Hatching Time and Alevin Growth Prior to the Onset of Exogenous Feeding in Farmed, Wild and Hybrid Norwegian Atlantic Salmon

The onset of exogenous feeding, when juveniles emerge from the gravel, is a critical event for salmonids where early emergence and large size provide a competitive advantage in the wild. Studying 131 farmed, hybrid and wild Norwegian Atlantic salmon families, originating from four wild populations and two commercial strains, we investigated whether approximately 10 generations of selection for faster growth has also resulted in increased somatic growth prior to the onset of exogenous feeding. In addition, we tested whether relaxed selection in farms has allowed for alterations in hatching time between farmed and wild salmon. Across three cohorts, wild salmon families hatched earlier than farmed salmon families, while hybrid families displayed intermediate hatching times. While the observed differences were small, i.e., 1–15 degree-days (0–3 days, as water temperatures were c. 5–6°C), these data suggest additive genetic variation for hatching time. Alevin length prior to exogenous feeding was positively related to egg size. After removal of egg size effects, no systematic differences in alevin length were observed between the wild and farmed salmon families. While these results indicate additive genetic variation for egg development timing, and wild salmon families consistently hatched earlier than farmed salmon families, these differences were so small they are unlikely to significantly influence early life history competition of farmed and wild salmon in the natural environment. This is especially the case given that the timing of spawning among females can vary by several weeks in some rivers. The general lack of difference in size between farmed and wild alevins, strongly suggest that the documented differences in somatic growth rate between wild and farmed Norwegian Atlantic salmon under hatchery conditions are first detectable after the onset of exogenous feeding.

外源摄食（exogenous feeding）的起始节点，即幼体从产卵砾石基质中孵出的时刻，是鲑科鱼类（salmonids）的关键生命事件——野外环境中，提早孵出与更大体型可使个体获得竞争优势。本研究以源自4个野生种群与2个商业品系的131个养殖、杂交及野生挪威大西洋鲑（Atlantic salmon）家系为研究材料，旨在探究针对快速生长的约10代选育，是否会在外源摄食起始前提升躯体生长速率。此外，本研究还检验了养殖环境中的松弛选择是否会导致养殖与野生大西洋鲑的孵化时间出现差异。在3个同期群（cohort）中，野生鲑家系的孵化时间早于养殖鲑家系，杂交家系的孵化时间则介于两者之间。尽管观测到的差异幅度较小——仅为1~15度日（degree-days，对应水温约5~6℃时，相当于0~3天的时间差），但这些数据表明孵化时间存在加性遗传变异。外源摄食前的卵黄囊仔鱼（alevin）体长与卵粒大小呈正相关。在剔除卵粒大小的影响后，野生与养殖鲑家系的卵黄囊仔鱼体长未出现系统性差异。尽管上述结果表明卵发育时间存在加性遗传变异，且野生鲑家系的孵化时间始终早于养殖鲑家系，但这类差异幅度极小，不太可能对自然生境中养殖与野生鲑的早期生活史竞争产生显著影响。考虑到部分河流中雌性个体的产卵时间可相差数周，这一点尤为凸显。养殖与野生卵黄囊仔鱼的体长整体无显著差异，这强烈表明，已有研究报道的养殖与野生挪威大西洋鲑在人工繁育条件下的躯体生长速率差异，仅会在外源摄食起始后才可被首次检测到。