Data from: Habitat-mediated effects of diurnal and seasonal migration strategies on juvenile salmon survival

Behavioral decisions during periods of vulnerability to predation risk, such as migrations during the juvenile life-history stage, may strongly affect the probability of survival. Habitats through which animals migrate are heterogeneous, and risk-reducing behaviors may be more important in some habitats than others. Using biotelemetry data, diurnal and seasonal riverine migration patterns of >3800 juvenile salmon across four species, 12 watersheds, and five years were quantified to evaluate possible effects of migration timing on survival from lower river reaches to coastal waters. In small, clear rivers most salmon avoided migrating during daylight hours, and average survival of fish migrating at night (55%; 95% confidence limits 50–61%) was twice that of fish migrating in daylight (24%; CL 17-31%). Conversely, in the large, heavily silted Fraser River neither preference for nocturnal travel nor effects of diurnal timing on survival were observed. Early ocean survival was also influenced by the timing of ocean entry, but in opposite directions for fish from the Fraser River and smaller rivers. In the Fraser River, average survival for later migrants (69%; CL 60–77%) was nearly twice that of earlier migrants (38%; CL 33–44%), likely related to seasonal increases in river flow. In contrast, in smaller rivers, average survival for earlier migrants (70%; CL 65–74%) was 3-fold greater than survival for later migrants (19%; 95% CL 14–25%). Together these results demonstrate that timing decisions affecting survival of juvenile salmon during their migration are likely mediated by landscape characteristics that plausibly influence the risk of predation.在面临捕食风险的脆弱阶段——例如幼体生命史阶段的洄游过程中，动物的行为决策会对其存活概率产生显著影响。动物洄游所途经的生境具有异质性，因此风险规避行为在不同生境中的重要程度存在差异。本研究利用生物遥测（biotelemetry）数据，对4个物种、12个流域、5年间共计3800余条幼鲑的日间与季节河流洄游模式进行量化分析，以评估洄游时间对鲑鱼从下游河段向近岸海域洄游过程中存活率的潜在影响。 在小型清澈河流中，多数鲑鱼会规避日间洄游；夜间洄游个体的平均存活率为55%（95%置信区间50~61%），是日间洄游个体（24%；置信区间17~31%）的两倍。与之相反，在大型高浊度的弗雷泽河（Fraser River）中，既未观测到鲑鱼偏好夜间洄游的现象，也未发现日间洄游时间对存活率产生影响。 幼体入海初期存活率同样受入海时间的影响，但弗雷泽河与小型河流的鲑鱼呈现出完全相反的趋势：弗雷泽河的晚洄游个体平均存活率为69%（置信区间60~77%），几乎是早洄游个体（38%；置信区间33~44%）的两倍，这一现象可能与河流径流量的季节性升高有关；而在小型河流中，早洄游个体的平均存活率为70%（置信区间65~74%），是晚洄游个体（19%；95%置信区间14~25%）的3倍以上。 综合上述结果可知，影响幼鲑洄游期间存活率的洄游时间决策，很可能由景观特征所介导，而这些景观特征可合理地影响捕食风险水平。