Data from: Reconstructing the migratory behavior and long-term survivorship of juvenile Chinook salmon under contrasting hydrologic regimes

The loss of genetic and life history diversity has been documented across many taxonomic groups, and is considered a leading cause of increased extinction risk. Juvenile salmon leave their natal rivers at different sizes, ages and times of the year, and it is thought that this life history variation contributes to their population sustainability, and is thus central to many recovery efforts. However, in order to preserve and restore diversity in life history traits, it is necessary to first understand how environmental factors affect their expression and success. We used otolith 87Sr/86Sr in adult Chinook salmon (Oncorhynchus tshawytcha) returning to the Stanislaus River in the California Central Valley (USA) to reconstruct the sizes at which they outmigrated as juveniles in a wetter (2000) and drier (2003) year. We compared rotary screw trap-derived estimates of outmigrant timing, abundance and size with those reconstructed in the adults from the same cohort. This allowed us to estimate the relative survival and contribution of migratory phenotypes (fry, parr, smolts) to the adult spawning population under different flow regimes. Juvenile abundance and outmigration behavior varied with hydroclimatic regime, while downstream survival appeared to be driven by size- and time-selective mortality. Although fry survival is generally assumed to be negligible in this system, >20% of the adult spawners from outmigration year 2000 had outmigrated as fry. In both years, all three phenotypes contributed to the spawning population, however their relative proportions differed, reflecting greater fry contributions in the wetter year (23% vs. 10%) and greater smolt contributions in the drier year (13% vs. 44%). These data demonstrate that the expression and success of migratory phenotypes vary with hydrologic regime, emphasizing the importance of maintaining diversity in a changing climate.

遗传与生活史多样性的丧失已在多个分类群中被证实，被认为是物种灭绝风险升高的首要诱因。幼鲑会以不同的体型、年龄与季节离开其出生河流，学界普遍认为这类生活史变异有助于维持种群可持续性，因此也是诸多种群恢复工作的核心内容。然而，若要保护并恢复生活史性状多样性，首先需要明晰环境因子如何影响这些性状的表达与个体存活成功率。本研究利用返回美国加利福尼亚中央谷斯坦尼斯劳斯河的成年奇努克鲑（Oncorhynchus tshawytcha）的耳石（otolith）87Sr/86Sr比值，重建了其在湿润年（2000年）与干旱年（2003年）作为幼体时的降海洄游体型。我们将旋转式螺旋诱捕器获取的降海洄游个体的出现时间、丰度与体型估算值，与同生群成年个体重建的对应数据进行了对比，借此得以估算不同水文情势下，各类洄游表型（仔鱼fry、稚鱼parr、降海鲑smolts）的相对存活率及其对成年产卵种群的贡献比例。幼体丰度与降海洄游行为随水文气候情势发生变化，而下游存活情况似乎受体型与时间选择性死亡所调控。尽管该系统中通常认为仔鱼存活率可忽略不计，但2000年降海洄游的成年产卵个体中，有超过20%是以仔鱼阶段完成降海洄游的。两个年份中，三类洄游表型均对产卵种群有所贡献，但其相对占比存在差异：湿润年仔鱼贡献占比更高（23% vs 10%），干旱年则以降海鲑贡献占比更高（13% vs 44%）。本研究数据表明，洄游表型的表达与存活成功率随水文情势发生变化，这凸显了在气候变化背景下维持生物多样性的重要性。