Identifying mechanisms leading to size-selective mortality in the early marine phase of juvenile Pacific salmon

点击翻译按钮获取中文摘要

Many stocks of Pacific salmon are experiencing continued and severe declines despite reductions in fishing effort and the implementation of conservation and mitigation initiatives. This prolonged decline of salmon stocks is particularly prevalent in Chinook salmon and sockeye salmon, where declines have been observed range-wide, but especially in southern BC. The sustained low return rates of southern British Columbia salmon stocks over the last 20 years have been attributed to a combination of unfavorable marine conditions during early life such as climate change, variability in prey availability, predation risk, disease, habitat degradation, or human stressors other than fishing. While the causes of Pacific salmon declines are likely multifactorial, it is generally recognized that persistently unfavourable ocean conditions and poor marine survival played a major role in these declines. Salmon mortality rates generally exceed 90-95% during the marine stages of their life. Much of this loss is thought to occur in coastal environments as a consequence of either heavy predation upon ocean entry and/or due to winter starvation. Here, we will take a multidisciplinary approach to elucidate the interacting effects of ocean conditions, growth, phenology, and physiological performance on variation in survival of early ocean juvenile Chinook and sockeye salmon. Specific objectives include: 1/ Quantifying the links between early growth and mortality by assessing the role of early marine growth and condition to survival of juvenile salmon from otolith microstructure, length frequency analyses, and energy dynamics; 2/ Experimentally testing how juvenile salmon respond to climate and food variability during early marine life; 3/ Testing the relationship between winter severity and resiliency to starvation on growth, survival, and vulnerability to predation as a function of size; and 4/ Quantifying energetic levels, stress indicators, parasite loads and diversity, and building genomic profiles related to stress conditions. Our general hypotheses include: 1/ Variability in the timing, size at entry, and early of growth of salmon entering the marine environment will be correlated to size before winter and survival. Earlier entry may match earlier climate-change induced spring blooms leading to increased growth rates and higher survival. And, faster growing individuals should exhibit less size-selective mortality. The outcome of the interplay between phenology and growth will depend on conditions such as temperature and food availability, which we will test experimentally; 2/ Lipid levels will increase with body size, vary with temperature and ration, but generally decline through the winter; 3/ We predict that food limitation and/or low growth will increase vulnerability to predation due to differences is risk-taking behaviour; and 4/ We predict strong responses to temperature extremes and low food rations. Under these conditions growth and lipid accumulation should be reduced, stress levels increased, parasite loads and diversity elevated, aggressive behavior more common, and vulnerability to predation increased. In addition we expect that those genes coding for proteins related to thermal and food deprivation stress such as heat shock proteins and growth hormone receptors will vary across tissues and treatments. Our results will provide missing information needed to evaluate when and where early marine mortality is acting, the extent of size selectivity, and the potential mechanisms responsible for the observed mortality. By assessing the extent and causes of early marine mortality in juvenile salmon, this project will improve our understanding of the factors limiting the production of Pacific salmon in the marine environment.