Shedd et al. (2022) - Evolutionary Applications - Reduced relative fitness in hatchery-origin Pink Salmon in two streams in Prince William Sound, Alaska in 2013-2016

Previous studies generally report that hatchery-origin Pacific Salmon (Oncorhynchus spp.) have lower relative reproductive success (RRS) than their natural-origin counterparts. We estimated the RRS of Pink Salmon (O. gorbuscha) in Prince William Sound, Alaska (PWS) using incomplete pedigrees. In contrast to other RRS studies, Pink Salmon have a short freshwater life history, freshwater habitats in PWS are largely unaltered by development, and sampling was conducted without the aid of dams or weirs resulting in incomplete sampling of spawning individuals. Pink Salmon released from large-scale hatchery programs in PWS have interacted with wild populations for more than 15 generations. Hatchery populations were established from PWS populations but have subsequently been managed as separate broodstocks. Gene flow is primarily directional, from hatchery strays to wild populations. We used genetic-based parentage analysis to estimate the RRS of a single generation of stray hatchery-origin Pink Salmon in two streams, and across the odd- and even-year lineages. Despite incomplete sampling, we assigned 1,745 offspring to at least one parent. Reproductive success (RS), measured as sampled adult offspring that returned to their natal stream, was significantly lower for hatchery- versus natural-origin parents in both lineages, with RRS ranging from 0.03 to 0.47 for females and 0.05 to 0.86 for males. Generalized linear modeling for the even-year lineage indicated that RRS was lower for hatchery-origin fish, ranging from 0.42 to 0.60, after accounting for sample date (run timing), sample location within the stream, and fish length. Our results strongly suggest that hatchery-origin strays have lower fitness in the wild. The consequences of reduced RRS on wild productivity depend on whether the mechanisms underlying reduced RRS are environmentally driven, and likely ephemeral, or genetically driven, and likely persistent across generations.

既往研究普遍认为，孵化场来源的太平洋鲑（Oncorhynchus spp.）相较于野生种群，其相对繁殖成功率（relative reproductive success, RRS）更低。本研究针对阿拉斯加威廉王子湾（Prince William Sound, PWS）的粉鲑（O. gorbuscha），采用不完全谱系数据估算其RRS。与其他RRS相关研究不同，粉鲑的淡水生活史较短，且威廉王子湾的淡水栖息地基本未受开发活动干扰；同时本次采样未借助水坝或堰网，导致产卵个体的采样并不完整。阿拉斯加威廉王子湾的大规模孵化场繁育项目所产出的粉鲑，已与野生种群共存超过15个世代。孵化场种群最初源自本地种群，但后续已作为独立亲鱼种群进行管理。基因流主要呈单向性：从孵化场逃逸个体流向野生种群。本研究通过基于遗传的亲权分析，估算了两个溪流中以及奇偶年类群内，单世代孵化场来源的逃逸粉鲑的RRS。尽管采样不完全，我们仍将1745个后代至少归属到了一个亲本。以返回其出生溪流的采样成年后代数量衡量的繁殖成功率（reproductive success, RS），在两个类群中均显著低于孵化场亲本与野生亲本。雌性个体的RRS介于0.03至0.47之间，雄性个体则介于0.05至0.86之间。针对偶数年类群的广义线性模型分析显示，在控制采样日期（洄游时间）、溪流内采样位置以及鱼体体长等变量后，孵化场个体的RRS依然更低，区间为0.42至0.60。本研究结果强烈表明，孵化场逃逸个体在野外的适合度更低。RRS降低对野生种群生产力的影响，取决于其背后的驱动机制：若为环境驱动，则该效应可能是短暂的；若为遗传驱动，则该效应可能会跨世代持续存在。