Fraser River sockeye salmon: data synthesis and cumulative impacts

River sockeye salmon but also, where possible, the observed patterns in non-Fraser Riversockeye salmon (though this evaluation is limited since the Cohen Commission did not collectstressor data for non-Fraser sockeye stocks). Based on the evidence available, a relativelikelihood is assigned to each broad category of stressor (e.g., contaminants, predators, etc.) ateach life stage, according to the framework shown in Figure 3.3-3. The conclusions from eachlife stage apply to the contribution of each broad impact factor to the overall pattern of changeobserved in Fraser River sockeye stocks. There may be cases in which the relative likelihoods ofparticular stressors do not all align perfectly with the relative likelihood assigned to the parentstressor category. For example, the evaluation of the overall impact of predators may not matchthe evaluation of particular predators. There may also be cases in which the results from thisevaluation framework might be different for individual stocks. However, the focus of the presentproject is to evaluate the likelihood that each broad factor has made a significant contribution tothe overall observed decline in the Fraser River sockeye salmon stock complex.Because this method is an inherently retrospective form of analysis, the results cannot be used tomake future predictions. Both Forbes and Callow (2002) and Burkhardt-Holm and Scheurer(2007) emphasize that it is unrealistic to expect these methods to be definitive in terms ofascribing causation. While such an approach may be able to explain retrospectively which factorsmost likely contributed to past patterns of change in productivity, the importance of particularfactors may be more or less important in the future and will vary within any given year in bothmagnitude and relative importance. Even if we had complete data on all of the factors potentiallyaffecting sockeye over the entire period of record for the stock productivity data, we would notbe able to necessarily predict in advance how these factors will combine in the future to affectproductivity. This is particularly true in the era of climate change, where the biophysicalstructure and functioning of ecosystems may move beyond the range of historical conditions. Asmentioned above in section 3.1, environment-recruitment correlations generally do not persistover time (Walters and Collie 1988, Walters 1989, Myers 1998).23