Fraser River sockeye salmon: data synthesis and cumulative impacts

coastal migration. Anomalous climate conditions in 2007 resulted in exceptional snowpackaccumulation in the mountain ranges of western BC during the winter, then a delayed but rapidsnowmelt in the spring that produced extreme levels of discharge into Queen Charlotte Sound.As a result of this large influx of freshwater into the Sound, salinity measurements were found tobe at or near record lows throughout the area. This anomaly was then maintained by an atypicalwind regime that kept these fresher waters “backed up” in Queen Charlotte Sound. The sea levelanomalies observed in Prince Rupert provide further support for this mechanism. The products ofthese factors were a fresher surface layer and a more stable ocean column that inhibited mixing,allowing the surface layer to warm to higher than average temperatures and potentially becomedepleted of nutrients. The spring of 2007 marked a year of very poor chlorophyll production inQueen Charlotte Sound, a factor that has been associated over time with poor survival rates ofsockeye salmon from Chilko Lake. The combination of a substantial reduction in food supplyand the higher energetic costs of migrating through warmer waters could potentially have led toincreased mortality of the cohort of Fraser River sockeye salmon that returned in 2009. However,by contrast, McKinnell et al. (2011) found that while some of these same physical and biologicalmeasures were higher than average in the Strait of Georgia in 2007, none of them exhibitedextreme levels. Although long records of many physical ocean properties are available, thereexists only a limited record for biological properties such as chlorophyll concentration (i.e. since1998). McKinnell et al. (2011; page ix and 135) noted that in 2007 there was typical survival ofacoustically-tagged hatchery-reared sockeye salmon from Cultus Lake northward through theStrait of Georgia in 2007, which is consistent with the non-extreme physical conditions discussedabove.McKinnell et al. (2011; page 110) emphasize that conditions in Queen Charlotte Sound werevery different in 2008 (affecting the 2010 returns) as compared to conditions in 2007 (affectingthe 2009 returns):“The summer of 2008 was the opposite of 2007. Sea surface temperatures along theNorth American coast were cool following what was the coldest year in the Gulf ofAlaska since 1972, and these cool anomalies persisted along the coast throughSeptember. Unlike the Strait of Georgia, migrating sockeye salmon in 2008 would havehad a very different thermal experience during their migration in 2008 compared to 2007once leaving the coastal straits. The temperature of surface seawater along the coast isoften an indicator of major ecological changes that accompany the warmer/colder ocean.”The much improved marine survival rates of Fraser sockeye in 2008 (relative to 2007) areconsistent with the hypothesis that sea surface temperatures strongly affect marine survival.62