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

许多太平洋鲑鱼种群正在经历持续和严重的下降，尽管捕捞活动减少，并实施了养护和缓解举措。这种鲑鱼库存的长期下降在Chinook鲑鱼和Sockeye鲑鱼中特别普遍，在整个范围内都观察到了下降，但特别是在卑诗省南部。过去20年来，不列颠哥伦比亚省南部鲑鱼种群的收益率持续较低，归因于早期生命中不利的海洋条件，如气候变化、猎物可获得性的变化、捕食风险、疾病、栖息地退化或捕鱼以外的人类压力因素。虽然太平洋鲑鱼数量减少的原因可能是多因素的，但人们普遍认为，长期不利的海洋条件和糟糕的海洋生存状况是造成这些数量减少的主要原因。鲑鱼在其生命的海洋阶段的死亡率一般超过90%-95%。据认为，这种损失大部分发生在沿海环境中，原因要么是进入海洋时的大量捕食，要么是由于冬季饥饿。在这里，我们将采用多学科的方法来阐明海洋条件、生长、物候和生理表现对早期海洋幼体Chinook和红鲑鱼生存变化的交互影响。 具体目标包括：1/通过从耳石显微结构、长度频率分析和能量动力学评估早期海洋生长和条件对幼鲑鱼生存的作用，量化早期生长和死亡之间的联系；2/通过实验测试幼鱼如何对早期海洋生物的气候和食物变化做出反应；3/测试冬季的严酷程度和饥饿的复原力对生长、存活和捕食能力的影响；4/量化能量水平、应激指标、寄生虫负荷和多样性，并建立与应激条件相关的基因组谱。 我们的一般假设包括：1/鲑鱼进入海洋环境的时间、进入时的大小和生长的早期的变异性将与越冬前的大小和生存相关。更早的进入可能与气候变化导致的更早的春季开花相匹配，从而提高生长速度和更高的存活率。而且，生长较快的个体应该表现出较少的体型选择死亡率。物候与生长之间相互作用的结果将取决于温度和食物可获得性等条件，我们将对此进行实验测试；2/脂肪水平将随着身体大小而增加，随温度和口粮的变化而变化，但通常在整个冬季都会下降；3/我们预测，食物限制和/或低生长将因食物种类的不同而增加被捕食的可能性；4/我们预测，极端温度和低口粮是冒险行为；以及4/我们预测对极端温度和低口粮做出强烈反应。在这些条件下，应该减少生长和脂肪积累，增加应激水平，增加寄生虫的负载和多样性，更常见的攻击行为，并增加对捕食的脆弱性。此外，我们预计那些编码与热和食物剥夺压力相关的蛋白质的基因，如热休克蛋白和生长激素受体，将在不同的组织和治疗中有所不同。 我们的结果将提供评估早期海洋死亡发生的时间和地点、大小选择性的程度以及造成观察到的死亡的潜在机制所需的缺失信息。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了解。

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.