Fraser River sockeye salmon: data synthesis and cumulative impacts

how effects at different life history stages can accumulate over the whole life cycle, increasingthe cumulative stress on an individual salmon to the point where it dies. The accumulation ofstress may be concentrated in one life history stage or distributed across multiple life historystages. The stress experienced within each life history stage may be insufficient to causemortality, but the cumulative effect of stressors in multiple life history stages can cause death.However, mortality events at an early life history stage can also result in a compensatoryreduction in competitive stress for those fish which survived, reducing their cumulative stressand increasing their chances of survival. These two sets of processes illustrated in Figure 3.3-1and 2.3-1 (i.e., cumulative impacts from many stressors within each life history stage, cumulativeeffects on each fish over its life) occur concurrently within each generation of sockeye.Table 4.7-1 summarizes the results of our analyses by life history stage. We found only twofactors (marine conditions and climate change) which were likely to have been a primary factorin the observed declines in Fraser sockeye productivity (recruits/spawner) over the last twodecades. While en route mortality has definitely had an impact on the sockeye fishery andnumbers of fish reaching the spawning ground, it is unlikely to have affected total productivity,since en route mortality is already included in the calculation of total recruits (i.e., recruits =spawners + en-route mortality + harvest). The effects of predators during the marine phase of thesalmon life cycle (stages 3 and 4 in Table 4.7-1) were judged to be possible primary contributorsto these declines. Due to lack of data it is not possible to draw conclusions about thecontributions of pathogens, which is a particularly important data gap that we discuss further inour recommendations (Section 5). Aquaculture was not considered in our report as theCommission Technical Reports on this potential stressor were not available, but will beconsidered in an addendum to this report. All other factors (i.e., forestry, mining, large hydro,small hydro, urbanization, agriculture, water use, contaminants, density dependent mortality,human activity and land uses in the Lower Fraser and Strait of Georgia) were judged to beunlikely as primary causes of long term productivity declines, though they may still have beencontributory factors. That is, stressors which we consider unlikely to be primary causes ofproductivity declines, may combine with other factors to create sufficient cumulative stress tokill salmon (i.e., through additive or greater than additive (synergistic) interactions) in somestocks in some years.The coastal migration phase of the sockeye’s life history provides a good example of multiplestressors interacting to cause cumulative impacts. There is indirect evidence that while oceantemperatures were not high enough to directly kill sockeye smolts in the summer of 2007, thesewarmer temperatures may have decreased the quantity and quality of available food andincreased other stressors (e.g., metabolic demands during inshore migration, vulnerability topredators, the level of pathogens and harmful algae); see McKinnell et al (2011) and Peterman et88