Fraser River sockeye salmon: data synthesis and cumulative impacts

2.3 Present Cumulative Effects AnalysisThe present cumulative effects analysis relies on data provided by each of the independentresearch projects investigating a different potential category of stressor. Some of the analysesexamine potential interactions among different types of stressors, but most of our quantitativework focuses on the relative impact of these different factors. Our analyses are be limited by: 1)the quality and extent of the data that are actually available; and 2) the degree of complexity inthe “true” underlying causes of the recent decline in productivity of Fraser River sockeye.We first address the issue of complexity. Figure 2.3-1 illustrates four different hypothetical pathsby which an individual sockeye salmon could be exposed to stressors over its lifetime, yet allleading to the same outcome - death as an adult. For the sake of simplicity, we have assumed it ispossible to integrate all stress factors into a single measure of cumulative stress where 0.0represents perfect health and 1.0 represents mortality. There are four scenarios described inFigure 2.3-1:o A1: the sockeye is severely affected by stressors as a fry (e.g., acquiring a disease thatalmost killed it and permanently affected its health), but does not suffer any further stressuntil, as an adult, a minor incremental impact results in death.o A2: the stressor that almost killed the sockeye as a fry killed off many other fry, reducingcompetition for food at the smolt stage, improving the health of surviving individuals;o B: the sockeye suffers moderate stress from many separate incidents over its lifetime,eventually dying at the same age as under the other scenarios even though none of thestressors experienced were even moderately severe.o C: the sockeye is only moderately affected by stressors over its lifetime until, as an adult,it is impacted severely by stressors that quickly result in death (e.g., low food, abundantpredators, high temperatures).In scenarios A and C, the rapid induction of severe stress could be either one severe stressor ormany stressors occurring simultaneously but within a similarly constrained window of time. Inscenario B, the slow induction of moderate stress could be either continued exposure to onestressor over the entire lifetime, or many different stressors occurring variably over time andspace.If scenario A or C accurately represent the “true” pathway, this relationship might be detected bytesting the fit to productivity indices of models that only include sets of factors limited to fry (A)and adult (C) life history stages. If scenario B represents the “true” pathway with one keystressor, this relationship might be detected by testing model sets limited to particular stressors or8