Fraser River sockeye salmon: data synthesis and cumulative impacts

examples illustrate this problem -- the collapse of Canada's Northern cod populations inthe early 1990s and the virtual disappearance of California sardine in the 1960s -- both ofwhich fueled long debates about the relative importance of fishing, environmentalchanges, and government regulations in causing those collapses.The sockeye stocks within the Fraser Basin have widely varying life history, genetic and habitatcharacteristics that create different levels of vulnerability to the stressors each stock encounters(described in Nelitz et al. 2011). Effects of stressors on survival at any life history stage dependon both the magnitude of the stress and the vulnerability of the salmon. Characteristics that varyacross stocks include: spawning habitat (inlets, outlets, lake shore, flow rates, substrateconditions, environmental conditions), nursery lakes (area, size, productivity, temperature, icebreak-up, duration of rearing), smolt out-migration (distance, timing, temperatures, arrival atestuary, residence time in estuary), coastal migration (timing, duration, route), and adultmigration (return route, age of return, timing, estuary residence time, timing of upstreammigration, upstream distances and duration, river temperatures and other environmentalcharacteristics, pre-spawn mortality rates). Many Fraser sockeye stocks are strongly cyclical(e.g., Late Shuswap, Quesnel, Scotch) whereas others are less so. Once mobile, each salmon hasa recurring choice – eat or hide. Sockeye stocks (and sub-populations within each stock) havedeveloped complicated and varying life histories that include moving between ranges of habitatsvarying in the risks they represent (Christensen and Trites 2011, pg. 5). Finally, we are observinglarge scale effects of climate change in both freshwater and marine environments, withinfluences on many of the above attributes and their interactive relationships.3.2 Unknowns, Unknowables, Knowledge Gaps, and Data LimitationsGiven all of the above challenges, what can fisheries science achieve that is helpful to both theCohen Commission and fisheries managers? First, science can test hypotheses, rejecting thosethat are unlikely or false. Even with considerable gaps in data and understanding, and mostlyindirect evidence, contrasts over space and time in both salmon stock productivity and thepotential stressors allow us to judge certain stressors to be unlikely to have been the primaryfactors causing declines in sockeye productivity or abundance. Other factors may be possible oreven likely, provided that they fulfill most or all of various criteria (i.e., have a plausiblemechanism by which survival could be affected; have generally exposed Fraser sockeye toincreased stress over the period of productivity declines; correlate over space, time and stockswith variations in productivity; and (ideally) have other corroborating evidence from cause-effectstudies). The procedure by which we evaluate alternative hypotheses is described below insection 3.3. Two key principles are: 1) hypotheses can be rejected as false or unlikely, but cannotbe accepted as true (only relatively more likely); and 2) correlation does not equal causation (one14