Fraser River sockeye salmon: data synthesis and cumulative impacts

Cumulative Stress (arbitrary units)1.0 MortalityHigh morbidity0.0AA1A2BEgg Fry Smolt Adult SpawnerCTime (Life-history Stages)Figure 2.3-1. Cumulative stress model. Lines illustrate four scenarios through which an individual sockeye salmonmight suffer from the cumulative impacts of exposure to stressors over its lifetime. Each scenario illustratesa different pattern in the number, severity and timing of stressors experienced, yet the timing of mortality isthe same for all three scenarios. For both Scenarios A and C, it is evident that eventual mortality isprimarily the result of one particular period of substantial stress, though in Scenario A this is not theproximate reason for death. Scenario A2 reflects the possibility that density-dependence effects on thepopulation might occur within a single generation (as compared to density-dependent effects that may alsooccur across generations), which could benefit surviving individuals. That is, in Scenario A2, the stressorthat almost kills this hypothetical individual may actually kill a substantial portion of the rest of thepopulation. If so, the surviving individuals may encounter improved conditions as smolts due to lowerdensity and less competition for resources. However, in Scenario B mortality is the result of manysubsequent impacts over the individual’s entire lifetime, none of which would have resulted in mortality ontheir own or even as a small subset of the cumulative impact. A roughly similar conceptual model could bedeveloped for an entire sockeye population, though it would be more appropriate to use overall survivalrates, which would decrease over time and life history stages.Concepts of cumulative effects are embedded throughout this report. First, we have alreadydiscussed above discuss the theory of cumulative effects. Second, the conceptual model (Figure3.3-1) embodies several characteristics of cumulative effects analyses: it provides a graphicalrepresentation of how the valued ecosystem component (i.e., sockeye salmon) is potentially10