1. Migration enables animals to access important resources throughout their lifetime but exists in a trade-off with elevated mortality risk. In spring, juvenile Atlantic salmon (smolts) migrate from their natal rivers for marine feeding grounds, with the timing of their marine entry a potentially important determinant of their long-term survival. However, there is relatively little known on how the interaction of biological and environmental factors affect smolt migration phenology at the individual level, and how these vary throughout the duration of the smolt seaward migration (run).
2. Using 15-year tag, recapture, and detection datasets of individual smolts (marked with passive integrated transponder tags) from a chalk stream in southern England, the influences of a range of biological and environmental variables were tested on the run timing of individual smolts, measured as the timing of their arrival in a lower river reach.
3. The probability of smolts arriving earlier in the lower river reach was elevated following winters that were relatively warm, and when there were larger positive daily changes in water temperature and discharge during the run. Early migrants tended to be larger individuals and from sites lower in the catchment, from where the smolts had to migrate relatively shorter distances. Later migrants were more likely to migrate in schools, but with schooling behaviour also more likely to occur during daylight than at night.
4. The relative influence of some of these variables altered throughout the run. Relative changes in daily water temperature were not important during the middle period of the smolt run but were important at the start and end of the run. Relative changes in daily discharge were most influential towards the end of the run, when even relatively small changes in discharge had a strong influence on migration.
5. These results reveal the importance of a wide range of biological and environmental variables on the phenology of smolt migrations, and how their influence can alter throughout the run. With predictions of annually increasing river temperatures, more frequent and intense discharge events, and associated shifts to earlier migration, these results emphasise that such changes in climate are likely to have substantial consequences on the future success of smolt migrations and thereby future numbers of returning adult spawners.

中文翻译：

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生物和环境对英格兰南部白垩溪流中大西洋鲑鲑鱼的迁移物候的影响

1. 迁徙使动物能够在其一生中获得重要的资源，但存在与死亡风险升高的权衡。春天，幼年大西洋鲑鱼（小鲑鱼）从它们出生的河流迁移到海洋觅食地，它们进入海洋的时间是它们长期生存的潜在重要决定因素。然而，关于生物和环境因素的相互作用如何在个体层面影响黑鲱迁移物候，以及这些在鲱鱼向海迁移的整个过程中如何变化（运行），人们知之甚少。
2. 使用来自英格兰南部粉笔流的个体小鲑鱼（用被动集成转发器标签标记）的 15 年标记、重新捕获和检测数据集，测试了一系列生物和环境变量对个体小鲑鱼的运行时间的影响，以它们到达下游河段的时间来衡量。
3. 在相对温暖的冬季之后，以及在运行期间水温和流量的每日正变化较大时，小河豚较早到达下游河段的可能性增加。早期的迁徙者往往是较大的个体，并且来自流域较低的地点，小马科动物必须从那里迁徙相对较短的距离。较晚的移民更有可能在学校迁移，但在白天的上学行为也比夜间更有可能发生。
4. 在整个运行过程中，其中一些变量的相对影响发生了变化。每日水温的相对变化在幼鱼运行中期并不重要，但在运行开始和结束时很重要。日流量的相对变化在运行结束时影响最大，即使流量相对较小的变化也会对迁移产生强烈影响。
5. 这些结果揭示了范围广泛的生物和环境变量对黑鲑迁移物候的重要性，以及它们的影响如何在整个运行过程中发生变化。随着对河流温度逐年升高、更频繁和更强烈的排放事件以及向更早迁移的相关转变的预测，这些结果强调，气候的此类变化可能对黑鲭迁移的未来成功产生重大影响，从而对未来返回的成虫数量产生重大影响。产卵者。