Fraser River sockeye salmon: data synthesis and cumulative impacts

in the Lower Fraser than other Fraser sockeye, yet this stock is the only one of 19 with increasingproductivity. This implies that conditions in the Lower Fraser River were not sufficientlystressful to cause productivity declines in Harrison sockeye, and suggests that Lower Fraserconditions were unlikely to be a primary driver of observed productivity declines in the otherFraser stocks that pass through the Fraser estuary much more quickly.4.3.4 Correlation/consistency with patterns in non-Fraser River sockeyeproductivityWe do not have indicators of potential stressors for non-Fraser sockeye stocks during the smoltmigration stage. Similar arguments to those presented in section 4.2.4 apply to habitat andcontaminant stressors during the smolt migration stage (i.e., we suspect they are not primarydrivers of observed productivity declines, but have no stressor data to test this hypothesis).4.3.5 Other evidenceOther evidence includes literature demonstrating thresholds, an understanding of the specificform of response of sockeye to certain stressors (i.e., helping to accept or reject differentsuspected causes), experiments demonstrating cause-effect linkages, or positive sockeyeresponses following the removal of a stressor. In general, the level evidence for Stage 2 is lessthan for Stage 1, due primarily to the challenges of experimentally evaluating responses ofrapidly migrating smolts to a continuing gradient of stressors. Evidence is strongest for suchstressors as contaminants and temperature, which are amenable to experimentation. Asdiscussed above, MacDonald et al. (2011) compared water contaminant concentrations duringthe smolt migration period with thresholds established from laboratory and field studies, andfound no evidence that contaminants encountered by smolts contributed to declining sockeyeproductivity. Studies of smolt health conducted in other rivers (e.g., Columbia River and otherstudies reviewed in Marmorek et al. 2004) are generally not applicable to the Fraser situation.Therefore, we are left with little other evidence to evaluate stressor hypotheses.4.3.6 ConclusionsTable 4.3-1 shows our conclusions regarding the effects of each stressor on life history stage 2(smolt migration from rearing habitats to the Fraser Estuary). Again, our conclusions relate to theoverall trends in sockeye productivity over the last two decades. This table is identical to Table4.2-1 for Stage 1, except that migrating smolts are judged to have no exposure to either mines orsmall hydro, compared to low exposure for eggs, alevins and fry.53