Fraser River sockeye salmon: data synthesis and cumulative impacts

2011). The thermal limit hypothesis describes the role that ocean temperature has in limiting thegeographic range of sockeye salmon in the North Pacific, and suggests how this range may bereduced by warming ocean temperatures due to climate change. But McKinnell et al. (2011,Section 3.5) critique this theory because it does not consider the ability of sockeye salmon toseek cooler ocean temperatures simply by moving deeper. Inter-annual and inter-decadalvariation in ocean temperature and biological productivity are known to have substantial impactson many marine species (McKinnell et al., 2011).Such variability in ocean conditions may be further exacerbated by climate change. Forexample, in recent decades the Pacific Decadal Oscillation has been exhibiting more frequentoscillations between phases (Hinch and Martins, 2011, Section 1.5). Hinch and Martins (2011)suggest that “it seems that interannual variations in climate conditions have contributed to theextreme variation in the abundance of returning adults that were observed in 2009 and 2010”.Habitat conditions in the Strait of Georgia, including marine conditions and human activitiesand development are obviously not factors during the open ocean phase, but the samemechanisms described in Section 4.4 are relevant for returning adults. Returning adults maypotentially be more resilient to some of the stressors encountered through the Strait of Georgia,especially since returning individuals are those that have survived the early marine phase andtwo winters in the open ocean.4.5.2 Exposure of Fraser River sockeye to stressorsThe ability to assess the exposure of Fraser River sockeye salmon to various potential stressorsin the open ocean and return journey to B.C. coastal waters is severely limited by lack ofknowledge of the distribution of sockeye salmon during this stage. McKinnell et al. (2011,Section 1.3) describe the lack of systematic monitoring and report that “there are virtually noobservations of Fraser River sockeye salmon during about 75% of their life at sea and the valueof coincidental samples taken during their emigration from the Strait of Georgia is debatable”.Aside from the following case where there is some information with which to evaluate exposure,one must simply assume that the Fraser River sockeye salmon have been exposed to themechanisms described above.The evidence presented by Christensen and Trites (2011) shows that for marine mammalpredators there has not been exposure to California sea lions because they are not present alongthe southern coast of BC during the summer. However, Steller sea lions have a large populationdistributed along with the northern and western coasts of Vancouver Island during the summerand the total population for BC and southeast Alaska may be upwards of 60,000 (Peterman et al.73