A COMPARISON OF THE WILLINGNESS OF 4 SPECIES OF PACIFIC SALMON TO RISK EXPOSURE TO A PREDATOR

Theory and recent experimental evidence indicate that prey animals should be willing to incur some level of predation risk while foraging. This level of risk will be determined by the costs (i.e., lost foraging opportunities) and benefits (i.e.. reduced probability of mortality) associated with predator avoidance. We tested the hypothesis that the optimal trade-off between obtaining additional food and avoiding predators is species specific with four species of Pacific salmon: chinook (Oncorhynchus tshawytscha), coho (O. kisutch), chum (O. keta), and pink salmon (O. gorbuscha). We also tested the relative vulnerability of the salmon to the predator used in our experiments. Our results demonstrated that chinook salmon were significantly more willing to risk exposure to a predator to obtain additional food than were chum and coho salmon. Pink salmon were intermediate between chinook and the other species. Furthermore, this difference exceeded that which could be explained by variation in length and weight between species. Experiments to test vulnerability to the predator indicated that coho salmon were significantly less vulnerable to the predator than were chinook and chum salmon. If these behavioral differences persist throughout their lives, there should also be differences in the population response of these salmon species to fluctuation in predator density.