Fraser River sockeye salmon: data synthesis and cumulative impacts

Freshwater predators on juvenile sockeye could potentially increase in response to large fryproduction (discussed above), or could increase in response to other natural or anthropogenicfactors that shift the species composition of fish or wildlife communities. Christensen and Trites(2011) cite studies from along the Pacific Coast indicating that various predators can potentiallyconsume significant numbers of sockeye in freshwater, including coho, chinook, cutthroat trout,rainbow trout, steelhead, Northern pikeminnow, Yellow perch, common mergansers, Caspianterns, and double-crested cormorants. They later shrink this list of suspects based on trends in theabundance of these predators.Disease is another potential form of delayed density dependent mortality (discussed above), orcould increase for other reasons. In his review of potential candidate diseases, Kent (2011,Executive Summary) noted that the IHN virus is well recognized as a lethal pathogen to fry, andrated IHN as “High Risk”. He also summarized studies indicating that warming temperatures,pollution and habitat alteration can potentially increase both the susceptibility of salmon todisease, as well as the abundance of certain pathogens (Kent 2011; pgs. 21-22 ).As discussed in Hinch and Martins (2011, Section 1.4) climate change can potentially affect thesurvival of eggs, alevins and fry by: shifting temperatures above the thermal optimum to whicheach stock has adapted (sockeye populations in the interior of B.C. prefer cooler water thancoastal stocks); increasing late fall stream flows and expanding the wetted area available forspawning (a positive effect); increasing winter stream flows and scouring more eggs from thegravel (a negative effect); and increasing rates of predation on sockeye fry rearing in lakes. Inaddition to these direct impacts on sockeye spawning and rearing, climate change can potentiallyexacerbate the impacts from other stressors (e.g., disease (discussed above); more extreme stormevents increasing forestry impacts on water and sediment delivery; climate-induced changes toseasonal patterns in stream flow combining with water withdrawals to worsen conditions foreggs, fry and smolts).The above stressors all have plausible mechanisms for potential impacts on Stage 1 of thesockeye life cycle. However, for the above factors to jointly affect egg-to-fry/smolt survival,these early life history stages must be exposed to some combination of these stressors in actualspawning and rearing locations within the Fraser Basin, and at levels that cumulatively combineto affect survival. This is much more difficult to determine, as discussed below. Levels ofexposure and survival must be inferred indirectly from incomplete information.41