1). The Chinese paddlefish had a DIC of 0.63 to 0.78, while the Chinese sturgeon had a DIC of 0.88 to 0.93, indicating that the paddlefish is more affected by the GD and has lower life history plasticity.

Yangtze sturgeon

We assumed a linear distribution of population density along the migration route, reflecting a decrease in newly hatched juveniles as they moved downstream. Approximately 75% of the annual juveniles were distributed upstream of the GD and 25% downstream. Figure 6A shows the density distribution of juveniles pre- and post-GD. The dotted lines at the GD site indicate that approximately 30% of the juveniles passing through the turbines were killed, while others survived and reached the middle and lower reaches of the Yangtze River. Since 1981, an estimated 25% of the juveniles that hatched in the upstream spawning grounds annually have been unable to migrate back upstream for reproduction or successfully reproduce below the dam. This has resulted in them becoming an invalid stock (type 3) (Fig. 5D). The barrier effect of the GD has caused the invalidation of 25% of the recruitment stock each year since 1981, leading to a persistent decline in population.

Using the valid stock method, we estimated the DIC of juveniles to be β = 0.75, as shown in Fig. 6A. Under the assumption that the GD has an equivalent effect on both the spawning stock and the recruitment stock, we can derive the DIC of the entire population of Yangtze sturgeon (Table 1).

Chinese sucker

Figure 6B (solid red line) shows that juveniles were mainly distributed along a 500-km stretch between Yibin and Chongqing and a 1675-km stretch between Yichang and the estuary. The Three Gorges section was just a migration corridor. Assuming an even distribution, we estimated that the number of juveniles in the middle and lower reaches of the Yangtze River was roughly three times the number in the upper reaches (75:25%). As 3- to 6-year-old subadults (solid blue line) moved closer to upstream spawning sites, their density increased. Approximately 65% of subadults were found in the upper reaches, whereas 35% were in the middle and lower reaches. Spawners were mainly located in the 500-km river section between Yibin and Chongqing, with a higher density in Yibin and a lower density in Chongqing (solid black line). Since the river closure for the GD on 4 January 1981, the Chinese sucker population has split into two independent subpopulations. The upstream subpopulation continued to reproduce but lost subadults below the GD (blue dotted line), causing a decrease in breeding size (black dotted line on the left side). In addition, approximately 30% of the juveniles passing through the turbines were killed each year, resulting in a decrease in juvenile densities above and below the GD. The 3-year-old subadults below the GD would reach reproductive age in 1985. Only 74% of the spawners below the GD reproduced, and 26% became invalid. The barrier effects of the GD on the Chinese sucker are manifested in the loss of juveniles passing through the turbines each year and in the invalid stock of partial spawners below the dam (Fig. 5E). The combination of these two factors was classified as type 5.

Jiang and Yu (47) reported 153 bycatch individuals in the Tongling section of Anhui Province from 1997 to 2002. Using the catch method, we estimated a DIC of 0.85 for spawners (1985–2002) and juveniles (1981–2002) below the GD (fig. S3). Before 1981 (pre-GD), Chinese suckers bred only in the upper reaches, and there was no evidence of breeding in the middle and lower reaches. After 1981 (post-GD), Chinese suckers were forced to breed below the GD, but the unfavorable environmental conditions prevented some spawners from reproducing successfully. We estimated that the breeding efficiency of Chinese suckers below the GD was 0.74, meaning that only 74% of the spawners participated in breeding activities each year. After 2003 (post-TGD), no upstream juveniles were received below the GD. The DICs below the GD decreased from 0.85 for spawners (1985–2002) and juveniles (1981–2002) to 0.74 for spawners and juveniles (2003–). There was a lack of relevant data for the upper reaches of the Yangtze River. We then assumed that the DIC for spawners above the GD/TGD was the same as below the GD, with a value of 0.85. From 2003 onward, newly hatched juveniles remained above the TGD and started to reproduce in 2009. To account for the potential mortality of unsuitable juveniles in the lentic water environment of the reservoir, we adjusted the DIC to 0.98. Consequently, DICs above the GD/TGD for spawners were 0.85 (1981–2009) and 0.98 (2010–), while DICs for juveniles were 0.85 (1981–2002) and 0.98 (2003–) (Table 1).

C. guichenoti

In Fig. 6C, we assume a linear density distribution of spawners from Jin’anqiao to Yibin. The Jin’anqiao spawning site was the largest, with the others decreasing linearly in size. Consequently, the Yibin section has the highest larval density, correlating with a migration distance of approximately 1100 km, during which fertilized eggs undergo development into feeding larvae and remain in the section. Approximately 17% of the eggs/larvae below the GD and TGD were considered invalid stock (type 3), suggesting that these two dams had less impact on the species (Fig. 5F). However, the Xiangjiaba and Xiluodu dams, located exactly on the demarcation line between the spawning ground and the nursery habitat, completely separated the spawning stock and the recruitment stock, disrupting their life cycle integrity. As a result, the minority of the spawning stock and the TRS became invalid (type 6). In addition, following the operation of the 12 major dams, the TSS and the TRS also became invalid (type 7). The population collapse of C. guichenoti was not due to the barrier effect of a single dam, but rather the superposition of multiple barrier effects from the Yangtze dams, manifested in increasing proportions of invalid stocks in both the spawning and recruitment stocks as the number of cascade dams increased.

We used the four methods to estimate the DICs for C. guichenoti to understand how a combination of methods can be used under different data scenarios as follows (Table 1):

In valid stock method, Fig. 6C shows that 17% of the total eggs/larvae were below the GD. After 1981 (post-GD), this 17% became invalid stock because they were unable to reproduce below the GD as they matured. The remaining 83% above the GD were considered valid stock. Therefore, the DIC of the eggs/larvae above the GD was 0.83 (1981–2002). After 2003, the TGD prevented eggs/larvae from passing through, causing an increase in their density in the reservoir. Taking into account minor mortality in the reservoir, the DIC of eggs/larvae above the TGD was estimated to be 0.98 (2003–2007).

In abundance method, since 1981, a survey of egg-eating fish below the GD has been used to study the reproductive behavior of Chinese sturgeon, as C. guichenoti is a major predator of their eggs. Yu et al. (48) provided data on the estimated number of subadults/adults of C. guichenoti from 1997 to 2001, allowing us to calculate a DIC of 0.8 (1981–2002) (fig. S4A).

In catch method, Tao et al. (49) reported catch numbers below the GD from 2005 to 2007, indicating a changing trend in the C. guichenoti stocks (subadults/adults) after 2003 (post-TGD), with a DIC of 0.32 (2003–) (fig. S4B).

In 2007–2008, the XJB and XLD’s river closures resulted in the disappearance of spawning grounds in the two reservoirs, and 15% of the spawners became invalid stock (Fig. 6C). We consider the Xiangjiaba and Xiluodu dams as one unit due to their proximity and the 1-year difference in their completion dates. Here, we propose two approaches to estimate the DICs for C. guichenoti affected by these dams.

In analogy method, the Chinese paddlefish spawners above the GD were totally valid (β = 0.63), and 85% of the spawners above the Xiangjiaba and Xiluodu dams were valid (Fig. 6C). Using this information, we estimated the DIC of spawners and eggs/larvae to be 0.54 (0.63 × 0.85), as they are equivalent in this context.

In catch method, Tang et al. (38) analyzed the survey data of eggs/larvae in the Geliping section of Panzhihua from 2006 to 2010, showing the effect of Xiangjiaba and Xiluodu dams on spawners. With a simplified model, the regression analysis yielded a DIC of 0.53 for spawning stock and eggs/larvae (fig. S4C).

The results indicate that the DIC is 0.54 for the analogy method and 0.53 for the catch method, both of which are reasonable. Thus, we chose the DIC value of 0.53 (2008–).

Trend analysis of dam-induced impacts using invalid stock types

We investigated the correlation between the invalid stock and the DIC. Our results suggest that invalid stock types can indicate the trend of dam-induced effects. Identifying these types helps to estimate the magnitude of impacts and compare them with data-limited fish.

Population decline accelerates as the proportion of invalid stock increases. Figure 7A shows that total invalidation (type 2, type 4, and type 7) causes larger population declines than partial invalidation (type 1 and type 3). Despite their different species and migration patterns, both Yangtze sturgeon and Chinese paddlefish show a consistent trend where type 3 (PRS) has a higher DIC of 0.75 compared to type 4 (TRS) with a value of 0.63.

Fig. 7. — (A) Comparison of DICs for partially invalid stocks and totally invalid stocks. (B) Comparison of the DICs for invalid spawning and recruitment stocks. (C) Comparison of the DICs for the GD, TGD, and the other cascade dams. (D) DICs of various invalid stock types.

Protecting recruitment stock is more important than protecting spawning stock. Figure 7B shows that the DICs for recruitment stock cases (type 3 and type 4) were lower (0.75 and 0.63) than for spawning stock cases (type 1 and type 2) (0.93 and 0.882). This trend was also observed in dual-factor cases. This finding is unexpected as the prevailing viewpoint prioritizes protecting spawning stock.

The cascade dams accelerate the rate of population decline to extinction. Figure 7C shows that the Chinese sturgeon population declined at an accelerated rate, as the DICs of GD, TGD, and other upper cascade dams were 0.93, 0.882, and 0.881, respectively. For C. guichenoti, the TGD had a slight positive effect. However, when other upper dams, such as the Xiangjia and Xiluodu dams were built, the fish population declined by almost half each year. In addition, different dams exerted distinct effects on the population of C. guichenoti. The Xiangjiaba and Xiluodu dams have the greatest impact (β = 0.53), followed by the GD (0.88) and TGD (0.98). Notably, the combination of TSS and TRS (type 7) represents the worst-case scenario for this species, as shown in Fig. 7D, with the lowest DIC (0.32) indicating the fastest population decline.

On the basis of the data from five species and 12 Yangtze dams, we conclude that further research on the invalid stock concept will provide a larger database to evaluate the impacts of dams on data-limited fish. We can extend the invalid stock concept to analyze the long-term impact of environmental factors such as climate change and water pollution on migratory fish. By conducting field and indoor experiments, we can assess the effects of high water temperatures or pollution on reproduction.

Recreating population decline processes of the five fish species

We have completed the validation of the simplified model based mainly on the Chinese sturgeon case (Methods). To use this model, we need to determine the initial population sizes of the species before the dam was built. The estimation methods depend on the availability of specific data, as data limitations vary. Now, we have very limited information on the abundance of the four fish species besides the Chinese sturgeon. There are two main categories of data available for analysis. The first is catch statistics, which includes Chinese paddlefish, Yangtze sturgeon, and Chinese sucker. Using an analogy method, we assumed that the exploitation rate of these three species mirrored that of Chinese sturgeon. Consequently, we used the amount of catch to estimate the population size. The second category includes statistics on eggs/larvae for C. guichenoti. Given the number of eggs/larvae at a particular station in a river, we extrapolated the total for that year using the eggs/larvae density distribution diagram along the river. The number of female spawners was then estimated from the mean fecundity, and the number of male spawners and the total number of spawners were estimated from the sex ratio of females to males.

Estimating initial population sizes

Wei and Yang (32) reported that the average annual catch of Chinese paddlefish in the Yangtze River in the 1970s was approximately 25 metric tons or 676 individuals. This is similar to the reported annual catch of 517 individuals for Chinese sturgeon in the 1970s, with an average annual exploitation rate of 23% (50). Both species were commercially harvested in the 1970s, primarily for the spawning stock, with a common catch range in the upper reaches of the Yangtze River. The exploitation rate refers to the ratio of individuals caught to the total number of individuals. Assuming the same rate for Chinese paddlefish in the 1970s, we estimated that the pre-GD spawning stock size was 2934 individuals. Using a sex ratio of 1:2 (34), we estimated that there were 978 females and 1956 males (text S4.2).

We collected catch data from the 1970s on the total catch and percentage of Yangtze sturgeon in the Yibin, Luzhou, and Hejiang river sections of Sichuan Province. The annual catch of Yangtze sturgeon in the 1970s was estimated to be 9700 kg (text S4.3). Assuming that the individuals harvested in this section of the river represent the spawning stock, and using the same exploitation rate as for the Chinese sturgeon (23%), we estimated the spawning stock of the Yangtze sturgeon to be 42,174 kg. With an average weight of 10 kg (25), the pre-GD spawning stock was estimated to be 4217 individuals. Using a sex ratio of 1:2.5, there were 205 females and 3012 males. Given that mature individuals comprised 6.7% of the total stock (25) and the assumed linear distribution of juvenile density (Fig. 6A), our estimate suggests that there were 58,723 individuals of the recruitment stock (juveniles and subadults) in the Yangtze River, of which 44,042 were in the upper reaches of the Yangtze River (above the GD) and 14,681 in the middle and lower reaches (below the GD). These estimates represent the pre-GD abundance of Yangtze sturgeon.

The historical abundance of the Chinese sucker is not well known due to a lack of catch data. Limited information suggests that from the 1950s to the 1970s, the Chinese sucker became less widespread, and its population notably decreased. The Yibin fisheries cooperatives in Sichuan Province provided the only catch statistics until the 1970s. We believe that overfishing in the Yibin spawning sites was responsible for the decline. After the ban on commercial fishing in 1983 and the listing of the Chinese sucker as a second-class protected animal in 1988, individuals of all ages reappeared in the Yangtze, indicating that overfishing did not cause the extinction of the species. We used mid-1970s estimates of pre-GD spawning stock size of 73 individuals (31 females and 42 males) and a juvenile abundance of 8.89 million individuals (text S4.4).

Before 2000, research on C. guichenoti was limited, resulting in a scarcity of relevant data. Using abundance data from 1997 to 2001 (48), we determined that the initial population size (subadults and adults) below the GD was 265,137 individuals in 1997 (fig. S4A). Data for the same period above the GD are not available. However, the construction of cascade dams in the Jinsha River has resulted in an increase in research endeavors since 2000. By analyzing the linear density distributions in Fig. 6C and considering the presence of 570 million eggs/larvae in Panzhihua in 2006 (38), we estimated the total number of eggs/larvae in that year to be 1.981 billion. Using an estimate of 22,817 eggs per female (39), we determined that the number of female spawners of C. guichenoti in the upper reaches of the Yangtze River was 86,821 individuals. In 2006, 63,373 males were present, estimated by a sex ratio of 1.37:1, resulting in a total of 150,194 spawners (text S4.5).

Dams trigger exponential population declines

We used the simplified model to reconstruct the population decline of each of the five fishes based on the estimated DICs (Table 1) and initial population sizes. In the case of the Chinese sturgeon, the males in the spawning stock were more affected by the Yangtze dams than the females and will be the first to go EW. Figure 8A shows that the Chinese sturgeon spawners declined from 2138 individuals (1033 males) in 1990 to 734 (122) in 2003 to 209 (82) in 2013 to 86 (25) in 2020 to 46 (10) in 2025.

Fig. 8. — (A) Chinese sturgeon spawners. (B) Chinese paddlefish spawners. (C and D) Spawning stock and recruitment stock of Yangtze sturgeon. (E and F) Spawners and juveniles of Chinese sucker. (G to I), Total resources (subadults/adults) below the GD, spawners, and eggs/larvae of *C. guichenoti*. See text S4 for details.

Figure 8B shows that the Chinese paddlefish spawners in the Yangtze River consisted of 2934 individuals (1956 males and 978 females) in 1980, which decreased to 40 individuals (27 males and 13 females) in 1993, and only one male in 2005. Given that 45 individuals were captured for scientific research between 1982 and 2003 (17), the spawners may have disappeared after 1993, meaning that the Chinese paddlefish stopped breeding and became functionally EX in 1993. Extinction occurred in 2005 or earlier.

Figure 8C shows that for the Yangtze sturgeon, the spawners consisted of 4217 individuals (1205 females and 3012 males) in 1980, decreased to 100 (28 and 72) in 1994, and to 18 (5 and 13) in 2000. Given the dispersed breeding manner of Yangtze sturgeon and bycatch mortality, we speculate that breeding activity stopped in 2000 or even earlier, explaining why “no naturally bred juveniles have been found throughout the river since 2000” (51). Figure 8D shows that the recruitment stock (juveniles and subadults) was 58,723 individuals (44,042 above the GD and 14,681 below the GD) in 1980, decreased to 1395 (1046 and 244) in 1994, to 248 (186 and 43) in 2000, and lastly to 14 (10 and 2) in 2010. If we include deaths due to bycatch and fishing for scientific research, all of which occur below the GD, these numbers are even lower. This explains why “no wild individuals have been found above the GD since 2010 and below the GD since 1994” (51).

Figures 8 (E and F) shows that the Chinese sucker spawners above the GD decreased from 73 individuals in 1980 to 3 in 2000 to 7 in 2010, while the spawners below the GD decreased from 26 in 1985 to 2 in 2000 to 0 in 2010. Thus, no juveniles have been produced naturally in the Yangtze River since 2010. The juveniles above the GD decreased from 2.22 million in 1980 to 100,000 in 2000 and to 26,000 in 2010, while the juveniles below the GD decreased from 6.67 million individuals in 1980 to 2.82 million in 1984. The number of juveniles increased to 4.67 million in 1985 due to the partial natural breeding scale (74%) below the GD, which began in 1985. After a transient rise in 1985, the juveniles continued to decline to 400,000 in 2000 and to 38,000 in 2010.

Between 1981 and 2003, the population of C. guichenoti below the GD was maintained by eggs/larvae from the upper spawning grounds in the Jinsha River that successfully passed through the turbines at the GD, as the species could not reproduce in the areas below the dam. However, after the construction of the TGD, the eggs/larvae were no longer able to pass through the turbines at the TGD. Consequently, the population below the GD disappeared by 2010. After the impoundment of the cascade dams (Xiangjiaba Dam in 2012 and Xiluodu Dam in 2013), the C. guichenoti population experienced a sharp decline. Figure 8G shows that the number of subadults/adults below the GD decreased from 265,137 individuals in 1997 to 70,000 in 2003, when the TGD was put into operation, and further decreased to only 8 in 2011. Figure 8 (H and I) shows that the spawners decreased from 150,194 individuals in 2006 to 38 in 2020, while eggs/larvae went from 1.981 billion individuals in 2006 to 500,000 in 2020.

Briefly, before the construction of the GD, the five fish species were economically important in the Yangtze River. However, since the operation of the GD in 1981, the populations of Chinese sturgeon, Chinese paddlefish, Yangtze sturgeon, and Chinese sucker have experienced a substantial decline. Although these four species have been legally protected from fishing since 1988, their populations have persistently dwindled. Similarly, until 2006, C. guichenoti was the primary catch in the upper reaches of the Yangtze River due to the relatively minor impact of the GD and TGD on this species. However, the construction of the cascade dams on the Jinsha River, notably the Xiangjiaba and Xiluodu dams, played a fatal role by severing the connection between the spawning stock and the recruitment stocks. Therefore, C. guichenoti went from an economically valuable species to an endangered species in a short time. As a result, C. guichenoti was officially classified as a second-class national protected species in 2021. Here, we propose extinction timelines for the five fish species. Specifically, the Chinese sturgeon was considered functionally EX in 2013 and will be EW in 2026. The Chinese paddlefish was functionally EX in 1993 and EX in 2005. In addition, the Yangtze sturgeon and the Chinese sucker were EW in 2010, while the C. guichenoti will be EW by 2030.

Migration patterns have varying effects on population declines. Among diadromous fishes, freshwater amphidromous fishes (e.g., Chinese paddlefish) are more affected by dams than anadromous fishes (e.g., Chinese sturgeon) because only a small proportion of the anadromous population develops into the spawning stock (about 5% for Chinese sturgeon) that enters the river to breed each year, while a large proportion of the recruitment stock remains in the sea. Therefore, even when breeding in the river ceases, the migratory phenomenon can persist for several years or even decades, creating an illusion of species population health or sustainability based on the presence of invalid stocks. Among riverine potamodromous species, the fish that cannot reproduce below the dam (e.g., Yangtze sturgeon and C. guichenoti) are more severely affected by dams than species that can reproduce below the dam (e.g., Chinese sucker), suggesting lower life history plasticity.

DISCUSSION

Scientific misjudgments underpinning the FRPs

After four decades of implementation of the FRPs, the five prominent fish species are facing imminent extinction. China has undertaken conservation efforts since 2015, with two action plans specifically aimed at conserving the Chinese sturgeon and the Yangtze sturgeon. However, these plans do not adequately address the negative impacts of dams and lack targeted measures (51, 52). In addition, China imposed a 10-year fishing ban on the Yangtze River in 2020, followed by enacting the Yangtze River Protection Law in 2021. This legislation designates 13 aquatic species, including the five fishes studied here, as CR (53). However, we believe that these measures will not be sufficient to recover the five fishes in the Yangtze River because it is imperative to address the dangerous situation created by the Yangtze dams. Our findings suggest that the FRPs for the Yangtze dams are scientifically based on unreliable judgments, resulting in a flawed fish conservation policy. Here, we present six major misjudgments that underlie China’s FRPs (text S5).

The first misjudgment concerns the assertion that overfishing is the primary cause of the population collapse of the five fishes. Our studies have shown that the Yangtze dams were primarily responsible for the decline of the Chinese sturgeon population (13, 30, 54, 55). However, IHB ichthyologists have chosen to disregard the failure of the FRPs and instead attribute the survival crisis of the five fishes, especially the Chinese sturgeon, to overfishing (56, 57). Our findings indicate that the Chinese sturgeon has not been overfished, as the average annual exploitation rate from 1972 to 2017 was 0.61%, with a peak of 3.6% in 1981, which is below the 5% threshold for overfishing (Fig. 9). Our research has consistently found no evidence to support overfishing as the primary factor contributing to the survival crisis of the five fish species. Instead, we highlight that the Yangtze dams, not overfishing, have disrupted the life cycles of the five fishes by blocking migration and hindering reproductive success. Moreover, the lack of targeted measures has led to the emergence of large invalid stocks. As a result, merely banning fishing without addressing the effects of dams is insufficient to save these five fish species.

Fig. 9. — (A) Total population size (30). (B) Total catch number and annual exploitation rate. ① Steady commercial fishing (pre-GD and before 1981), with an average exploitation rate of 1.6%. ② Initial stage of post-GD (1981–1982), with an average exploitation rate of 2.8%. ③ Fishing restricted to scientific purposes (1983–2008), with an average exploitation rate of 0.3%. ④ Comprehensive fishing ban (2009–present), with an average exploitation rate of 0.04%. An exploitation rate of 5% is a threshold for overfishing of the Chinese sturgeon. See text S5 for details.

The second misjudgment involves the claim that “breeding below the dam” indicates species conservation success. In January 1981, as the river closure of the GD approached, the pending fish rescue measures triggered a major debate on how to protect migratory fish in China. A particular concern arose regarding the Chinese sturgeon’s ability to continue breeding after the river was dammed. The IHB reported that its scientists caught eggs, larvae, and mature individuals below the GD in the fall of 1981 and 1982, suggesting that Chinese sturgeon could adapt to the dam-modified environment and continue to reproduce. Therefore, the IHB recommended restocking as the primary measure instead of fish passage. The discovery of “mature” Chinese paddlefish and Chinese sucker below the GD implies that their wild populations would not be extirpated (58–60). However, our findings indicate that the Chinese paddlefish population declined rapidly and became EX after the construction of the GD because the juveniles were unable to return to their spawning grounds after migrating to the sea. In addition, our findings show that breeding below the dam does not prevent extinction and is not as important as some ichthyologists expect. The spawning stocks of Chinese sturgeon and Chinese sucker experienced notable declines, with a large portion becoming invalid stock each year. Here, we emphasize that previous studies have used the presence of invalid stocks to suggest the existence of a viable and sustainable population, but this conceals the inevitable decline of these populations toward extinction (14, 58–60).

The third misjudgment is regarding the claim that restocking is a viable strategy for mitigating the adverse impact of dam-induced fish depletion. Restocking involves introducing artificially bred fish into existing populations to replenish diminished spawning biomass. In 1982, the IHB suggested that restocking could effectively offset the decline of the Chinese sturgeon population caused by the GD (14). As a result, the Institute of Chinese Sturgeon was established in 1983 with a specific focus on researching artificial breeding and release technology. In addition, many professional institutions have researched the artificial propagation and release of Chinese sturgeon and other migratory species, making restocking a popular solution to China’s dam impact on fish species. Since 1983, more than seven million Chinese sturgeon of various sizes have been released into the Yangtze River (13, 61). However, there is currently no strong evidence that any of the released individuals have returned to the Yangtze River and engaged in reproductive activities. Therefore, we have argued that the restocking efforts for the Chinese sturgeon have been unsuccessful (62), but not because of the alleged insufficiency in the quantity and size of released fish, as some ichthyologists have suggested (57, 61, 63). Restocking may be a better solution to overfishing than addressing the dam barrier effect. The study’s findings suggest that the success of FRPs is considerably dependent on the preservation of the fish life cycle, specifically the facilitation of reproduction in natural habitats.

The fourth misjudgment concerns the claim that replacing mainstream habitats with tributaries can protect flagship species. Since 2000, Chinese ichthyologists have proposed a fish conservation strategy called “replacing mainstream habitats with tributaries” to mitigate the negative impacts of cascade dams on the mainstream of the Jinsha River. This involves designating key tributaries as dam-free, free-flowing rivers by either prohibiting dams or dismantling small dams. Cao (64), an ichthyologist at the IHB, noted that if dams block migrating fish in the Jinsha mainstream, then they will swim back to the tributaries to reproduce. This approach was developed by the TGD-FRP in the 1990s. In the TGD reservoir area, 44 endemic species were observed, 29 of which were also found in the Chishui River (11). The Chishui River has been identified as a potential alternative habitat for fish species affected by the TGD (fig. S5). However, this approach cannot save the five flagship fishes or modify their life cycle because they primarily migrate in the mainstream of the Yangtze River and not in its tributaries.

The fifth misjudgment pertains to the assertion that the FRPs were backed by solid science. The selection of fish rescue targets for dams is critical. However, the prioritization of these targets is flawed due to knowledge gaps in fish migration taxonomy and life cycle integrity. The GD-FRP has focused exclusively on the Chinese sturgeon, ignoring the Chinese paddlefish, the Yangtze sturgeon, and the Chinese sucker. Our findings indicate that the priority of the GD-FRP should be determined by the severity of the impact, with the Chinese paddlefish ranking highest (β = 0.63 to 0.78), followed by the Yangtze sturgeon (0.75), Chinese sucker (0.74 to 0.85), Chinese sturgeon (0.88 to 0.93), and C. guichenoti (0.88 to 0.98). The Chinese paddlefish, Yangtze sturgeon, and Chinese sucker are more threatened by the GD than the Chinese sturgeon. Unfortunately, they were not prioritized in the GD-FRP, resulting in their extinction before the TGD-FRP was implemented. In the mid-1990s, a national rare fish reserve was established in the Hejiang-Pingshan section of the upper reaches of the Yangtze River as a key measure of the TGD-FRP. However, the barrier effect of the GD had resulted in the extinction or near extinction of protected species such as the Chinese paddlefish, Yangtze sturgeon, and Chinese sucker before the implementation of the TGD-FRP, so this reserve exists in name only. Our findings contradict the purported success and scientific foundation of the GD- and TGP-FRPs (56), indicating the opposite.

The sixth misjudgment concerns the assertion that fishways are unnecessary in dams. The 1982 GD-FRP suggested that fishways were not needed for the Chinese sturgeon (14). The TGD, built in 1993, followed this idea and did not include fishways. Despite a fish passage policy implemented by the Chinese government after 2000, 10 dams built since then upstream of the TGD still lack fishways. The underlying reason is that the IHB’s fish rescue strategy, which lacks fishways for the GD and TGD, has received considerable scientific and technical awards and recognition. In addition, it has been widely supported by the Chinese hydropower industry as an exemplary approach to fish conservation in the hydropower sector. The results of this study indicate that fish passage is the most favorable approach to mitigate the barrier effect of dams on migratory fish. Although doubts have been raised regarding the efficacy of fish passage (65), we argue that the implementation of fish passage is imperative in cases where the dam poses a threat to migratory fish populations. Consequently, we advocate the restoration of upstream and downstream connectivity in the Yangtze River. In particular, China’s fish passage policy needs improvement, as fishways built after 2000 only consider the upstream migration of the spawning stock, ignoring the downstream migration of the recruitment stock (larvae, juveniles, and post-spawners) (66).

Reforming China’s FRP for dams

Most of the world’s rivers are dammed, including new dams under construction. Protecting migratory fish from the threats posed by dams is a common challenge worldwide, including in China. However, despite China’s efforts to protect the Yangtze River, the negative impacts of dams on migratory fish have received limited attention due to the complex nature of the issue. To protect fish species, it is critical to learn from four decades of FRPs and focus on reforming China’s dam-related FRP.

China needs to recognize scientific misjudgments to redirect research toward innovation. The failure of the FRPs serves as a stark reminder of the need to conduct a comprehensive examination of the reasons behind the ineffectiveness of China’s fish conservation strategies and their purported “scientific results.” China has invested heavily in scientific research on dam-related fish conservation over the past 40 years. However, many of these scientific research efforts have proven inadequate to support the conservation of flagship fish species, primarily due to the misguided direction of basic research. To reformulate FRPs for the Yangtze dams, China must prioritize correcting misconceptions about past FRPs, including the six major misjudgments mentioned above.

China needs to strengthen oversight of dam owners to clarify their obligations to protect fish. These five fishes were the main targets of the FRPs for the Yangtze dams. Who exactly is responsible for saving these fishes? According to China’s environmental protection laws, the hydropower companies (dam owners) are responsible for protecting the affected fish. However, because of the complexity of factors affecting fish, such as overfishing, water pollution, and navigation, the responsibility of hydropower companies has often not been recognized, and they have been eager to fund authoritative institutions to absolve themselves of blame, rather than seeking ways to implement their responsibility to protect fish. We suggest that China strengthen the supervision of hydropower companies and incorporate fish conservation in the strict rating criteria so that the companies treat fish conservation as a true concern of the companies themselves.

China needs to use adaptive management to handle intricate ecological projects effectively. This involves a comprehensive set of procedures and institutional arrangements, including an “implementation, monitoring, evaluation, and improvement” cycle to continuously review and improve fish rescue strategies and countermeasures. Since 1995, China has established an ecological and environmental monitoring network for the TGD (20), which has proven to be useless for conserving these flagship fishes. To solve this problem, we propose to develop and implement an ecological operation plan for the Yangtze cascade dams to restore the reproductive hydrological conditions of the affected migratory fish species based on an adaptive management system and the monitoring network to constantly improve the operation program.

China also needs to improve international cooperation for innovation and knowledge sharing. Conflicts between dams and migratory fish are common in rivers worldwide (1–9). Therefore, international cooperation is urgently needed to address these challenges and share experience in maintaining the balance between fish conservation and dam management. In addition, international rivers that are undergoing hydropower development, such as the Amazon, Congo, and Mekong, require cooperative efforts to conserve migratory fish populations effectively. International cooperation is urgently needed for China to enhance fish passage and ecological operation techniques (66, 67).

The five fishes: Hope or concern?

Now, the five fish species have already lost their best chance for recovery. The destruction of spawning grounds, feeding grounds, and migration corridors can harm migratory fish and cause population extinction with the loss of any one of these three habitat elements (68). Unfortunately, all three habitats for the five species have been destroyed by the Yangtze dams. The Chinese paddlefish were EX in 2005, while the remaining four species are on the verge of extinction, with only hatchery-bred individuals surviving. Therefore, it is imperative to mitigate the barrier effects of dams and supplement natural populations through restocking and rewilding. How can the barrier effect of dams be mitigated? There are three potential solutions to address this issue: removing the dams, constructing fish passages, or implementing the ecological operation of the dams. However, the feasibility of removing the Yangtze dams in China remains impractical despite the growing prevalence of dam removal initiatives in the United States and Europe (69). The construction of fishways for these dams presents huge technical obstacles that make success unlikely in the short term. Consequently, the sole viable alternative lies in the ecological management of the cascade dams. Hence, our recommendations for the five fish species must be based on the existing circumstances.

To save the Chinese sturgeon, it is imperative to reduce the water temperature in the spawning area below the GD by 2.7°C from October 22 to November 15 (30). To achieve this goal, we propose to integrate the ecological operation of the cascade dams with local cooling techniques to maintain an appropriate water temperature range (18° to 20°C). In addition, as a complementary measure, we recommend improving the release efficiency of the Chinese sturgeon restocking program so that the species can return to the Yangtze River and reproduce successfully. The Yangtze sturgeon, Chinese sucker, and C. guichenoti are riverine potamodromous fish species that primarily breed in the upper Yangtze River, including a 400-km section from Yibin to Chongqing between the Xiangjiaba Dam and the end of the TGD reservoir, where some free-flowing features still exist. This study shows that several individuals from the three species were able to complete their life cycles in the 400-km section. It is imperative to investigate the life history plasticity of these species and develop strategies to mitigate the barrier effects of the dams. Conservation efforts for these species could be enhanced by promoting the successful completion of their life cycles, particularly their reproductive behavior, within the section.

METHODS

Fish migration taxonomy

The study of modern fish migration has a history of over 100 years since the publication of Meek’s book in 1916 (70). Taxonomic nomenclature and definitions were widely accepted until Myers’ work in 1949 (71). Later, McDonald and others (72–77) further improved the definitions, evolution, and application of fish migration classification. Fish migration taxonomy was based on the marked differences in salt content between freshwater and saltwater. Diadromous fish migrate between freshwater and saltwater, potamodromous fish migrate entirely within freshwater, and oceanodromous fish migrate entirely within seawater (71, 72). The life cycle of a migratory fish includes various growth stages related to its migration, from fertilized egg to larva, juvenile, subadult, adult, spawner, and post-spawner. This life cycle can vary greatly depending on species, habitat, and other environmental factors (72). However, definitions of migration classification are assigned based solely on the most distinctive migration in the specific life stage of a given fish (e.g., reproductive migration) and do not take into account other migrations that occur throughout the life cycle of migratory fish. Therefore, it is impossible to understand the entire life cycle of fish by relying on the classification of migration alone.

Here, we propose a three-tiered (category-type-subtype) framework for fish migration taxonomy (Fig. 2). In this framework, migratory fishes are divided into three categories—diadromy, potamodromy, and oceanodromy. Diadromy is subdivided into three types, namely, anadromy, catadromy, and amphidromy, of which the amphidromy is further subdivided into two subtypes based on the spawning environment, namely, freshwater amphidromy and marine amphidromy. Previous studies have defined the three types (anadromy, catadromy, and amphidromy) and the two subtypes (freshwater amphidromy and marine amphidromy) of diadromy (71, 73, 74). Here, we propose to subdivide potamodromy, which is largely dam related and less studied, into two types: riverine potamodromy and lacustrine potamodromy.

Definitions of migration patterns

Diadromy was first used by Myers in 1949 to describe “truly migratory fishes that migrate between the sea and freshwater” (71). Diadromy is the earliest described migration pattern, and diadromous fish are the most well-studied category of migratory fish. McDowall (74, 75) refined this definition to stipulate that diadromous migrations must occur regularly, be physiologically mediated movements between two biomes, occur at predictable times, occur during a characteristic life history phase in each species, involve most individuals in a given population, be typically obligatory, and involve two reciprocal migrations, from freshwater to the sea and the reverse. Myers (71) defined anadromy as “diadromous fishes that spend most of their lives in the sea and migrate to freshwater to breed.” McDowall (74, 75) refined the definition as follows: “diadromous fishes in which most feeding and growth are at sea before the migration of fully grown, adult fish into freshwater to reproduce; either there is no subsequent feeding in freshwater, or any feeding is accompanied by little somatic growth; the principal feeding and growing biome (the sea) differs from the reproductive biome (freshwater).” Myers defined catadromy as “diadromous fishes that spend most of their lives in freshwater and migrate to the sea to breed.” McDowall refined the definition as follows: “diadromous fishes in which most feeding and growth are in freshwater before the migration of fully grown, adult fish to sea to reproduce; there is either no subsequent feeding at sea or any feeding is accompanied by little somatic growth; the principal feeding and growing biome (freshwater) differ from the reproductive biome (the sea).” The term amphidromy was proposed by Myers to describe diadromous life histories where migrations between freshwater and seawater were not directly associated with reproduction. This difference in migration purpose (reproductive versus not reproductive) is the critical distinction between amphidromy and the other two types of diadromy. In addition, amphidromy is characterized by an obligate period of growth in an osmotic environment different from the hatching environment; Myers referred to this osmotic transition as “osmoregulatory migration.” Myers defined amphidromy as “diadromous fishes whose migration from freshwater to the sea, or vice-versa, is not for breeding but occurs regularly at some other definite stage of the life cycle.” McDowall refined the definition as follows: “diadromous fishes in which there is a migration of larval fish to sea soon after hatching, followed by early feeding and growth at sea, and then a migration of small post-larval to juvenile fish from the sea back into freshwater; there is further, prolonged feeding in freshwater, during which most somatic growth from juvenile to adult stages occurs, as well as sexual maturation and reproduction; the principal feeding biome is the same as the reproductive biome (freshwater).” Unfortunately, both of these definitions are inadequate and problematic when used to describe the entire life cycle. Myers’ definition is accurate but simplistic, while McDowall’s definition only applies to specific growth stages, such as larva and post-larva. See text S2 for the definition of oceanodromy.

Potamodromous fish are considered the most numerous migratory fish species in rivers worldwide (76, 77) and are the most common migratory fish species affected by dams. However, potamodromy is far less well studied than diadromy, and there is no further classification available to distinguish potamodromous phenotypes with complex life cycles (72). Therefore, we subdivided potamodromy into two types based on the spawning environment: riverine potamodromy and lacustrine potamodromy. We defined the riverine type as potamodromous fishes that must migrate to the upper reaches of the river for reproduction until they complete reproduction in the riverine running water environment under specific conditions. Juveniles drift downstream with the current and enter lentic areas of the river or lakes to feed until their gonads reach a specific stage and begin their upriver migration for reproduction. Most migratory fish species in the Yangtze River and other rivers in China exhibit riverine potamodromy, such as the four major carp species (Mylopharyngodon piceus, Ctenonpharyngodon idellus, Hypophthalmichthys molitrix, and Aristichthys nobilis). We also defined the lacustrine type as potamodromous fishes that must reproduce in the lake and usually lay eggs on aquatic plants. At a specific growth stage, they regularly migrate to the lotic rivers and return to the lake after a period. An example of fish species exhibiting lacustrine potamodromy is the roach (Rutilus rutilus) in Lake Krankesjon, a shallow lake in southern Sweden (78). These lacustrine potamodromous fishes are typically characterized by partial migration behavior.

Six descriptive life cycle models of migratory fish

The life cycle of migratory fish includes migrations of various stages. Previous studies of fish life cycles have often relied on the accumulation of survey data, which is limited by sample size, the inability to track juveniles, and the geographic limitations of fishery data. As a result, a well-rounded understanding of fish migrations may not be achieved, and erroneous inferences may be made. Therefore, we suggest using migration patterns and typical cases to create life cycle models for the fish. This will establish a life cycle framework that aligns with the migratory characteristics of the fish and avoids omitting key migration aspects based solely on survey data.

We conducted case studies on the entire life cycles of typical fish species whose migration patterns were well known. We then generalized six descriptive life cycle models of migratory fish corresponding to six migration types/subtypes of diadromy and potamodromy, excluding oceanodromy, which is not relevant to dams. Here, we show three life cycle models of anadromous, freshwater amphidromous, and riverine potamodromous fishes related to this study. See text S2 for the other three models.

First, the life cycle model of anadromous fish involves diadromous fish that migrate between freshwater and saltwater environments, spending most of their lives in the sea or saltwater lakes for feeding. Adults that have reached a specific stage of gonadal development and are ready to breed begin to enter the river and migrate upstream, feeding little during this upriver migration. Post-spawners of semelparous species die (e.g., salmonids), while post-spawners of iteroparous species migrate rapidly downriver to the sea for feeding. After hatching, larvae and juveniles likewise migrate downstream to the sea or saltwater lakes, feeding and growing during their downriver migration and then undergoing osmoregulation in the estuary before ultimately entering the sea.

Second, the life cycle model of the freshwater amphidromous fish involves diadromous fish that migrate between freshwater and saltwater environments, spending most of their lives in freshwater for feeding and reproduction. After hatching, larvae and juveniles must migrate downstream to the sea for early feeding and growth in saltwater until they reach a specific growth stage and complete osmoregulation. At this point, the fish return from the sea to the river for further feeding, prolonged upriver migration, somatic growth (e.g., from subadult to adult and spawner), sexual maturation, and ultimately reproduction in the river. After reproduction, post-spawners disperse downstream to feed, mostly in freshwater, until their gonads are close to maturity and they begin the upriver migration to reproduce again.

Last, the life cycle model of the riverine potamodromous fish involves potamodromous fish that migrate within freshwater and spend most of their lives in the lentic river areas or lakes for feeding. Sexually mature adults migrate to the upper reaches of the river for reproduction until they complete reproduction in the riverine running water environment under specific conditions. They lay adhesive and demersal eggs or floating eggs. The eggs/larvae drift with the current, or juveniles migrate downstream, and they enter river lentic areas or lakes for feeding and to develop into juveniles or subadults. Until their gonads reach a specific stage, they begin the prolonged upriver migration for reproduction. After reproduction, post-spawners migrate downstream to river lentic areas or lakes to feed until their gonads are close to maturity and they begin the upriver migration to reproduce again.

A procedure for determining migratory fish life cycle

We propose a procedure for determining the entire life cycle of a migratory fish species by combining a life cycle model with sporadic limited data. First, we use the three-tiered taxonomic system to confirm the migration pattern of a given fish and then identify its corresponding life cycle model. Second, we collect historical catch and survey data at various growth stages and verify the reliability of these data. We determine the spawning, feeding, and overwintering areas of the fish and the upper and lower limits of the migration path by collecting data on the reproduction, distribution, age structure, and abundance of the fish. This information is gathered from literature and field interviews with local fishermen. Third, we use the available data to specify the details of the bidirectional migration, including the onset times of juvenile downstream migration and adult (or subadult) reproductive migration. We also identify endogenous factors, such as gonadal development, and exogenous factors, such as water temperature or hydrological variations that trigger a shift in the bidirectional migrations. Last, we will obtain a general understanding of the entire life cycle of the species.

A simplified model of dam barrier effects

The alteration of fish resources involves the transition of generations and the progression of age groups characterized by varied natural mortality rates and fishing mortality rates. Age-structured population models that track dynamic changes in females and males in each age group can be used to quantify interannual variations in age distributions and population structure. Fish breeding behavior and population size usually fluctuate depending on environmental factors, such as river hydrological change (79). However, barring direct human interference or drastic changes in the natural environment, the age structures of fish, particularly spawners, are relatively stable (30). The effect of a given dam on the breeding activity and population size of a migratory fish depends on both dam attributes (e.g., its location, flow regulation capacity, inundation range, and impoundment time) and fish attributes (e.g., migration pattern and spatiotemporal distribution in the river). To quantify the dam barrier effect on population fragmentation, we derived a universal population model with age-specific survival rates and valid stock ratios. Solving the universal model for a specific fish species requires a substantial amount of biological and ecological data, including survival rates and valid stock ratios for all age cohorts. However, obtaining these data can be challenging (text S3).

In the absence of sufficient data, we propose a simplified mathematical model of the dam barrier effect that requires less data (text S3): N_t = N₀ ∙ β^t−T, where T is the first year of a given stock decline caused by dams, indicating a time lag between dam construction and a given stock decline; t is the number of years since time T; N₀ is the initial number of individuals in a given stock, such as the spawning stock, the recruitment stock, or the total population; N_t is the number of individuals in a given stock in the year t; and β is the dimensionless DIC (0 ≤ β ≤ 1). The DIC is an integrated coefficient that reflects the dam barrier effect on the survival rate and the proportion of valid stock.

The simplified model offers a valuable analytical approach for studying the barrier effects of dams on data-poor migratory fish populations. Figure S6 illustrates the impact of DIC (β) on the relative population sizes (N/N₀) at T = 0. The results indicate that if β is less than 0.6, then the population size will become EX in less than a decade. Conversely, if β is greater than 0.8, the population size can be sustained for over 20 years.

Estimation methods for the DIC

The key to using the simplified model is the estimation of the DIC, which depends on the strength of the dam barrier effect and the life history plasticity of the migratory fish. A larger value of β indicates a higher degree of life history plasticity and greater adaptability of the migratory fish to the dam barrier effect, implying a slower rate of population decline and vice versa. To estimate the key parameter, we recommend four methods based on the data status: the catch method, the abundance method, the valid stock method, and the analogy method. The DICs may vary for a given fish depending on its life stage (e.g., spawner or juvenile) and spatial location (e.g., upstream or downstream of the dam). Therefore, separate estimates must be made for each specific stock and its corresponding spatial location. To ensure accuracy, a combined approach may be necessary due to the different attributes of the collected data. However, assessing the data availability remains a challenge when using these methods. It is important to consider the influence of factors other than dams on the data, which may be small but cannot be entirely ignored.

The catch method is an estimation approach based on catch data. It assumes that the multi-year catch data in the fixed river section, under the same fishing conditions after dam construction, reflect the changes in population size. Therefore, the simplified model can be used to perform regression analysis on the data and obtain the DIC. In this study, we applied this method to specific stocks of Chinese paddlefish, Chinese sucker, and C. guichenoti.

The abundance method is an estimation approach based on population or stock size results from various years under the influence of dams. It is usually applied to well-studied fish. These abundance data have been estimated in previous studies and can be used to estimate DICs for a given fish. In this study, we applied this method to the Chinese sturgeon and the specific stock of C. guichenoti.

The valid stock method is an estimation approach based on valid stocks. By analyzing the entire life cycle of fish, as well as fishing and survey data, we can estimate the distribution ranges and patterns of spawning and recruitment stocks along the migration route before dam construction. On the basis of the information about the dam, we can estimate the proportion of invalid stocks in the spawning and recruitment stocks after dam construction. This allows us to obtain the DIC of the specific stock, which represents the proportion of the valid stock. In this study, we applied this method to specific stocks of Yangtze sturgeon and C. guichenoti.

The analogy method is an estimation approach based on a database containing numerous case studies. In the absence of available data, we searched for similar cases of other fish species to estimate the DIC for a given fish species. On the basis of the case studies in this study, we used the DIC of Chinese paddlefish by analogy to estimate the DIC (β = 0.53) of C. guichenoti, which was consistent with the result of the catch method (0.54).

Verification of the simplified model

To assess the accuracy and applicability of the simplified model, we compared our results with the capacity-gonad-confined population model (CGPM) for the Chinese sturgeon, which is a highly precise model based on age-structured functions (30). Figure S7 (C and D) shows that the results of the simplified model were slightly lower than those of the CGPM in the early stages of dam operation. Over time, however, the discrepancy between the two model estimates decreased and the estimated species extinction times became more consistent. Therefore, we concluded that the simplified model provides a valuable estimate of dam effects on population size, even in cases where detailed biological parameters are not available. The predictions of our model were also verified by fishing and survey data, including the extinction time of the Chinese paddlefish and Yangtze sturgeon (18, 51). This confirmation demonstrates that the simplified model can accurately forecast the impact of the dam on the migratory fish population with limited data. See text S4 for details.

Limitations of this study

This study has certain limitations, such as the need for larger sample sizes of fish to improve the accuracy of the precision of fish life cycle models. It is also necessary to refine the parameter estimation methods for the simplified model to accommodate various scenarios of data-limited fish. In addition, further refinement and quantification of the invalid stock is needed to establish a more robust correlation between the invalid stock and the DIC.

Acknowledgments

We thank H. Zhang at the Yangtze River Fisheries Research Institute for sharing the Chinese sturgeon bycatch data. We extend our gratitude to L. Wang at the China Institute of Water Resources and Hydropower Research, C. Li at the Chongqing Academy of Environmental Sciences, and T. Mu (former Head of Yibin Fishery Administration Office in Sichuan Province) for valuable assistance in conducting field surveys of the spawning grounds and stocks of the five fish species in 2017 and 2023. We are also grateful to X. Ke (former Director of Sichuan Aquatic Research Institute, Sichuan Province) and other anonymous experts who were interviewed by us in 2017 and provided technical details of their involvement in the GD-FRP.

Funding: This research was supported by the National Natural Science Foundation of China (52079148).

Author contributions: Conceptualization: Z.H. Methodology: Z.H. Investigation: Z.H. and H.L. Visualization: Z.H. and H.L. Funding acquisition: Z.H. Project administration: Z.H. Supervision: Z.H. Writing—original draft: Z.H. and H.L. Writing—review and editing: Z.H.

Competing interests: The authors declare that they have no competing interests.

Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

Supplementary Materials

This PDF file includes:

Supplementary Text S1 to S5

Figs. S1 to S13

Tables S1 to S7

Legends for movies S1 to S5

References

sciadv.adi6580_sm.pdf^{(4.2MB, pdf)}

Other Supplementary Material for this manuscript includes the following:

Movies S1 to S5

sciadv.adi6580_movies_s1_to_s5.zip^{(38.8MB, zip)}

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Associated Data

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Supplementary Materials

Supplementary Text S1 to S5

Figs. S1 to S13

Tables S1 to S7

Legends for movies S1 to S5

References

sciadv.adi6580_sm.pdf^{(4.2MB, pdf)}

Movies S1 to S5

sciadv.adi6580_movies_s1_to_s5.zip^{(38.8MB, zip)}

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PERMALINK

Dams trigger exponential population declines of migratory fish

Zhenli Huang

Haiying Li

Roles

Abstract

INTRODUCTION

Fig. 1. Major cascade dams in the mainstream of the Yangtze River.

RESULTS

Identifying migration patterns and life cycles of the five fish species

Fig. 2. Three-tiered framework of fish migration taxonomy and six descriptive life cycle models of migratory fish, which are related to dams.

Fig. 3. Migration patterns, spawning grounds and migration paths of the five fishes.

Chinese sturgeon

Fig. 4. Life cycles of the five fishes.

Chinese paddlefish

Yangtze sturgeon

Chinese sucker

C. guichenoti

Estimating the invalid stocks and DICs for the five fish species

Fig. 5. Seven types of invalid stocks created by the barrier effects of the 12 Yangtze dams on the five fishes.

Chinese sturgeon

Table 1. DICs for the five fishes in the Yangtze River.

Chinese paddlefish

Yangtze sturgeon

Fig. 6. Schematic showing the population density distributions of three riverine potamodromous species in the Yangtze River before and after the construction of the Yangtze dams.

Chinese sucker

C. guichenoti

Trend analysis of dam-induced impacts using invalid stock types

Fig. 7. Invalid stock types reflecting the trends of dam-induced impacts.

Recreating population decline processes of the five fish species

Estimating initial population sizes

Dams trigger exponential population declines

Fig. 8. Population decline processes for the five fishes in the Yangtze River.

DISCUSSION

Scientific misjudgments underpinning the FRPs

Fig. 9. Estimated exploitation rate of the Chinese sturgeon.

Reforming China’s FRP for dams

The five fishes: Hope or concern?

METHODS

Fish migration taxonomy

Definitions of migration patterns

Six descriptive life cycle models of migratory fish

A procedure for determining migratory fish life cycle

A simplified model of dam barrier effects

Estimation methods for the DIC

Verification of the simplified model

Limitations of this study

Acknowledgments

Supplementary Materials

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Other Supplementary Material for this manuscript includes the following:

REFERENCES AND NOTES

Associated Data

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