Seasonal spatio-temporal variation in habitat quality and abiotic conditions leads to animals migrating between different environments around the world. Whereas mean population timing of migration is often fairly well understood, explanations for variation in migratory timing within populations are often lacking. Condition-dependent tradeoffs may be an understudied mechanism that can explain this differential migration. While fixed condition-specific thresholds have been identified in earlier work on ontogenetic niche shifts, they are rare in differential migration, suggesting that thresholds in such systems can shift based on temporally variable environmental conditions. We introduced a model based on size-specific tradeoffs between migration and growth in seasonal environments. We focused on optimal migratory timing for first-time migrants with no knowledge of an alternative habitat, which is a crucial stage in the life history of migratory salmonids. We predicted that optimal timing would occur when individuals move from their natal habitats based on a seasonally variable ratio of predation and growth. When the ratio becomes slightly more favorable in the alternative habitat, migratory movement can occur. As it keeps shifting throughout the season, the threshold for migration is variable, allowing smaller individuals to move at later dates. We compared our model predictions to empirical data on 3 years of migratory movement of more than 800 juvenile trout of varying size from natal to feeding habitat. Both our model and empirical data showed that large individuals, which are assumed to have a lower predation risk in the migratory habitat, move earlier in the season than smaller individuals, whose predicted predation-to-growth ratio shifted to being favorable only later in the migratory season. Our model also predicted that the observed difference in migratory timing between large and small migrants occurred most often at low values of growth differential between the two habitats, suggesting that it was not merely high growth potential but rather the tradeoff between predation and growth that shaped differential migration patterns. We showed the importance of considering condition-specific tradeoffs for understanding temporal population dynamics in spatially structured landscapes. Rather than assuming a fixed threshold, which appears to be absent based on previous work on salmonids, we showed that the body-size threshold for migration changed temporally throughout the season. This allowed increasingly smaller individuals to migrate when growth conditions peaked in the migratory habitat. Our model illuminates an understudied aspect of predation as part of a condition-dependent tradeoff that shapes migratory patterns, and our empirical data back patterns predicted by this model.

中文翻译：

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季节性淡水环境中依赖于大小的权衡促进了鲑鱼的差异迁移

栖息地质量和非生物条件的季节性时空变化导致动物在世界各地不同环境之间迁移。尽管人们通常对平均人口迁移时间相当了解，但通常缺乏对人口内迁移时间变化的解释。依赖于条件的权衡可能是一种未被充分研究的机制，可以解释这种差异迁移。虽然在早期关于个体发育生态位变化的工作中已经确定了固定的条件特定阈值，但它们在差异迁移中很少见，这表明此类系统中的阈值可以根据时间变化的环境条件而变化。我们引入了一个模型，该模型基于季节性环境中迁移和增长之间的特定规模权衡。我们专注于不了解替代栖息地的首次移民的最佳迁徙时间，这是迁徙鲑鱼生活史中的关键阶段。我们预测，当个体根据捕食和生长的季节性变化比率从出生地迁出时，最佳时机将会出现。当替代栖息地的比率变得稍微有利时，就会发生迁徙运动。由于它在整个季节不断变化，迁移的门槛是可变的，允许较小的个体在以后移动。我们将我们的模型预测与 800 多条不同大小的幼鳟鱼从出生到觅食栖息地 3 年迁徙运动的经验数据进行了比较。我们的模型和经验数据都表明，大型个体，被认为在迁徙栖息地具有较低捕食风险的个体在季节中比较小的个体移动得更早，其预测的捕食与生长比率仅在迁徙季节后期才转变为有利的。我们的模型还预测，观察到的大型和小型迁徙者迁徙时间的差异最常发生在两个栖息地之间的低增长差异值，这表明形成差异的不仅仅是高增长潜力，而是捕食和增长之间的权衡迁移模式。我们展示了考虑特定条件权衡对于理解空间结构化景观中的时间人口动态的重要性。而不是假设一个固定的阈值，根据以前对鲑鱼的研究似乎不存在这个阈值，我们发现迁移的体型阈值在整个季节随时间变化。当迁移栖息地的生长条件达到顶峰时，这使得越来越小的个体迁移。我们的模型阐明了捕食的一个未被充分研究的方面，作为形成迁徙模式的条件相关权衡的一部分，我们的经验数据支持该模型预测的模式。