Predator foraging mode shifts affecting mortality of juvenile fishes during the subarctic spring bloom

We examined some processes affecting foraging mode shifts among Pacific herring (Clupea pallasi) and walleye pollock (Theragra chalcogramma) during the spring bloom in Prince William Sound, Alaska. The duration of the bloom of the copepod Neocalanus was reduced when the surface layer became strongly stratified in May. Stronger surface layer stratification observed in 1995 and 1996 was associated with higher salinities below 50 m depth compared with 1994. Functional response models for Pacific herring and adult walleye pollock feeding on large calanoid copepods indicated that these predators began to switch to alternative prey as the mean biomass of large calanoid copepods declined below about 0.2 and 1.0 g/m(3), respectively. Analysis of covariance indicated that Pacific herring and walleye pollock tended to switch from large calanoid copepods to nekton prey after the copepod bloom declined (p = 0.028). Mortality of pink salmon (Oncorhynchus gorbuscha) was negatively correlated with the duration of the copepod bloom during the juvenile life stage (p = 0.013), as well as with juvenile growth rate (p < 0.001), juvenile body weight at release (p < 0.001), and the number of juveniles released (p < 0.001). Our results indicate that bottom-up processes affecting the duration of the spring bloom and juvenile growth also modify top-down processes involving foraging mode shifts toward piscivory and mortality of juvenile fishes.