Timing is everything: survival of Atlantic salmon (Salmo salar) postsmolts during events of high salmon lice densities

Atlantic salmon in aquaculture act as reservoir hosts and vectors of parasites like salmon lice and this parasite is shown to harm wild salmonid populations. In the present study, n=29817 tagged Atlantic salmon were studied in four release trials. Half of the released fish were given prophylactic treatment against lice, the other half represented sham control fish. We used a nested design comparing years with low and high lice density and seasonal dynamics in infestation pressure. The released Atlantic salmon thus experienced highly variable lice infestation pressures, which we linked to survival and growth in returning fish. The fish were released in a protected "National Salmon Fjord" and n=559 Atlantic salmon were recaptured after spending 1-4 years at sea. In most experimental groups 1 – 2.5 % of the fish were recaptured at return. However, survival of unprotected fish was extremely low for the trial released at the highest density of lice: only 0.03 % of these Atlantic salmon returned to the river, compared to 1.86 % in the protected group. Synthesis and applications . We document that a high lice density can cause a more than 50 times higher mortality risk in Atlantic salmon on their sea migration, even in a fjord with protected status. Fine-tuned and hard-to-predict year-to-year differences in timing, both for the wild smolt migration and the population build-up of lice released from aquaculture, means life or death to wild salmon. Management actions such as spatial segregation of farmed fish and lice (e.g. closed farm pens), and/or moving farms away from vulnerable habitats for wild salmonids (fjords and coastal areas), may be needed to ensure sustainable co-existence of wild and farmed Atlantic salmon.

水产养殖中的大西洋鲑（Atlantic salmon）可作为鲑虱（salmon lice）等寄生虫的储存宿主与传播媒介，该类寄生虫已被证实会对野生鲑科（salmonid）鱼类种群造成损害。本研究针对4次放流试验中的n=29817尾标记大西洋鲑展开分析。其中半数放流个体接受了鲑虱预防性防控处理，剩余个体作为假对照处理组。本研究采用嵌套设计，对比了鲑虱密度高低年份以及侵染压力的季节动态差异。受试大西洋鲑所经历的鲑虱侵染压力存在显著异质性，我们将该异质性与回游个体的存活率及生长情况进行了关联分析。本次试验在受保护的“国家鲑鱼峡湾”开展放流，最终有n=559尾大西洋鲑在海洋中栖息1至4年后被重捕。多数试验组的回捕率介于1%至2.5%之间。但在鲑虱密度最高的试验批次中，未接受防控处理的个体存活率极低：仅0.03%的该组大西洋鲑返回河流，而同期防控组的回捕率为1.86%。研究总结与应用：本研究证实，即便在受保护的峡湾中，高鲑虱密度也可使大西洋鲑在海洋洄游阶段的死亡风险提升50倍以上。野生幼鲑降海洄游的时间节点，以及水产养殖源鲑虱种群的年度增殖动态均存在难以精准预判的细微年度差异，这对野生鲑类而言关乎生死存亡。为实现野生与养殖大西洋鲑的可持续共存，或需采取针对性管理措施，例如对养殖鱼类与鲑虱实施空间隔离（如采用封闭养殖网箱），或将养殖设施搬离野生鲑科鱼类的脆弱栖息生境（峡湾与近岸海域）。