Fraser River sockeye salmon: data synthesis and cumulative impacts

The results show that the global model (M1) had the greatest level of support. The fact that thismodel was ranked highest even with 18 more variables than the next model indicates that thevariables associated with only a single project are not sufficient to explain the pattern inproductivity. When comparing the relative performance of models representing individualstressor categories the predators/alternate prey model (M5) has the greatest level of supportfollowed closely by the Lower Fraser/Strait of Georgia project (M7). The predator/alternate preymodel reflects two possible mechanisms for affecting Fraser sockeye: changes in the abundanceof predators and/or changes in the availability of alternate prey.As discussed above, each stressor category is only represented by the available data. Many ofthose categories are not adequately represented. For example, “climate change” (M6) is onlyrepresented by a single variable – the delayed effect of lower Fraser River water temperature forreturning adults – that does not fully capture the potential impacts of climate change on sockeyesalmon over their entire life. A second example is the impact of pathogens and disease, whichcould potentially be an important factor but are simply not represented within any of thesemodels due to the lack of data. Therefore it is not possible to make any conclusions at all aboutthe relative importance of pathogens and disease compared to other stressors, which is a majorweakness.Overall, we do not believe that organizing the data by stressor category is as useful as organizingthe data by life history stage. The boundaries between categories are arbitrary and many of themlack sufficient data. In the analysis organized around stressor category, none of the models haddefinitively higher support than the global model, which included all available variables. Bycontrast, when the exact same data were organized by life history stage, several of the modelsachieved a higher level of support than the global model.The results of this analysis raise the important issue (mentioned in Section 3.2) about correlationversus causation. An example of this issue is found within the predators model. Christensen andTrites (2011) show that the abundance of both Steller sea lions and Harbour seals have increasedsubstantially over the past few decades. Because the timeframe of that increase corresponds withthe period over which Fraser River sockeye salmon have been declining in productivity, thesemeasures are likely to exhibit a high degree of correlation. While it is possible, this does notnecessarily mean that the increase in pinnipeds is driving down the sockeye salmon population.Alternative explanations are: some other factor is affecting both populations; or, pinnipedpopulations have been recovering from low abundance since the banning of hunting and culling,and sockeye productivity is being driven by something else. Further study is required todetermine which of these possibilities are most likely. Any factor with a strong temporal trendover the same period of time as the strong temporal downtrend in sockeye salmon productivity98