Size- and Sex-Selective Mortality of Adult Sockeye Salmon: Bears, Gulls, and Fish Out of Water

Abstract

Pacific salmon Oncorhynchus spp. exist as reproductively isolated breeding populations and display marked variation in size and age at maturity within and among populations. We conducted a detailed study of mortality on adult sockeye salmon O. nerka in Hansen Creek, a small stream in southwestern Alaska, to test the prediction that predation by brown bears Ursus arctos and death by stranding during upstream migration would selectively remove males and large (old) individuals. During the 3 years of this study, 13.7% of the mature salmon died in shallow water at the creek's mouth before reaching the spawning grounds. Males had proportionally higher mortality at the mouth than females (17.6% versus 10.8%), and the fish of both sexes that died at the mouth were larger and older than those that survived to ascend the creek. Of the fish that died at the mouth, females were more often attacked (chiefly scavenged) by glaucous-winged gulls Larus glaucescens than males, and the gulls tended to attack fish that were smaller than average. Of the fish that reached the spawning habitats in Hansen Creek, those killed by bears were larger and older than those that died of senescence. Thus, there was highly size-selective mortality in this population. Data from four other nearby sockeye salmon populations confirmed the tendency for bears to kill predominately large salmon and males. Data from 14 other populations revealed that the predation intensity was generally, but not always, higher on males than females. Overall, the results indicated that there can be substantial selective mortality in salmon populations in small streams, which may influence their life history evolution.