Известия ТИНРО (Jun 2015)
Feeding habits of pacific salmons in the eastern Bering Sea in 2003-2012
Abstract
Change in prey for pacific salmons in the eastern Bering Sea is considered between the periods of relatively warm (2003-2006) and relatively cold (2007-2012) conditions on the base of data obtained in the frames of BASIS research program. In the warm period, mean biomass of the large-sized, medium-sized and small-sized fractions of zooplankton was 287.9, 258.2, and 109.2 mg/m3, respectively. In these conditions, fish prey was the basis of diet for pink salmon (70-68 % of ration), chum salmon (64-84 %) and sockeye salmon (45-87 %). Pink, chum, sockeye, and coho salmons had similar diet with the dominance of juvenile walleye pollock (similarity 70-90 %), in contrast to the diet of chinook salmon (40-70 % similarity with others). In the cold period, the large-sized zooplankton was much more abundant (1094.8 mg/m3 on average) mostly due to higher abundance of Copepoda and Chaetognatha; euphausiids, hyperiids, and pteropods were more abundant, as well. In these conditions, zooplankton, mainly euphausiids Thysanoessa raschii and hyperiids Themisto libellula and T. pacifica , dominated in the diet of plankton-eating salmons: 86-92 % of the ration for pink salmon, 88-92 % for chum salmon, and 74-81 % for sockeye salmon; all these species and chinook juveniles had similar diet (70-80 % similarity). On the contrary, predatory salmons as coho and chinook adults still preyed on fish. Mean daily rations in the warm/cold periods are estimated as 12.1/9.4 % of body weight for sockeye salmon with the length 10-20 cm, 8.6/7.3 % for sockeye salmon 20-30 cm, 8.3/13.8 % for chum salmon 10-20 cm, 8.6/10.0 % for chum salmon 20-30 cm, and 8.8/10.3 % for pink salmon 10-25 cm. The prevalence of nekton or planktonic food does not affect on daily rhythm of feeding intensity: juvenile salmons feed mainly in the daytime, so they have the maximal index of stomachs fullness in the evening or early night (100-230 ‱, both in the warm and cold periods). Generally, the salmons prefer fish larvae and juveniles in conditions of low zooplankton abundance and large-sized zooplankton in conditions of its high abundance. Selectivity on euphausiids, hyperiids and pteropods is observed only in conditions of high zooplankton abundance and is an indicator of sufficient food supply.
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