Fraser River sockeye salmon: data synthesis and cumulative impacts

also needs an underlying mechanism that can logically (and defensibly) link the cause withobserved effect).There are several challenges in this process of evaluating alternative hypotheses. The firstchallenge is data limitations, which include incomplete time series of information (both withineach stage of the life cycle and over multiple years), incomplete spatial coverage for all stocks,poor quality data (imprecise or inaccurate measurements), crude indicators that do not reallyreflect the condition of interest (e.g., air temperatures rather than the water temperatures wheresalmon eggs are incubating), and inconsistent methods of measurement. There are 36Conservation Units in the Fraser Basin (CUs). We only have estimates of spawning abundanceand en-route mortality for about half of these CUs, and juvenile production estimates for aboutone quarter of these CUs. With the exception of a few detailed studies (available for only a fewyears and stocks), we do not have any estimates of survival rates or abundance between the timethat fry or smolts are sampled, and the time that adults return to be counted at Mission two tothree years later. When it comes to explanatory factors, we would ideally have data that are intergenerational(i.e., across 40 years to provide a pre-decline base period), intra-generational (acrosslife history stages and locations), and inter-stock (to explain why some have done well whileothers declined). Statistical analyses of multiple factors (to see which ones are best correlatedwith productivity patterns) require data on all of the factors for all of the stocks and yearsincluded in the analysis. As difficult as it is to retrospectively deduce which factors were more orless likely to have caused historical patterns, the one advantage that we have over predicting thefuture is that there is only one past.The second challenge is gaps in basic knowledge or understanding. We generally do not knowhow, where or when sockeye die. The few situations in which we can definitively determine thecauses of mortality are comparatively rare (i.e., fish harvests, stomach analyses of predators,intensive telemetry studies showing that fish died while experiencing conditions beyondestablished thresholds). In most cases, mortality must be inferred indirectly based on informationon the sockeye’s exposure to different stresses, but there are uncertainties in both fish migrationpatterns and the stresses experienced by each group of fish. McKinnell et al. (2011; pg. 4) pointout:“During the period of years of interest to the Commission, there are virtually noobservations of Fraser River sockeye salmon during about 75% of their life at sea, andthe value of coincidental samples taken during their emigration from the Strait of Georgiais debatable.”Little is known about the potential impact that abundant predators may have on relatively rareprey. In such situations, it may be possible for the abundant predator to have a very large impacton, for example, a weak and declining sockeye stock, despite that prey being a minor and15