Change in chum salmon (Oncorhynchus keta) stomach contents associated with fluctuation of pink salmon (O-gorbuscha) abundance in the central subarctic Pacific and Bering sea

The abundance and stomach contents of salmonids (Oncorhynchus spp.) and the biomass of prey organisms were examined in the central subarctic Pacific and Bering Sea in the summer of 1991 and 1992. Salmonids were caught by surface longline using the same level of fishing effort. Chum (O. keta) and pink (O. gorbuscha) salmon were the predominant species, representing 44% and 36% of the total catch (n = 1275) in 1991. In 1992, chum salmon composed 85% of the total catch (n = 603), but the catch of pink salmon decreased to 1% of the total catch due to the odd/even year fluctuation of Asian pink salmon abundance in the study area. It was found that chum salmon changed their dominant diet from gelatinous zooplankton (pteropods, appendicularians, jellyfishes, chaetognaths, polychaetes and unidentified materials) in 1991, when pink salmon were abundant, to a diet of crustaceans (euphausiids, copepods, amphipods, ostracods, mysids and decapods) in 1992, when pink salmon were less abundant. Local crustacean biomass (wet weight; mg m(-3)) had significant negative correlation with the CPUE (catch number per 30 hachi) of pink salmon in 1991 (r = -0.586; P = 0.026) and that of chum salmon in 1992 (r = -0.616; P = 0.014). There may be a limitation in the available prey resource for production of salmonids.