Fraser River sockeye salmon: data synthesis and cumulative impacts

general, the potential mechanisms in this stage are similar to those described for Stage 3 (Section4.4) above.The high risk pathogens described earlier are known or suspected to potentially affect bothjuveniles and adults. However, although Parvicapsula minibicornis has been documented to behighly prevalent in sockeye salmon smolts, it is not found in adults, which suggests thatmortality due to this pathogen occurs within the early marine phase (Kent, 2011). Overall, verylittle is known about pathogens and disease in the marine environment (Kent, 2011).The potential mechanisms for predators to affect sockeye salmon populations are the same asabove (increased abundance, increased predation rate, or decreases in alternate prey) but theassemblage of potential predators is different. Christensen and Trites (2011) identify salmonshark and daggertooth as key predators of adult sockeye salmon that could plausibly have had anincreasing impact on sockeye salmon populations. Sockeye salmon is known to comprise a largeportion of the diet of salmon sharks. Blue sharks do not specialize on sockeye salmon, but do eatsalmon in general and are much more abundant than salmon sharks. On the return journey backto the Fraser, there are many marine mammals that will prey on adult salmon. In some cases,sockeye salmon do not appear to be a substantial portion of any of their diets but, as illustratedby the spiny dogfish example in Section 4.4.2, this does not necessarily imply that such predatorsdo not have an impact on sockeye salmon. The effect of any predator on sockeye depends on thepredator’s abundance, the proportion of the predator’s diet which consists of sockeye salmon,and the resulting total biomass of sockeye consumed by the predator. Data collected by P.FOlesiuk (unpublished) on Steller sea lion scat samples was presented at both the PSC workshopand the Cohen Commission workshop. These data show that adult salmon is a commoncomponent of the diet of Steller sea lions, being found in approximately 12-30% of samples,varying by season (Peterman et al. 2010, A. Trites, workshop presentation). These samples werenot identified by species. Other research has suggested that salmon represent approximately 10%of the overall diet of Steller sea lions, and that sockeye salmon contribute to 9% of that portion(i.e. 0.9% of the total diet; A. Trites, workshop presentation). However, since Steller sea lionsconsume a large amount of biomass, a small proportion of that consumption could still plausiblyhave a meaningful effect on sockeye salmon. The technical report on predators did not includeestimates of total sockeye consumed by Steller sea lions. The bird predators described above arenot relevant to this life stage. Returning adult sockeye salmon are a very different prey then theirearlier post-smolt forms: larger, but fewer and faster.The biological and physical ocean conditions are fundamentally important for the health andsurvival of maturing sockeye salmon in the North Pacific Ocean. For example, oceantemperature has a critical influence on bioenergetics for sockeye salmon (McKinnell et al.,72