Facultative migration occurs when, in response to prevailing conditions, individuals in a population may (or may not) undertake a migration. The brown trout (Salmo trutta) is a species that exhibits facultative migration, where some individuals within populations may move to mainstem rivers (fluvial–adfluvial migration), lakes (lacustrine–adfluvial migration), estuaries (partial anadromy) or sea (anadromy) to feed, while others remain resident. This study attempts to separate two alternative hypotheses for the population structuring that underpins the expression of facultative migration in this species: (a) that anadromous and nonanadromous fish comprise two gene pools; (b) that individual genetic variation or individual variation in gene–environment interactions is responsible for the expression of different life‐history tactics within the same gene pool. The study design involved sampling and analyses of anadromous and nonanadromous brown trout from three independent tributary rivers known to produce (sea‐run) trout within the same catchment. Results indicate that, in all cases, population genetic divergence was linked to geographical location and not to life‐history tactics. Two genetically distinct coexisting population pairs were identified in two separate tributaries. Despite similar environmental conditions in both tributaries, the frequency of each life‐history tactic (anadromy vs. nonanadromous) within these population pairs differed significantly. The results of this study support the hypothesis that facultative migration in brown trout is likely to be driven by a quantitative threshold trait, where the threshold value varies both among populations and among individuals within populations.

中文翻译：

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跨越鳟鱼生活史策略和较小空间尺度的遗传结构（Salmo trutta）

当响应于当前的情况，人口中的某个人可能（或可能不）进行迁移时，就会发生兼职迁移。褐鳟（Salmo trutta）是一种表现出兼性迁徙的物种，种群中的某些个体可能会迁移到主河（河流-河流迁移），湖泊（湖水-河流迁移），河口（部分厌氧）或海洋（厌氧）觅食，而其他人则仍然存在居民。这项研究试图为支撑该物种兼性迁移表达的种群结构分离两个备选假说：（a）无性和非无性鱼类包括两个基因库；（b）个体遗传变异或基因与环境相互作用的个体变异负责同一基因库中不同生命史策略的表达。该研究设计涉及对来自三个独立支流河的已知和非厌氧性褐鳟的采样和分析，已知这三个支流在同一流域内生产（海产）鳟鱼。结果表明，在所有情况下，人口遗传差异都与地理位置有关，与生活史策略无关。在两个独立的支流中确定了两个遗传上不同的共存种群对。尽管两个支流的环境条件相似，但这些人口对中每种生活史战术（厌食与非厌食）的发生频率都存在显着差异。这项研究的结果支持以下假设：褐鳟的兼职迁移可能是由定量阈值特征驱动的，该阈值在不同种群之间以及不同种群中的个体之间都存在差异。