Zeidberg and Robison. 10.1073/pnas.0702043104. |
Fig. 4. Seasonal structure in the abundance of Pacific hake, Merluccius productus (in gray) and Humboldt squid, Dosidicus gigas (in red) in Monterey Bay, California. Observations summed by month and divided by ROV hours of deployment. (Upper) From 1989 to 1997, hake observations remained high year-round, with a peak in December. For the period from 1997-2005, Dosidicus observations were inversely related to the upwelling index (www.pfel.noaa.gov), offshore flow (106 m-3 100 km, inverted right y axis). For hake in the period since the squid invasion, the seasonal structure of their population changed drastically. Observations of hake dropped by an order of magnitude in December, as sightings of Dosidicus increased.
Movies
. Video sequences of D. gigas foraging.
Movie 1. During a head-down descent, D. gigas sees a fish below it, stops by flaring its arms and curling the leading edge of its fins, then pivots to its left to track the prey. Once aligned, it pulls back the tips of its arms and rapidly projects the feeding tentacles forward to grasp the fish. Then it stops again, spreads the arms and curls the tentacles back toward the mouth for ingestion.
Movie 2. This footage, shot during an ROV descent, shows several squid feeding on myctophid lanternfishes (Stenobrachius leucopsarus and Tarletonbeania crenularis), where they use their long feeding tentacles for prey capture; and on krill (Euphausia pacifica), where the squid uses eight arm tips to catch the multiple small prey items. Chromatophore flashing by the squids is common when several are feeding in close proximity.
Movie 3. The final sequence shows D. gigas eating a large Pacific grenadier (Coryphaenoides acrolepis). The squid visually targets the fish as it jets forward and performs a tentacular strike. The tentacles hold the fish with their sucker rings while the squid jets forward, enclosing the fish within its eight arms. Then the squid bites the fish with its beak as pieces of flesh float away from its brachial crown.
SI Text
Linking Dosidicus and hake.
The numbers of Dosidicus we observed increased significantly shortly after El Niño events, and our observations of hake decreased significantly during and 1 month after each increase in Dosidicus abundance (Fig. 3, time-series cross-correlation analysis, P<0.05). Similar analysis found the decreases in hake abundance do not appear to be directly linked to these El Niño events (P>0.05). This pattern contrasts with reports of increased poleward flow and a northern-range shift of hake in the waters off Oregon and Washington that are associated with a proposed regime shift after the 1997-1998 El Niño (1, 2). Although we believe the predatory pressure of this squid on hake is linked to the decrease in observed hake abundance in the shelf and slope environment of Monterey Bay, the possibility remains that the long-term decrease in hake observations is reflecting similar decreases in the fishery (3) related to a regime shift. The increase of hake in our observations between 1999 and 2001 may be due to the large recruitment event of hake that occurred throughout their range in 1999. Commercial landings of hake off central and southern California have been in overall decline since the 1997-1998 El Niño (4, 5).
Trophic ecology of Dosidicus and tunas.
Tuna and billfish have been targeted by major commercial fisheries in the eastern Pacific Ocean (EPO) since the mid-20th century. Assessments of commercially exploited species have been made by several researchers, with some projecting complete collapse by 2048 (6). Ninety percent of the catch weight of this fishery has been comprised of just four species: skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares), bigeye (Thunnus obesus), and albacore (Thunnus alalunga) tunas. At present, skipjack is the only species not approaching maximum sustainable yield. Longline fishing, which targets the largest, oldest, and most fecund individuals dominated the first 25 years of the fishery, removing 40% of fish >175-cm forklength (FL) by 1980. Currently, biomass of these large fish is <17% of that expected without fishing. Since 1980, the purse seine fishery has targeted mostly smaller younger fish (7).
Dosidicus gigas
represents the largest biomass of cephalopods in the EPO (8). This squid competes with small tunas for micronektonic fishes and crustaceans as forage. Dosidicus, particularly in its younger stages, is a major prey item of bigeye, yellowfin, and bluefin (Thunnus orientalis) tunas (9-11). The proportion of squid in yellowfin diets is second in the index of relative importance and first in percentage by number. As the yellowfin grow, scombrid fishes make up a greater portion of their diet. The biomass of cephalopods and micronektonic fishes consumed by yellowfin decreases from 24,000 metric tons at 20-55 cm FL, to 3,000 metric tons at FL >125 cm (11). The increased commercial targeting of small yellowfin tuna has thus substantially removed both predators and competitors of Dosidicus from the EPO.
Dosidicus
comprised the largest percent volume of prey in bigeye tuna stomachs in the EPO, with larger fish consuming more squid than smaller individuals (9). The size distribution of both of both yellowfin and bigeye tunas has changed in the last 20 years as purse seining and the use of fish-attracting devices has increased. Average weight of individual yellowfin decreased from 25 to 15 kg between 1985 and 2000 (12). For bigeye, there has been a dramatic decrease, from >50 to <20 kg, in the weight of individual fish captured in the fishery since 1994 (13). Because the fisheries first removed the largest individuals and subsequently moved on to the smaller fish, predation and competition have decreased for Dosidicus in the center of its range, providing an opportunity for its populations to increase and expand. Although fish populations comprised of more small individuals may lead to greater biomass in the EPO overall (7), the shift in a population's structure to younger fish will have cascading effects in terms of reproductive output and predation. The range expansion of Dosidicus may be one of these effects.
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