Balancing natural and sexual selection in sockeye salmon: interactions between body size, reproductive opportunity and vulnerability to predation by bears

Traits that increase reproductive success, such as body size and sexual dimorphism, may compromise survival, leading to opposing pressures of natural and sexual selection. Discrete populations exposed to different balances between selective forces should differ in phenotypic traits associated with natural and sexual selection. We used two proximate populations of sockeye salmon (Oncorhynchus nerka) that differ in body size as a model for studying this kind of balancing selection. We hypothesized that large body size would enhance potential reproductive success through relationships with duration of nest guarding in females, and dominance and duration of reproductive life in males, but that it would be opposed by probability of premature death, chiefly from predation by bears. Longevity on the breeding grounds was primarily controlled by predation, which varied between creeks and years. Pick Creek salmon experienced less predation than those in Hansen Creek and also tended to live longer before being killed, giving Pick Creek females a higher probability of completing egg deposition and males a greater opportunity to breed than those in Hansen Creek. In addition, Hansen Creek salmon were subjected to strong, size-selective predation and also selective mortality from stranding as they ascended the mouth of the creek, whereas we found no evidence of size-selective mortality among Pick Creek salmon. Male dominance in courtship for access to females favoured large salmon, except when predation was very intense. These patterns of balancing selection were consistent with the larger body size of sockeye salmon in Pick Creek. We also found that premature mortality, especially predation by bears, can significantly truncate the reproductive opportunities of salmon, raising a cautionary note regarding controlled studies in which predation cannot occur.