Predation
Predation contributes to Steelhead Trout mortality across all life stages in both freshwater and marine environments. Predation contributes to the mortality of Steelhead Trout and other salmon along the sea going migration routes of Thompson and Chilcotin Steelhead Trout (Thomas et al. 2017, Nelson et al. 2018). Berejikian et al. (2016) suggests that predation by Harbour Seals (Phoca vitulina) contributes to mortality of migrating juvenile Steelhead Trout off Washington State. Data are very limited, but Harbour seals have also been known to consume outmigrating smolts in one river estuary of the Strait of Georgia (Thomas et al. 2017). In other systems, seabirds have been identified as important predators of steelhead (Hostetter et al.
2011 and 2012); however this has not been assessed for the Fraser River system. Interestingly, susceptibility of steelhead smolts to bird predation has also been linked to condition and disease (Hostetter et al 2011 and 2012). However, there is a large degree of uncertainty in the mortality rate and level of impact of predators such as pinnipeds on Steelhead Trout. There is further uncertainty regarding the impact of pinnipeds vs other predators or factors which may also contribute to Steelhead Trout declines, in addition to the extrapolation of localized pinniped predation to a coast wide impact. Uncertainty also arises because it is not clear whether in fact predation by any predator is compensatory or additive; that is there are no other factors or predators which would contribute to mortality if a particular predator was removed. Moreover, marine survival rates for steelhead suggest large decreases in survival during periods of low pinniped abundance, and for some stocks, periods of increasing marine survival under high pinniped abundance (Kendall et al. 2017). Although the working paper demonstrated a strong negative correlation between pinniped population growth and steelhead population decline, there was not consensus that there is a causal relationship between the two. Similarly the recommendation for a pinniped cull in the working paper was not supported and requires further investigation. Furthermore, there is insufficient evidence fully understand the potential unintended ecosystem consequences of reducing pinniped predation. Predation also occurs on Steelhead eggs, alevins and fry during freshwater incubation and rearing. Fish predators likely include but are not limited to Bull Trout (Salvelinus confluentus), whitefish, Cutthroat Trout (Oncorhynchus clarkia), juvenile Coho (Oncorhynchus kisutch) and Chinook (Oncorhynchus tshawytscha), Prickly Sculpin (Cottus asper) and Northern Pikeminnow (Ptychocheilus oregonensis). In the Nicola system, Northern Pikeminnow is likely the top predator on fry and parr. The mortality rates associated with freshwater predation have not been estimated. Competition and reduced prey in the Ocean Reduced prey availability resulting from higher levels of competition can increase foraging times and lead to higher mortality. Ruggerone and Irvine (2018) show that competition for prey species on which Steelhead Trout depend has increased due to increases in naturally and hatchery-produced Pink, Chum, and Sockeye salmon. The RPA results also suggest that increased competition may be reducing marine survival rates for Thompson and Chilcotin DUs.
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