Numerous factors such as predation, disease, injury, and environmental conditions (e.g., river flows, hydropower operations) can influence survival rates of fish. Although mortality due to predation is commonly assumed to be additive and result in a directly proportional reduction on survival rates, compensatory processes may work to counteract or negate the effects of predation mortality on survival rates. We applied a random effects model to a long‐term, mark‐recapture‐recovery data set on anadromous steelhead (Oncorhynchus mykiss) from the Snake River Basin in the northwestern United States to assess whether avian predation mortality constitutes an additive or compensatory source of mortality. Specifically, our assessment focused on predation mortality due to double‐crested cormorants (Phalacrocorax auritus) and Caspian terns (Hydroprogne caspia) on colonies in the Columbia River estuary. In addition, we evaluated several candidate environmental indices to examine potential interactions between the effects of predation versus environmental conditions on steelhead survival rates. Average predation rates were 3.3% for the double‐crested cormorant colony and 17.0% for the Caspian tern colony. For both colonies, the estimated correlation between the predation rate and survival rate of steelhead was near zero, indicating that mortality due to avian predation is compensatory. Models that included variables for river flow, juvenile migration timing, and an index of forage biomass in the ocean accounted for 56–59% of the variation in steelhead survival, whereas avian predation rates accounted for <1% of the variation. Management efforts to reduce the abundance of the bird colonies are unlikely to improve the survival or conservation status of steelhead; however, results indicate that steelhead survival could be improved by hydropower management decisions that increase river flows and reduce juvenile migration delays. © 2020 The Wildlife Society.

中文翻译：

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斯蒂德黑德的鸟类捕食与补偿性死亡率一致

诸如捕食，疾病，伤害和环境条件（例如，河流流量，水力发电作业）之类的许多因素都会影响鱼类的成活率。尽管通常认为捕食造成的死亡率是可加的，并直接导致成活率成比例地降低，但补偿程序可能会抵消或消除捕食死亡率对成活率的影响。我们将随机效应模型应用于来自美国西北部Snake河盆地的顽固性黑头（Oncorhynchus mykiss）的长期标记回收数据，以评估禽肉捕食死亡率是构成死亡率的累加还是补偿性来源。 。具体来说，我们的评估重点是由于双冠cor造成的捕食死亡率（ala（ala）和里海燕鸥（Hydroprogne caspia））在哥伦比亚河河口的殖民地上。此外，我们评估了几种候选环境指数，以研究捕食与环境条件之间的相互作用对杆头存活率的潜在影响。双冠cor殖民地的平均捕食率为3.3％，里海燕鸥殖民地的平均捕食率为17.0％。对于这两个殖民地，估计的掠食率与硬皮鱼的存活率之间的相关性都接近于零，这表明禽类捕食引起的死亡率是补偿性的。包括河流流量，青少年迁徙时机和海洋中饲草生物量指标在内的模型占硬皮动物存活率变化的56–59％，而鸟类捕食率占变化率的<1％。减少鸟群数量的管理工作不太可能改善硬头鱼的存活或保护状况；但是，结果表明，通过增加水流流量并减少少年移民延误的水电管理决策，可以提高硬头鱼的生存。©2020野生动物协会。