| Summary: | In 1990s, reduction of Japanese chum salmon (Oncorhynchus keta) body size was observed. The change of zooplankton density as prey, and/or their population density effect are proposed by many studies. In order to investigate this body size reduction of Japanese chum salmon in the North Pacific, we developed a bioenergetics model for chum salmon. Our model was based on NEMURO.FISH (Ito et al.,2004;Megrey et al.,2002) using respiration and consumption terms (Trudel et al.,2004; Ware,1978;Beauchamp et al.,1989) and assumed that SST and prey zooplankton density are the determining factors of the reduction of body size. The model reproduced the body size of the 1972 and 1991 year classes of chum salmon, respectively. Reproduced body size of the 1972 year class is larger than that of 1991 year class. This result shows a good agreement with the observations in the Bering Sea. Moreover, our model reproduces the trend of body size in l970-2000 well. The prey density, especially in the Eastern North Pacific, gives larger influence to the change of body size rather than the SST does. This suggests that size reduction of Japanese chum salmon in 1990s was partly affected by prey zooplankton density.
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