Data from: Effective size of a wild salmonid population is greatly reduced by hatchery supplementation

Many declining and commercially important populations are supplemented with captive-born individuals that are intentionally released into the wild. These supplementation programs often create large numbers of offspring from relatively few breeding adults, which can have substantial population-level effects. We examined the genetic effects of supplementation on a wild population of steelhead trout (Oncorhynchus mykiss) from the Hood River, Oregon, by matching 12 run-years of hatchery steelhead back to their broodstock parents. We show that the effective number of breeders producing the hatchery fish (broodstock parents; Nb) was quite small (harmonic mean Nb=25 fish per brood-year vs. 373 for wild fish), and was exacerbated by a high variance in broodstock reproductive success among individuals within years. The low Nb caused hatchery fish to have decreased allelic richness, increased average relatedness, more loci in linkage disequilibrium, and substantial leve ls of genetic drift in comparison to their wild-born counterparts. We also documented a substantial Ryman-Laikre effect whereby the additional hatchery fish doubled the total number of adult fish on the spawning grounds each year, but cut the effective population size of the total population (wild and hatchery fish combined) by two thirds. We demonstrate that, in this system, the Ryman-Laikre effect is most severe when (1) more than 10% of fish allowed onto spawning grounds are from hatcheries and (2) hatchery fish have high reproductive success in the wild. These results emphasize the tradeoffs that arise when supplementation programs attempt to balance disparate goals (increasing production while maintaining genetic diversity and fitness).

许多处于衰退状态且具有重要商业价值的硬头鳟种群，会通过人工繁育个体的人为野放行为得到种群补充。这类种群补充项目通常仅依托少量繁育亲本即可产生大量子代，进而对种群整体产生显著影响。我们通过将12个洄游年份的孵化场繁育硬头鳟（steelhead trout，*Oncorhynchus mykiss*）与其繁育亲本组进行匹配，研究了种群补充对美国俄勒冈州胡德河野生硬头鳟种群的遗传效应。研究表明，孵化场鱼苗对应的繁育亲本（即亲鱼群体；繁育有效个体数（effective number of breeders, Nb））的规模极小——每繁育年度的调和平均Nb为25尾，而野生种群的调和平均Nb为373尾，且由于同一年度内不同亲鱼个体的繁殖成功率差异极大，这一问题进一步加剧。繁育有效个体数偏低导致孵化场鱼苗相较于野生个体，出现了等位基因丰富度下降、平均亲缘关系升高、更多位点处于连锁不平衡（linkage disequilibrium, LD）状态，以及显著的遗传漂变现象。我们还观测到了显著的莱曼-莱克尔效应（Ryman-Laikre effect）：每年新增的孵化场鱼苗使产卵场的成年鱼类总数翻倍，但却将种群总有效规模（野生与孵化场个体合计）降低了三分之二。本研究证实，在该研究系统中，莱曼-莱克尔效应在以下两种情况下最为显著：（1）产卵场中超过10%的个体来自孵化场；（2）孵化场个体在野外具备较高的繁殖成功率。上述研究结果凸显了种群补充项目在平衡多重目标时所面临的权衡取舍：既要提升种群产量，又要维持遗传多样性与适合度。