Genetic divergence and diversity reflect a predominant freshwater resident life history in Rainbow Trout from southwestern Alaska

Rainbow Trout Oncorhynchus mykiss in southwestern Alaska occupy coastal watersheds near the northern boundary of the species native range and support a world class wild trout sport fishery. Although low freshwater temperatures and a short growing season in this region may favor anadromy, these populations appear to exhibit a freshwater resident life history strategy. In this study we used genetic data to evaluate two hypotheses regarding the influence of the presumed migratory behavior of these Rainbow Trout on reproductive isolation among and within watersheds. The results were largely consistent with the predictions but there were exceptions. The data supported the hypothesis that the freshwater resident behavior precludes marine-mediated gene flow resulting in large genetic divergence and low admixture among watersheds. The estimate of FCT (among-watershed differentiation, 0.350) was large and reflected over 96% of the variation among all sampled aggregations (FST = 0.363). However, evidence of admixed individuals in two adjacent watersheds and five first generation migrants among five watersheds suggests that the potential for coastal migration with gene flow exists in these populations. The data also supported the hypothesis that aggregations formed within watersheds during the spawning period (May-June) represent reproductively isolated populations. The pairwise estimates of FST and the G-test results revealed population structure in four of the six watersheds tested. However, not all aggregation pairs were found genetically distinct and there was notable variation in the pairwise FST estimates (0.000 – 0.067). In summary, the data reflected the predicted results for each hypothesis, but also revealed exceptions that, consistent with tagging studies, demonstrate the complexity of migratory behavior in southwestern Alaska Rainbow Trout. We discuss the implications of these results for fishery management and conservation.

栖息于阿拉斯加西南部的虹鳟（Rainbow Trout，学名*Oncorhynchus mykiss*）分布于该物种自然分布范围北界附近的沿海流域，支撑着世界级野生鳟鱼休闲渔业。尽管该区域较低的淡水水温与较短的生长季可能有利于溯河洄游（anadromy）习性的形成，但这些种群却表现出淡水定居型生活史策略。本研究利用遗传数据，评估了两项关于该虹鳟推测的洄游行为对流域间及流域内生殖隔离影响的假说。研究结果大体符合假说预测，但亦存在例外情况。数据支持如下假说：淡水定居习性阻断了海洋介导的基因流（marine-mediated gene flow），进而造成流域间遗传分化显著、种群间基因混合（admixture）程度较低。流域间分化指数（FCT）的估算值为0.350，数值较高，且其解释度覆盖了所有采样集群间总变异的96%以上（群体分化指数FST=0.363）。然而，在两个相邻流域中发现了基因混合个体，且在5个流域中检测到5个第一代迁移个体（first generation migrants），这表明该种群存在沿海洄游并伴随基因流的可能性。数据同时支持另一假说：在产卵期（5-6月）于流域内形成的集群，代表了生殖隔离的种群。两两FST估算值与G检验（G-test）结果显示，在受试的6个流域中，有4个流域存在种群遗传结构（population structure）。但并非所有集群对均表现出遗传分化，且两两FST估算值存在显著差异，区间为0.000~0.067。综上，数据验证了两项假说的预测结果，但同时也揭示了例外情况；与标记放流研究的结论一致，这表明阿拉斯加西南部虹鳟的洄游行为具有复杂性。本研究最后讨论了上述结果对渔业管理与物种保护的启示。