Climate Change - Cohen Commission

1.4.5. Returning adults
The relationship between river temperature and survival during the spawning migration has been
extensively studied. Migration mortality of adults (termed en route mortality) becomes elevated
at temperatures > 18 o C (Crossin et al. 2008; Farrell et al. 2008; Keefer et al. 2008; Macdonald
et al. 2000a,b, 2010; Martins et al. 2011; Naughton et al. 2005). High levels of en route
mortality can be caused by a combination of temperature-mediated factors. First, warm river
temperatures increase energy use in sockeye salmon and migration failure can occur if energy
reserves fall below a critical threshold (Rand et al. 2006). Second, exposure to high water
temperature increases the rate of development of pathogens in sockeye salmon (Crossin et al.
2008; Gilhousen 1990), causing physiological stress, decreased swimming performance and
disease (Bradford et al. 2010; Wagner et al. 2005). Finally, warm temperature reduces aerobic
scope in sockeye salmon, limiting the fish‟s ability to allocate energy to essential tissues during
the migration (Lee et al. 2003). Under extreme water temperatures, aerobic scope is reduced to
such an extent that continued migration can lead to anaerobic activity, exhaustion and death by
lactic acidosis or cardiac collapse (Farrell et al. 2008; Farrell 2009).
Recent physiological studies on adult migrants have suggested that Fraser River sockeye salmon
stocks can differ in their thermal tolerance, reflecting local adaptation to temperatures
experienced during historic migrations (Farrell et al. 2008; Farrell 2009; Lee et al. 2003). The
relationship between water temperature and spawning migration mortality can vary substantially
among stocks (Macdonald et al. 2010; Martins et al. 2011). Furthermore, recent studies have
revealed that female Fraser River sockeye salmon mortality is higher than that of males under
warm temperatures (Crossin et al. 2008; K. Jeffries, Department of Forest Sciences, University
of British Columbia, Ph.D. thesis in progress; E. Martins, Department of Forest Sciences,
University of British Columbia, unpub. data). This may occur because females which are near
fully mature have higher baseline stress levels than males at a similar level of maturation
(Hruska et al. 2010a; Roscoe et al. in press), so additional stressors experienced at that time
could have greater detrimental effects to females. Survival during the spawning migration can
substantially improve when sockeye salmon are able to behaviourally thermoregulate by making
use of thermal refuges, such as deep portions of lakes and cold tributaries (Mathes et al. 2010).
However, thermal refuges in the lower Fraser River are virtually absent, which limits the ability
of sockeye salmon from most stocks to behaviourally thermoregulate during the early portions of
their up-river migration (Donaldson et al. 2009).
Survival of adult migrating sockeye salmon in the Columbia River system was found to be
higher under high flow conditions, a finding that likely reflects the inverse relationship between
temperature and discharge (Naughton et al. 2005; Keefer et al. 2008). In the Fraser River, high
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