Fraser River sockeye salmon: data synthesis and cumulative impacts

Historically, the majority of returning adults migrated through the Strait of Juan de Fuca on thereturn journey to the Fraser River, whereas a smaller portion would return via the “northerndiversion” through Johnstone Strait. Returning via the Strait of Juan de Fuca potentially resultsin reducing the duration of exposure to potential stressors within the Strait of Georgia andeliminating exposure to potential stressors within Queen Charlotte Strait and Johnstone Strait onthe return journey. However, the balance of these alternate behaviours also appears to havechanged substantially over the past few decades. Figure 4.5-2 demonstrates that the proportion ofsockeye salmon returning via the “northern diversion” has increased markedly over time, fromapproximately 10-20% in the 1950s and 1960s, to upwards of 80% in many of the years since thelate 1970s. The obvious increase in the variability evident in Figure 4.5-2 suggests that someunderlying driver changed in the late 1970s. The reason that sockeye salmon might “decide” totake one route versus the other and the point at which the decision is made both remainunknown, but it appears that greater use of the northern diversion may be associated with warmercoastal ocean temperatures (McKinnell et al., 2011, Section 4.6).Figure 4.5-2. Time series of the percentage of Fraser River sockeye salmon that migrate via the northern diversion(Johstone Strait). Source: Pacific Salmon Commission unpublished data.Whereas the Cohen Commission scientific projects are looking at changes in sockeyeproductivity and returns as the primary response variable, changes in both ocean residency andmigration route are other potential responses that may be driven by similar factors andconditions. Understanding how these other response patterns have been changing over time, and75