Population dynamics of spotted scat, Scatophagus argus (Linnaeus 1766) from the Sundarbans of Bangladesh

Abstract

In Bangladesh, the mangrove areas of the Sundarbans are a hotspot for the spotted scat (Scatophagus argus), as the estuarine tributeries are significant zones for fishing, local marketing and foreign trading. Due to the widespread capture of spotted scats in the wild for a variety of uses, parameters of population were estimated using length-frequency and body weight data gathered from various sites in the southwestern forest areas of the country to comprehend the stock's status. Between May 2022 and April 2023, 1280 S. argus individuals were sampled from Khulna, Bagerhat and Satkhira. To gain insight into the population dynamics of S. argus, estimations of the growth parameters, recruitment pattern, mortalities, and level of exploitation (E) were made. For S. argus, a bit of negetive allomentric growth (b = 2.95) was observed. Moreover, the growth performance index (φ′), the asymptotic length (TL_∝) and the growth coefficient (K) were 2.83, 34.65, cm and 0.56 yr⁻¹, respectively. Natural mortality (1.08 yr⁻¹) was lower than fishing mortality (1.12 yr⁻¹). In the wild, this species recruits throughout the year, reaching its peak between October and January. By fishing gear, 50 % of the S. argus were caught between 7.71 cm and 9.43 cm. Moreover, high fishing mortality was assumed in both the small group (11–15 cm) and the large group (29–33 cm) of S. argus by virtual population analysis (VPA). It was discovered that the E for S. argus was 0.51, over the maximum permissible limit (E_max = 0.48). In summary, it is discovered that the S. argus population is overexploited. Therefore, in order to preserve this popular fish species and avoid local extinction, spotted scat resources should be taken into account under appropriate fisheries management guidelines. These findings will guide policymakers to manage the species accordingly.

Highlights

• •Population dynamic parameters of Scatophagus argus were estimated.
• •The asymptotic length was 34.65 cm, and the growth coefficient was 0.56 yr⁻¹.
• •Natural mortality was 1.08 yr⁻¹, whereas fishing mortality was 1.12 yr⁻¹.
• •E for S. argus is 0.51, over the E_max (0.48).
• •In conclusion, it is discovered that the S. argus population is overexploited.

1. Introduction

The Spotted scat, scientifically known as Scatophagus argus (Linnaeus 1766), is a species of aquarium fish that falls within the Scatophagidae family, categorized under the Perciformes order and is well-liked worldwide because of its vibrant beauty, resilience, moderate development, and peaceful nature [[1], [2], [3]]. Because of the many black dots that cover its body, it is sometimes referred to as the "spotted scat" [4]. Other names for it include leopard pomfret, argus fish, and butterfish [5], spade fish, and spotted spade fish [6]. "Indian discus" is another term commonly used to indicate this species in India's ornamental fish markets. This teleost is a type of euryhaline species, also known as "Chitra" or "Bistara" in local dialect, and is mostly found in the nearshore waters of Bangladesh's southwest coast as well as in the waters of the Indo-Pacific region [[7], [8], [9]]. Spotted scat's natural habitats include the Indo-Pacific region, particularly India, Sri Lanka, Australia, China, the Philippines, and the Malay Archipelago [[10], [11], [12], [13], [14], [15]].

In aquarium fish markets, S. argus is highly sought after and is said to command a competitive price [16]. South and Southeast Asian nations also love to eat these fish [[17], [18], [19]] because of their high protein content and excellent taste [20,21] and nutritional qualities [19]. In local marketplaces, large-sized fish have seemingly been reported to command prices comparable to those of table fish [6]. S. argus is therefore a well-liked candidate in aquaria of brackishwater [1]; as a cultivated species that are commercially significant [22] and is also a remarkable food fish in Southeast Asia [19]. Based on past experiences, it appears that our coastal farmers have a high demand for S. argus fish fry since they have been growing it for many years alongside prawns and other brackish water finfish. Thus, the natural population of fry in the Sundarbans is clearly threatened, in addition to increased fry harvesting and other anthropogenic interferences such habitat degradation, climate change, destructive and non-selective gears, poison fishing with pesticides and so on [[23], [24], [25]].

Restoring the abundance of vulnerable species in the wild can be accomplished through proper stock management. In fisheries management, input and output restrictions are often used to preserve fish species by maintaining sound health of their wild stocks [26]. Thus, basic data of a species such as growth, recruitment pattern, mortality, and exploitation rate are required before managing a stock. However, prior studies on spotted scat stock in the Sundarbans mangrove habitats are scant, even though this ornamental species has the potential to develop into a profitable industry that generates foreign exchange as well. Given these contexts, present study aims to evaluate essential population dynamic parameters of S. argus. Also, this paper presents relative yield per recruit of this species with the target of facilitating the formulation of management and conservation policies, as well as the future growth of the Spotted scat fishery in the Sundarbans.

2. Materials and methods

2.1. Sampling

The present study focused on the Sundarbans and its surrounding mangrove forest regions. The sample locations were selected in accordance with the quantity of the targeted spotted scat species in Khulna, Bagerhat, and Satkhira districts, which are located in Bangladesh's southwest. After that, the research area's sample sites were determined to be Hodda (22°1′24ℙN, 89°25′12ℙE) of Koyra, Karamjal (22°26′24ℙN, 89°34′49ℙE) of Mongla and Gabura (22°11′24ℙN, 89°18′00ℙE) of Shyamnagar (Fig. 1). Sampling was carried out throughout the year with the collaboration of fishermen and local people by using hooks and lines, traps, and nets to harvest S. argus from the wild from May 2022 to April 2023. A total of 1280 samples were collected during the study period. Consequently, 74, 100, 88, 46, 149, 233, 126, 69, 90, 84, 101, and 120 samples were obtained in May, June, July, August, September, October, November, December, January, February, March, and April, respectively. The authors were aware of animal ethics clearance from the concerned authorities. However, the protocol of the experiment was approved by Bangladesh Fisheries Research Institute (BFRI), and the research was conducted by considering the guidelines of the ‘Animal Welfare Act 2019’ approved by the National Parliament of Bangladesh.

Fig. 1.

Mapping of sampling sites along with the Sundarbans mangrove areas in Bangladesh.

2.2. Data collection

The species S. argus was confirmed by following a standard identification procedure [27], based on morphological and meristic features of the S. argus. After that, body weight (BW) of S. argus was determined with an hydrostatics electronic balance JA50002A that had a 0.01 g precision and TL was measured with a stainless steel ruler that had a 0.10 cm accuracy.

2.3. Data analysis

A overview of some characteristics and the way in which a species grows is given by the connection between TL and BW. The spotted scat's TL-BW relationship was established by using the following formula:

$$\text{BW}=\mathrm{a}\times \mathrm{T}{\mathrm{L}}^{\mathrm{b}}$$

where, W is BW in g, L is TL in cm, a and b are constants. For the purposes of the linear regression analysis (log BW = log a + b × log TL), the log-transformed values of TL and BW were calculated. To determine the significance of coefficient b at a 5 % significance level, the student's t-test was done.

A computer program, FAO-ICLARM Stock Assessment Tools (FiSAT-II) was used to determine the TL_∝ and K of the von Bertalanffy equation for growth considering the TL of a S. argus individual. To determine the values of TL_∝ and K, the FiSAT-II programme included Electronic Length Frequency Analysis (ELEFAN-I) by applying the following formula:

$${\text{TL}}_{\mathrm{t}}={\text{TL}}_{\infty }\left(1‐{\mathrm{e}}^{‐\mathrm{K}\left(\mathrm{t}‐{\mathrm{t}}_0\right)}\right)$$

where, t indicates the age of the spotted scat (yr), TL is the mean total length at age t (cm), t₀ is the hypothetical age when TL is zero, and K represents a growth coefficient (yr⁻¹). According to Pauly and Munro [28], the growth performance index (φ′) of S. argus was derived from TL_∝ and K, is as follows:

$${\mathrm{\phi }}^{\prime }=\mathrm{Log}\mathrm{K}+2\mathrm{Log}{\text{TL}}_{\propto }$$

The Length Converted Catch Curve method of Beverton and Halt [29] was used to determine the total mortality (Z), taking into account the habitat temperature (T) of 23.40 °C throughout the study year. In order to forecast natural mortality, an empirical relationship was used (M) in accordance with Pauly [30], as shown below:

$$\begin{gathered} {\log }_{10}\mathrm{M}=‐0.0066‐0.279{\log }_{10}\mathrm{T}{\mathrm{L}}_{\infty }‐0.6543{\log }_{10}\mathrm{K}+0.4634{\log }_{10}\mathrm{T} \\ \mathbf{Note}\mathbf{:}\mathit{T}\mathit{h}\mathit{i}\mathit{s}\mathit{e}\mathit{q}\mathit{u}\mathit{a}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\mathit{a}\mathit{p}\mathit{p}\mathit{e}\mathit{a}\mathit{r}\mathit{d}\mathbf{in}\mathbf{the}\mathbf{PDF}\mathbf{version}\mathit{i}\mathit{n}\mathit{m}\mathit{u}\mathit{l}\mathit{t}\mathit{i}\mathit{p}\mathit{l}\mathit{e}\mathit{l}\mathit{i}\mathit{n}\mathit{e}\mathit{s}\mathbf{,}\mathit{p}\mathit{l}\mathit{e}\mathit{a}\mathit{s}\mathit{e}\mathit{f}\mathit{o}\mathit{r}\mathit{m}\mathit{a}\mathit{t}\mathit{i}\mathit{t}\mathit{i}\mathit{n}\mathbf{a}\mathit{s}\mathit{i}\mathit{n}\mathit{g}\mathit{l}\mathit{e}\mathit{l}\mathit{i}\mathit{n}\mathit{e} \end{gathered}$$

Natural mortality (M) was subtracted from total mortality (Z) to get fishing mortality (F) value in yr⁻¹. Both F and Z values were incorporated to the equation of Gulland [31] to estimate expoitation level (E), as follows:

$$\mathrm{E}=\mathrm{F}/\mathrm{Z}$$

The recruitment pattern was estimated using the FiSAT-II length-frequency data management program by taking a backward projection approach on the TL axis of a batch of TL-frequency data. The likelihood of spotted scat capture was calculated using the "probability of capture" procedure, and to calculate a projected length structured virtual population analysis (VPA), the FiSAT-II analyzer was utilized.

Here, using the FiSAT-II routine, the VPA and cohort analysis was performed. In a VPA study, the input values were L_∞, K, M, F, a (constant), and b (exponent), with the assumption that the t₀ value was zero. Fry [32] reported this technique for the first time, and Jones [33] and Pauly [34] later made modifications. Pauly and Soriano [35] updated a model to estimate the relative yield-per-recruit (Y'/R) and relative biomass-per-recruit (B'/R) based on Beverton and Holt [36], which was then incorporated into the FiSAT-II program. Furthermore, this analysis yielded values for the maximum relative yield-per-recruit and the highest permissible limit of exploitation (E_max). The exploitation rate E_0.1 was estimated at which the slope of Y'/R is 1/10 of its value at E = 0 and 50 % of the unexploited relative biomass per recruit (B'/R) is simultaneously represented by the computed exploitation rate, E_0.5.

3. Results

3.1. TL-BW relationship

Table 1 shows the relationship between TL and BW of S. argus. The TL-BW relationship was established by considering the logarithmic form of the equation, which is BW = a × TL^b. To create a scatter diagram and obtain a curvilinear relationship, all total length values were plotted against the corresponding body weight values.

Table 1.

TL-BW association of sampled Scatophagus argus from the Sundarbans of Bangldesh.

Size (N)	TL (cm)		BW (gm)		Maturity (%)		a	b	r	R2	Allometry	P
	↓	↑	↓	↑	†	‡
1280	1.40	32.30	2.24	1130.04	65	35	0.03	2.95	0.90	0.98	Negative	0.00

↓ = Minimum; ↑ = Maximum; † = Immature; ‡ = Mature.

Plotting the computed body weight against the total length of the S. argus produced parabolic curves. On the other hand, linear lines were obtained by plotting the log TL values against their corresponding log BW values. The b value was estimated as 2.95. It showed negative growth allometry. For S. argus, the Pearson correlation coefficient (r) was 0.90. It suggests that there are very positive and significant association between TL and BW of this species.

3.2. Length

On each sampling day, the TL-frequency data were classified into size intervals of 5 mm and frequency distributions in different categorized sizes for calculating descriptive statistics of S. argus are shown in Table 1. The minimum and maximum TL were 1.40 cm and 32.3 cm, weighted 2.24g and 1130.04g for S. argus. Two-thirds of the total sampled S. argus were immature and the rest were mature individuals.

The FiSAT-II output, clearly showed that the extreme lengths (TL_max) were revealed as 33.00 and 38.43 cm in observed and predicted observations, respectively (Table 2). The range of TL_max lies between 32.33 and 44.52 cm, with a 95 % confidence interval.

Table 2.

Population parameters of Scatophagus argus at the Sundarbans in Bangladesh.

Observed Lmax (cm)	TL∝ (cm)	K(yr−1)	${\mathrm{\phi }}^{\prime }$	M(yr−1)	F(yr−1)	Z(yr−1)	E	Emax	Status
33.00	34.65	0.56	2.83	1.08	1.12	2.20	0.51	0.48	E > Emax

3.3. Growth

Growth performance index (φ′) was calculated and obtained as 2.83 (Table 2). Growth coefficient (K) and growth performance index (φ′) of S. argus population were estimated and calculated values were estimated.

The von Bertalenffy asymptotic length (TL_∝) was 34.65 cm and the K was 0.56 yr⁻¹ of S. argus. Growth curves with the length frequency distribution were created using these two growth factors (Fig. 2).

Fig. 2.

Von Bertalenffy growth curves of S. argus in the Sundarbans of Bangladesh.

3.4. Mortalities

The total mortality (Z) of S. argus was estimated at 2.20 yr⁻¹, by means of length converted catch curve analysis (Fig. 3). Here, the natural mortality (M) and fishing mortality (F) were 1.08 yr⁻¹ and 1.12 yr⁻¹, respectisvely (Table 2). Also, these two mortalities were added to get total mortality (Z), as shown in Table 2.

Fig. 3.

Length converted catch curve of S. argus in the Sundarbans of Bangladesh.

3.5. Exploitation levels

Expoitation level (E) of S. argus was 0.51 (Table 2). In contrast, the maximum permissible limit of exploitation (E_max) of S. argus was computed and obtained value (0.48) is displyed in Fig. 7. Seemingly, maximum permissible limit of expoitation (E_max) of S. argus was lower than exsisting expoitation level (E).

Fig. 7.

Relative yield per recruitment, Y'/R, (arbitrary units) of S. argus in the Sundarbans of Bangladesh.

3.6. Recruitment

Generally, throughout the research period, S. argus recruitment was seen throughout the entire year. Overall, the S. argus recruitment pattern in the study region indicated a continuous recruitment cycle with a single annual main peak occurring from October to January (Fig. 4).

Fig. 4.

Annual recruitment pattern of S. argus in the Sundarbans of Bangladesh.

3.7. Probability of capture

In managing a fishery, one of the most helpful factors to evaluate a fish stock is probability of capture. It illustrates how various sizes of S. argus are susceptible to various types of gear in a particular area at a certain time (Fig. 5). The gears posed a threat to 25 % of 7.71 cm TL, 50 % of 8.57 cm TL, and 75 % of 9.43 cm TL, according to the probabilities of capture study of S. argus. Consequently, it can be inferred that half of the S. argus that were caught were found to have between 7.71 and 9.43 cm of overall total length range.

Fig. 5.

Possibilities of capture on the basis of the length (TL) groups of S. argus in the Sundarbans of Bangladesh.

3.8. Virtual population analysis

The findings from the analysis of length-based VPA are illustrated in Fig. 6. The fishing mortality in relation to mean length is depicted in the figure. It indicates that the majority of the medium-sized spotted scats were fished within the total length range of 11.00 cm and 15.0 cm, with the values of F between 1.17 and 1.18. Similarly, The largest F peaks of large-sized S. argus ranged in length from 29 to 33 cm, with a F value of 1.12 yr⁻¹.

Fig. 6.

Length structured virtual population analysis (VPA) of S. argus in the Sundarbans of Bangladesh.

3.9. Relative yield-per-recruit

An illustration of the current level of exploitation of S. argus in the Sundarbans could possibly be made using the two-dimensional depiction of the E and Y'/R (Fig. 7). The input parameters for the analysis were L₅₀/L_∞ of 0.247 and M/K of 1.920. Furthermore, by applying knife edge selection, the relative Y'/R and B'/R analyses of S. argus were estimated. The evaluation showed that 0.483 was the highest exploitation rate (E_max). Besides, E_0.1 was calculated as 0.405. Moreover, The E_0.5, an exploitation level of S. argus when B/R reduces to 50 % compared to its virgin biomass, was 0.287 (Fig. 7). During the study period, E was 0.51, which was higher than E_max (0.48). Therefore, the situation indicated that the S. argus population was overexploited in the mangrove estuary (Table 2).

4. Discussion

In Bangladesh, stock assessment initiatives for S. argus are notably scarce within the Sundarbans regions. Worldwide, only a handful of studies on S. argus have focused on various small geographical areas. In contrast, this study concentrates on three southwestern districts encompassing the Sundarbans within Bangladesh's borders. Consequently, S. argus specimens were collected from the mangrove areas of the Sundarbans to determine significant population parameters based on their total length-frequency data.

In the regression analysis, S. argus's coefficient of determination (R²) was extremely near to 1 (Table 1). Therefore, it is possible to characterize the relationship between TL and BW as being extremely positive. The exponent (b) value is a crucial indicator for assessing a species' growth pattern. Nonetheless, for any species, ecological variables (such as food availability, water quality parameters, sample size, and length range) can result in changes in slope (b) values [37,38]. A species grows in all dimensions such as length, width, and weight when b equals 3. When a species shows negative allometric growth (b < 3), it means that its adult body shape is longer than its juvenile form [39]. On the other hand, increasing body width causes one to gain weight when positive allometric growth (b > 3) occurs [40]. S. argus, on the other hand, displayed somewhat negative allometric growth (Table 1).

The curves of growth were obtained directly from the measurements of length-frequency, and most of the peaks were on or near the line for the S. argus (Fig. 2). The calculated TL_∝ was 34.65 cm, while the K value was 0.56 yr⁻¹ (Table 2). In Indonesia, the longest TL_∝ of spotted scat was calculated as 19.16 and 17.80 in West Java and Central Java, respectively [41,42]. However, the TL_∝ of S. argus in Thailand was estimated as 17.87 [43]. The highest TL_∝ (34.65 cm) of S. argus was recorded in the Sundarbans, Bangladesh (Table 2). In general, higher species growth is indicated if the value of K is higher than 1 [44]. Because the K value in the S. argus population was less than 1, slower growth rate can be assumed in this investigation (Table 2).

Overall, the study on S. argus showed that recruitment occurs throughout the year, peaking mostly from October to January. Likewise, recruitment of S. argus was also found throughout the year, with a noticeable peak between May and August [43]. Similarly, Sholichin et al. [42] revealed that the recruitment of S. argus happened all year long with a peak in June. In contrast, the recruitment of S. argus has happened all year long, with two peaks also in February–April and July–September [41]. Gears made a very small group of S. argus vulnerable with TL ranging from 7.71 cm to 9.43 cm (Fig. 5). The VPA discovered high fishing mortality in the both small-sized group (11–15 cm) and large-sized group (29–33 cm) of S. argus (Fig. 6).

A variety of factors, including the stock dynamic, growth, recruitment, and mortality, are typically combined to create biological reference points [45]. Because of their important and meaningful characteristics, they can provide a valuable philosophy regarding the state of the fish stock [46]. Higher fishing mortalities (1.12 yr⁻¹) were observed than natural mortalities (1.08 yr⁻¹) of S. argus (Table 2). For taking a fruitful decision, exploitation levels explain the condition of a fishery, whether it is more harvestable or not [47]. It illustrates the spotted scat's non-equilibrium stock status. In most cases, the yield was optimized when F = M [31]. The value of E (0.51) indicated overfishing of S. argus during the study period. Theoretically, when E is 0.50, then the stock of any aquatic species is at its optimum level. According to Gulland [31], it seemed that the yield is optimum until fishing mortality exceeds natural mortality. Particularly, the stock is predicted to be over fished if E value is higher than 0.50. Hence, the fishing mortality rate equal to the natural mortality rate represents the optimal value of exploitation [31]. As like as present study, exploitation levels (E) of S. argus fishery exceeded 0.50 in India, Indonesia and Thailand [[41], [42], [43],48]. Thus, the remaining fishing pressure above the optimum level of E in all cases indicates the importance of this species in commercial aspects. During the study period, spotted scat population was being overfished, as the calculated E value of the species also exceeded maximum permissible limit of exploitation, E_max = 0.48 (Table 2). Lastly, it is conceivable that S. argus was experiencing overfishing, and continued indiscriminate exploitation might exacerbate the threat to its population, potentially leading to collapse in the near future.

5. Conclusion

Spotted scat is one of the most demanded and delicious species for farming in the Sundarbans mangrove areas. Therefore, in order to fulfill demand at various aquaculture farms, fishermen capture juveniles straight from mangrove estuaries for delivery to the markets all year long. Thus, the S. argus fishery has the possibility of becoming more vulnerable. In order to examine the population status in the mangrove zones, a spotted scat stock assessment was conducted. Length-weight data were gathered from different parts of the Sundarban mangrove forest in order to assess recruitment patterns, growth characteristics, death rates, and exploitation levels to determine population parameters of S. argus. Overall, S. argus growth was found to be somewhat sluggish as the K value is low. Fishing mortality is higher than the natural mortality. Therefore, existing exploitation rate became higher than the maximum permissible limit of exploitation. This situation can drag this stock down the path of disappearance forever. Though The recruitment of spotted scats was year-round, peaking from October to January; nevertheless, high fishing pressure was noticed on two size groups of the S. argus population. Thus, sustainabe harvest is advised for a healthy stock in future. In conclusion, immediate restoration strategies should be launched by policymakers since overexploitation of the S. argus population showed an alarming condition of this stock in the Sundarbans of Bangladesh.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Md. Hashmi Sakib: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Shawon Ahmmed: Investigation, Data curation. Md. Reaz Morshed Ranju: Investigation, Data curation. Md. Moshiur Rahman: Investigation, Data curation. Md. Masudur Rahman: Data curation. Md. Latiful Islam: Writing – review & editing, Validation, Supervision, Resources. Md. Zulfikar Ali: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.