| Summary: | Can variation in prey density drive changes in the intensity or direction of selective predation in natural systems? Despite ample evidence of density-dependent selection, the influence of prey density on predatory selection patterns has seldom been investigated empirically. We used 20 years of field data on brown bears (Ursus arctos) foraging on sockeye salmon (Oncorhynchus nerka) in Alaska, to test the hypothesis that salmon density affects the strength of size-selective predation. Measurements from 41,240 individual salmon were used to calculate variance-standardized selection differentials describing the direction and magnitude of selection. Across the time series, the intensity of predatory selection was inversely correlated with salmon density; greater selection for smaller salmon occurred at low salmon densities as bears’ tendency to kill larger-than-average salmon was magnified. This novel connection between density dependence and selective predation runs contrary to some aspects of optimal foraging theory and differs from many observations of density-dependent selection because (1) the direction of selection remains constant while its magnitude changes as a function of density and (2) stronger selection is observed at low abundance. These findings indicate that sockeye salmon may be subject to fishery-induced size selection from both direct mechanisms and latent effects of altered predatory selection patterns on the spawning grounds, resulting from reduced salmon abundance. Hansen Creek, Alaska Survey Data for sockeye salmon (Oncorhynchus nerka)These data are a compilation of daily stream surveys in conjunction with a long-term study of the evolutionary ecology of sockeye salmon (Oncorhynchus nerka) in Hansen Creek, a tributary of the Wood River system of Bristol Bay, Alaska. Data pertaining to tagged fish from 1990 to 1993 have been compiled from Dr. Ruggerone. Data from 1997-2012 are daily records of all dead salmon observed at the mouth of Hansen Creek and within the stream proper and have been ...
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