Fraser River sockeye salmon: data synthesis and cumulative impacts

more likely to suffer en-route and pre-spawning mortality mortality. The genomic signal detectedby Miller et al. was also present in smolts during both 2007 and 2008 (Miller, handout providedto June 2010 PSC Workshop), yet those years of entry had very different marine survival rates(based on the very large difference in observed vs expected adult returns in 2009 vs. 2010). Asdiscussed by McKinnel et al. (2011) ocean conditions in Queen Charlotte Sound and the Gulf ofAlaska were much cooler in 2008 than in 2007. It may be that when ocean conditions are poor(as in 2007) disease contributes to mortality, but when ocean conditions are good (as in 2008) thefish survive despite carrying diseases.As noted by McKinnel et al (2011), biologists rarely observe death by natural causes of FraserRiver sockeye at sea. Therefore, unlike with autopsies of humans, we can generally only infer thecumulative causes of mortality through indirect evidence. Even with very detailed informationon the exposure of salmon to different stressors (e.g., Petrosky and Schaller 2010), it is difficultto draw strong conclusions on the relative contributions of each factor to observed patterns ofsurvival. Often there are multiple explanations that are generally consistent with the observeddata.4.7.2 Quantitative analyses across the entire life cycleWe performed quantitative analyses across the entire life cycle from two different perspectives –by life history stage and by stressor category. An overview of our methodology is described inSection 3.3.6, a technical description of the methodology is presented in Appendix 3 (Section3.5.2), and the detailed results are reported in Appendix 4. In this section we provide anoverview of the results of our quantitative analyses and a broad discussion of the majorconclusions. First, we describe our analysis of the relative importance of different life stages inexplaining variation in sockeye productivity. Second, we describe our analysis of the relativeimportance of categories of potential stressors across all life stages, as organized by the CohenCommission Technical Reports.The Relative Importance of Different Life History StagesWe analyzed data across all projects and life stages that were available for brood years 1969-2001. This particular time period was the result of the trade-off between choosing a window ofanalysis short enough to include a wide selection of variables and long enough to generatemeaningful results with respect to the observed decline in the productivity of Fraser Riversockeye salmon. To assess the relative importance of each life history stage, multiple modelswere tested, each of which contained all of the available variables associated with a particularlife history stage (Table 4.7-2). The life history stages examined include: incubation andfreshwater rearing, outmigration, coastal migration, ocean rearing, return to the Fraser, upstream91