Fraser River sockeye salmon: data synthesis and cumulative impacts

There are many marine predators that may consume sockeye post-smolts as they migrate fromthe mouth of the Fraser northward along the coast. Christensen and Trites (2011) identify spinydogfish, coho salmon, chinook salmon, juvenile sablefish, humbolt squid, and arrowtoothflounder as potential fish predators. Potential bird predators include common tern, arctic tern,pelagic cormorant, Brandt’s cormorant, gulls, and common murre. Mortality due to predation islikely to have always been high during this life stage as the sheer abundance of post-smoltsmigrating up the coast would have always attracted a diversity of predators, but if the level ofpredation has increased in recent decades, the impact on the sockeye salmon population mayhave also increased. Marine mammal predators (including pinnipeds and cetaceans) have beendocumented eating salmon post-smolts but there is no evidence of marine mammal predation onsockeye salmon post-smolts (Christensen and Trites, 2011). However, knowledge on the diet,abundance, distribution and biology of potential predators is often scarce. For example, littleinformation is known on the diet of Pacific white-sided dolphins, but salmon might represent 30-60% of their diet during June through November (Christensen and Trites, 2011). Salmon is alsoknown to be an important prey species for Steller sea lions, although the evidence suggests thatSteller sea lions predominantly eat adult salmon rather than juveniles. Knowledge of specificpredator-prey associations is largely based on diet information for predators but such informationis largely qualitative and often non-existent for particular predators of interest (Christensen andTrites, 2011). Overall, Christensen and Trites (2011) emphasize that even if it could be shownthat the aggregate rate of predation on sockeye salmon has increased substantially over the pastseveral decades, it would likely still not be possible to determine whether predation itself wascontributing to the decline of sockeye salmon or predators are simply acting as the“executioners” of sockeye salmon that were already less healthy and slower due to some otherunderlying driver.Christensen and Trites (2011) also put forth an alternate theory regarding the potential impact ofpredation on sockeye salmon. They suggest that if there have been substantial declines in thepopulations of alternate prey species that are physically comparable to sockeye salmon,predators that might otherwise not eat or not prefer sockeye salmon might increase theirconsumption. In this situation, it would be possible for predators to have an increased impact onsockeye salmon while not actually increasing in abundance.Competition is another plausible mechanism of potential importance discussed by Christensenand Trites (2011). Ruggerone et al. (2010) summarize trends in wild and hatchery populations inthe North Pacific, and discuss the potential for a “tragedy of the commons” effect due toincreased numbers of fish competing for a finite pool of food resources. In section 4.7, weconsider the extent to which total pink salmon abundance (wild plus hatchery) can explain57