Variation in life history traits within and across species is known to reflect adaptations to different environmental drivers through a diversity of mechanisms. Trait variation can also help buffer species and populations against extinction in fluctuating environments and against anthropogenic disturbances. Here, we examine the distribution and drivers of Ocean‐type Chinook salmon (Oncorhynchus tschawytscha) juvenile migratory life histories. We defined alternative migratory strategies according to whether individuals reared in the stream (natal rearing) or left shortly after hatching to rear elsewhere (non‐natal rearing). We then evaluated the frequency of migratory strategies across 16 populations with time series extending up to 25 years and evaluated the environmental variables that influenced variation in migration strategy. We found bimodal migration patterns and abrupt transitions in migrant sizes across all populations, supporting the widespread nature of alternative migratory strategies. Additionally, we found that the amount of freshwater rearing habitat available to juveniles, relative juvenile density and spring flow patterns significantly influenced the overall migration pattern for populations. Smaller streams and higher conspecific densities generally produced more non‐natal rearing migrants and larger streams and lower conspecific densities producing more natal rearing migrants. Our results shed light on previously unexplored patterns of juvenile migratory strategies and encourage broader consideration for how current conservation actions perform at protecting juvenile migratory diversity.

中文翻译：

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替代洄游策略在亚年幼的奇努克鲑鱼中很普遍

已知物种内和物种间生活史特征的变化通过多种机制反映对不同环境驱动因素的适应。性状变异还有助于缓冲物种和种群在波动环境中的灭绝和人为干扰。在这里，我们研究了海洋型奇努克鲑鱼 (Oncorhynchus tschawytscha) 幼年迁徙生活史的分布和驱动因素。我们根据个体是在溪流中饲养（出生饲养）还是孵化后不久离开到别处饲养（非出生饲养）来定义替代迁移策略。然后，我们评估了时间序列长达 25 年的 16 个种群的迁移策略频率，并评估了影响迁移策略变化的环境变量。我们发现所有人口中的双峰迁移模式和迁移规模的突然转变，支持替代迁移策略的广泛性质。此外，我们发现幼鱼可用的淡水饲养栖息地数量、相对幼鱼密度和春季流动模式显着影响了种群的整体迁移模式。较小的溪流和较高的同种密度通常会产生更多的非出生抚养移民，而较大的溪流和较低的同种密度产生更多的出生抚养移民。我们的结果揭示了以前未探索的幼鸟迁徙策略模式，并鼓励更广泛地考虑当前的保护行动如何在保护幼鸟迁徙多样性方面发挥作用。