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

尽管减少了捕捞努力并实施了保护和缓解举措，许多太平洋鲑鱼种群仍在经历持续和严重的下降。这种鲑鱼种群的长期减少在奇努克鲑鱼和红鲑鱼中尤为普遍，在整个范围内都观察到这种减少，但在不列颠哥伦比亚省南部尤其明显。在过去的20年里，不列颠哥伦比亚省南部鲑鱼种群的持续低回报率归因于生命早期不利的海洋条件，如气候变化、猎物数量的变化、捕食风险、疾病、栖息地退化或捕捞以外的人类压力因素。虽然太平洋鲑鱼数量下降的原因可能是多方面的，但人们普遍认为，持续不利的海洋条件和海洋生存能力差在这些下降中起了主要作用。在它们生命的海洋阶段，鲑鱼的死亡率通常超过90-95%。据认为，这种损失大部分发生在沿海环境中，这是由于入海时遭到大量捕食和/或由于冬季饥饿造成的。在这里，我们将采取多学科的方法来阐明海洋条件、生长、物候和生理性能对早期海洋幼鱼奇努克和红鲑鱼生存变异的相互作用。具体目标包括：1通过评估早期海洋生长和条件对幼年鲑鱼存活的作用，从耳石微观结构、长度频率分析和能量动态来量化早期生长和死亡率之间的联系；2/通过实验测试幼年鲑鱼在早期海洋生物中对气候和食物变化的反应；3/测试冬季严酷程度与生长、生存和捕食脆弱性之间的关系；和4/量化能量水平、应激指标、寄生虫负荷和多样性，并建立与应激条件有关的基因组图谱。我们的一般假设包括：1/鲑鱼进入海洋环境的时间、大小和生长早期的变化将与冬季前的大小和生存相关。提前进入可能与提前的气候变化导致的春季开花相匹配，从而提高生长速度和存活率。而且，生长更快的个体应该表现出更低的尺寸选择性死亡率。物候和生长之间相互作用的结果将取决于温度和食物供应等条件，我们将通过实验进行测试；2/脂质水平会随着体型的增加而增加，随温度和日粮的变化而变化，但一般在冬季下降；3/我们预测，由于冒险行为的差异，食物限制和/或低生长将增加对捕食的脆弱性；4/我们预测极端温度和低口粮会引起强烈反应。在这些条件下，生长和脂质积累减少，应激水平增加，寄生虫负荷和多样性增加，攻击行为更常见，对捕食的脆弱性增加。此外，我们预计那些编码与热和食物剥夺应激相关的蛋白质的基因，如热休克蛋白和生长激素受体，在不同的组织和治疗中会有所不同。我们的结果将提供所需的缺失信息，以评估何时何地早期海洋死亡率的作用，大小选择性的程度，以及负责观察到的死亡率的潜在机制。通过评估幼年鲑鱼早期海洋死亡的程度和原因，该项目将提高我们对海洋环境中限制太平洋鲑鱼产量的因素的理解。

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.