Data from: Adoption of alternative migratory tactics: a view from the ultimate mechanism and threshold trait changes in a salmonid fish

Partial migration, in which a portion of the population migrates while the rest of the population remains as residents, is a common form of migration. Alternative migratory tactics (AMTs) of partial migration are often determined by polygenic threshold traits. However, the ultimate mechanisms that drive inter-population variations in threshold traits are not well understood. We present a simple schematic model to explain how the threshold trait changes with fitness consequences under opposing natural and artificial selection forces. We conducted a field test to evaluate the effects of migration difficulty (as a natural selective force) and selective captive breeding (as an artificial selective force) on threshold traits of a partially migratory fish. Male masu salmon (<i>Oncorhynchus masou</i>) in the Shari River system have AMTs divided into three population categories of hatchery, wild/above the waterfall, and wild/below the waterfall (control). The wild/above the waterfall salmon live in a high-migration-cost situation, and the threshold trait changed in a direction that promoted residency. In hatchery salmon, which are produced by migrant-selective captive breeding, the threshold trait changed in a direction that promoted migration. In contrast, Dolly Varden charr (<i>Salvelinus malma</i>) displayed only resident tactics, and the threshold trait did not differ between the populations above and below the waterfall, indicating that environment did not explain the variation in the threshold trait. Our results support the model and suggest that opposing natural and artificial selection forces drive variations in the threshold traits and migratory patterns in the studied species. Our conceptual framework for the ultimate mechanism may help to better understand adoption of AMTs and production of diverse intraspecific traits in migratory animals.

部分迁徙（partial migration）是一种常见的迁徙形式，指种群内一部分个体发生迁徙，其余个体则以居留者身份留存原地。部分迁徙的替代性迁徙策略（Alternative Migratory Tactics，AMTs）通常由多基因阈值性状（polygenic threshold traits）决定。然而，驱动阈值性状种群间差异的终极机制仍未得到充分阐释。本研究提出一个简易图解模型，用以阐释在自然选择与人工选择这两股对立选择压力下，阈值性状如何随适合度效应发生改变。我们开展了一项野外实验，以评估迁徙难度（作为自然选择压力）与选择性人工繁育（作为人工选择压力）对一种部分迁徙鱼类阈值性状的影响。沙里河水系的雄性樱鳟（Oncorhynchus masou）存在替代性迁徙策略，其种群可分为三类：繁育场种群、瀑布上游野生种群以及瀑布下游野生种群（对照组）。瀑布上游的野生种群处于高迁徙成本的生境中，其阈值性状朝促进居留的方向发生改变。而由选择性繁育迁徙个体得到的繁育场种群，其阈值性状则朝促进迁徙的方向发生改变。与之相对，花羔红点鲑（Salvelinus malma）仅表现出居留策略，且瀑布上下游种群的阈值性状并无差异，这表明环境因素无法解释阈值性状的变异。本研究结果验证了所提出的模型，并表明自然选择与人工选择的对立压力驱动了研究物种的阈值性状与迁徙模式的变异。本研究提出的终极机制概念框架，或有助于更好地理解迁徙动物的替代性迁徙策略采用方式，以及种内多样性状的产生机制。