In many populations of Atlantic salmon, individuals return to rivers months or even a year prior to spawning. This premature arrival in fresh water by far exceeds the time needed for upstream migrations to spawning areas. Leaving the ocean early means forgoing substantial growth and incurring high metabolic costs of fasting in freshwater. Differences in timing of return migration has been shown to have a genetic basis, and different sea age classes return to rivers at different times. Previously suggested explanations regarding the cause for this behaviour have included both environmental factors and life history trade-offs, but these hypotheses have not been tested. Here, we combine data on marine growth and mortality of Atlantic salmon with the calculations of energetic expenditures of fasting in freshwater to explore how timing of river entry affects the probability of survival and energy recourses available for spawning. Our results show that to maximize the product of survival until spawning and energy available at spawning, multi sea winter fish in general must enter rivers earlier than one sea winter fish. And that for a large range of sea survival rates, the optimum river entry date for multi sea winter fish is as early as possible. This strongly suggests that the phenomena of premature arrival in Atlantic salmon is adaptive behaviour.

中文翻译：

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大西洋鲑鱼回程的最佳时机

在许多大西洋鲑鱼种群中，个体在产卵前几个月甚至一年返回河流。这种过早到达淡水的时间远远超过了上游迁徙到产卵区所需的时间。过早离开海洋意味着放弃实质性的生长，并导致在淡水中禁食的高代谢成本。返回迁徙时间的差异已被证明具有遗传基础，不同的海龄级别返回河流的时间也不同。先前提出的关于这种行为原因的解释包括环境因素和生活史权衡，但这些假设尚未得到检验。在这里，我们将大西洋鲑鱼的海洋生长和死亡率数据与淡水禁食能量消耗的计算结合起来，探索入河时间如何影响生存概率和产卵可用的能量资源。我们的结果表明，为了最大限度地提高产卵前的生存率和产卵时可用的能量，多海冬鱼通常必须比单海冬鱼更早进入河流。而对于大范围的海洋成活率来说，多海冬鱼的最佳入河日期越早越好。这强烈表明大西洋鲑鱼过早抵达的现象是适应性行为。