Fraser River sockeye salmon productivity and climate: A re-analysis that avoids an undesirable property of Ricker’s curve

In descending order of importance, artificial spawning channels, density-dependent mortality, carryover mortality, and climate have significant influences on the average productivity of Fraser River sockeye salmon (Oncorhynchus nerka). When factors that are known or have been hypothesized to affect Fraser River sockeye salmon productivity are included in a single analytical framework, no significant change in average productivity occurred in 1976/1977, however, beginning in 1989 average productivity was significantly lower. In the one lake (Chilko) in the Fraser River basin where pre-smolt survival can be distinguished from post-smolt survival, this decline arose from freshwater causes. After accounting for other factors that have a greater influence, Fraser River sockeye salmon productivity tends to be slightly lower in years when the intensity of the Aleutian low pressure region is stormier in winter, although the effect is not strongly expressed in any particular population. A footnote to the study was the realization that estimates of Ricker’s density-dependent mortality parameter, β, are influenced by both the numerical properties of the equation and by population biology; density-dependent and density-independent influences on the estimates of the parameter are confounded.