Fraser River sockeye salmon: data synthesis and cumulative impacts

4. better integration of existing and future data sets affecting freshwater spawning andrearing habitats (a more general need, discussed in section 9.2)4.3 Stage 2: Smolt OutmigrationThis stage covers sockeye from the time they leave their nursery lake (as fry, pre-smolts orsmolts) to the time they reach the mouth of the Fraser River.4.3.1 Plausible mechanismsMost of the plausible mechanisms discussed for stage 1 (egg-to-smolt stage) also apply to stage2, since migrated smolts can be exposed to degraded habitats, contaminants, pathogens, elevatedtemperatures and the effects of delayed density dependence. Nelitz et al. (2011) point out thatsockeye salmon smolts are cued to migrate towards the ocean in response to changingenvironmental conditions, which includes responding to day length, lake springtime temperatures(related to the timing of ice break-up in nursery lakes), and springtime peak flows, all of whichare influenced by year to year climate fluctuations and climate change. Earlier outmigrationcould lead to a mismatch between the arrival of salmon smolts in the Fraser estuary and Strait ofGeorgia, and the timing of plankton blooms that are essential for growth and survival in Stage 3(coastal migration).4.3.2 Exposure of Fraser River sockeye to stressorsThere are four key points to consider regarding the exposure to stressors during this stage. First,the duration of potential exposure to stressors is much less in Stage 2 than in Stage 1. Whereassockeye spend on average 1.75 years in stage 1 (e.g, spawning in August-November of 2004 andleaving their rearing lake in May of 2006), they generally spend only two months in Stage 2migrating downstream the ocean (e.g., during May and June 2006), and will be exposed to awide range of conditions during this migration. Second, both the duration of exposure and thestressors experienced (i.e., the vulnerability of a stock’s migratory habitat) vary with thedistances over which sockeye smolts must migrate. There is a 10-fold variation in migratorydistances across the Fraser Conservation Units, from 111 km for Cultus Lake sockeye, to 1182km for Nadina sockeye (Nelitz et al. 2011, Table 18). Third, all sockeye stocks pass through theFraser estuary, and are exposed (though briefly) to the cumulative effects of habitat disruption inthis region (Johannes et al. 2011). Fourth (and counter to the third point), while “dilution is notthe solution to pollution”, the substantial volumes of water in the lower Fraser River have animportant dilution effect on contaminant concentrations (MacDonald et al. 2011; Johannes et al.2011).50