Potential Effects of Contaminants on Fraser River Sockeye Salmon

The eggs <strong>of</strong> sockeye salmon incubate within the stream-bed or lake-bed substrate forextended periods <strong>of</strong> time. Time to hatching is variable, depending primarily on thetemperature regime at the spawning location. Data from several sources indicate thatsockeye salmon eggs require between 350 and 720 temperature units (i.e., degree days, inoC) and up to 170 days to hatch (Foerster 1968; Velsen 1980). After hatching, alevins (oryolk-sac fry) may remain in the gravel for several months, with emergence usuallyoccurring 140 to 225 days following fertilization <strong>of</strong> the eggs (i.e., at 1000 to 1150temperature units; Mead and Woodall 1968).Emergent sockeye salmon fry exhibit a variety <strong>of</strong> migratory behaviours, depending on thestock and the spawning location under consideration. For example, fry emerging fromtributaries located upstream <strong>of</strong> the nursery lake move downstream with the current, whilethose emerging from downstream spawning locations initially move laterally to thestreambanks (to avoid being swept downstream) and then migrate upstream to the nurserylake (Burgner 1991). Fry emerging from lakeshore spawning sites tend to move <strong>of</strong>fshoreto deeper water. Upon arrival in the nursery lake, sockeye salmon fry rear for one to twoyears before migrating to the ocean. At least one stock (Harrison River) does not utilizerearing habitats within a freshwater nursery lake, but instead rears within the HarrisonRiver, Lower Fraser River, and/or Fraser Estuary before migrating to the ocean(Johannessen and Ross 2002). Accordingly, most stocks <strong>of</strong> sockeye salmon utilizefreshwater rearing habitats for at least one year prior to downstream migration.In their second year <strong>of</strong> life, most sockeye salmon in the Fraser River Basin undergo anumber <strong>of</strong> morphological, physiological, and behavioural alterations to prepare formigration to the ocean. This process, termed smoltification, usually occurs in the springwhen break-up (i.e., ice melt) and spring overturn occur in the nursery lake. Sockeyesalmon smolts typically initiate downstream migration in late April to late June, dependingon the stock (Hartman et al. 1967). Downstream migrants may average 40 km/day,possibly requiring up to 30 days to reach the estuary. Some sockeye salmon reside in theestuary or nearshore areas for some time after outmigration, but most stocks enter theStrait <strong>of</strong> Georgia by the end <strong>of</strong> May (Burgner 1991). Distribution <strong>of</strong> sockeye salmon to<strong>of</strong>fshore waters is usually complete by autumn. Sockeye salmon reside in the marineenvironment for one to four years, with most fish returning to the Fraser River after twoto three years <strong>of</strong> ocean residence (Burgner 1991).Adverse effects on ecological receptors can occur when stressors and receptors arepresent in the same place and at the same time. As such, determination <strong>of</strong> exposure <strong>of</strong>sockeye salmon to contaminants in the Fraser River Basin requires an understanding <strong>of</strong> the7