The negative association of sea lice from fish farms on recreational fishing catches of Atlantic salmon

The question of whether and to what extent sea-louse (L. salmonis) infestations from salmon farms influence wild Atlantic salmon survival has been subject to sustained scientific debate and political controversy. Documenting the population-level effects of sea lice on wild salmon remains inherently challenging. We employ comprehensive sea-lice data and recreational catch data from Norway to assess the impact of farm sea-louse infestations on wild salmon catches in different production areas (PAs). Our analysis finds a significant correlation between declines in wild Atlantic salmon catches and increasing amounts of adult female sea lice per km2. The effect is most pronounced in PA 4 on the west coast of Norway, an area within the government’s ”traffic light” management system where out-migrating salmon smolts are deemed to face high exposure to louse-induced mortality risk. Our model predicts below-average catches when the total sea louse load exceeds the government’s limit of 0.1 average adult female sea louse per farmed fish within some production areas. Furthermore, our results indicate that the risk of below-average catches increases by approximately 47% when salmon farms exceed this limit (estimated risk ratio of 1.47, 95 % CI [1.10, 1.96]).

Synthesis and Applications: Our study expands the existing body of evidence demonstrating a negative association between fish farming and the ecosystem services provided by wild salmon stocks. It has important implications for aquaculture management. First, it shows that farm sea lice directly affect peoples, rights holders, and interest groups, such as landowners and anglers, who rely on viable populations of wild salmon. Second, it suggests that a >0.1 sea lice limit may be insufficient to prevent subpar catches in some areas. Furthermore, in the context of allowing further growth in farmed salmon biomass, setting absolute sea-louse limits for entire production areas may be a more effective regulatory instrument than setting average lice limits per farmed fish. Our findings contribute to advancing a scientific basis for setting appropriate louse limits on farm and area scales.

Methods
The data pertains to the total number of sea lice (L. slamonis) in salmon farm production areas and wild Atlantic salmon catch statistics for Atlantic salmon in Norway.

1. We employ comprehensive sea-lice (L. salmonis) data from salmon farms in Norway and recreational river catch data of Atlantic salmon from Norway to assess the impact of farm sea-louse infestations on wild salmon catches in different production areas (PAs). 
2. We have estimated the total amounts of lice within each corresponding PA for each year. To facilitate a comparison of relative lice levels across PAs, we factored in the size of PAs and computed the total number of female sea lice per km2, denoted as the farm sea-louse load.  
3. The official government statistics of salmon catches in Norway document the nominal river catches of adult Atlantic salmon, caught by rod.  We aggregated the catches for each Sea-winter class (SW) from all rivers within each PA. The aggregated values were then standardized (subtracting the mean and dividing by SD) across the period examined. This standardized variable resulted in a normally distributed continuous variable that expresses both positive and negative values, signifying years when catch levels for each specific SW-class within a PA deviate from the average.

鲑鱼养殖场产生的鲑疮痂鱼虱（Lepeophtheirus salmonis，简称L. salmonis）感染是否会以及在多大程度上影响野生大西洋鲑的存活，这一问题长期以来一直是学界持续争论和政治争议的焦点。量化鱼虱对野生鲑鱼种群层面的影响，本质上仍颇具挑战性。本研究借助挪威境内的全面鱼虱监测数据与休闲捕捞数据，评估不同生产区域（Production Areas，简称PAs）内养殖场鲑疮痂鱼虱感染对野生大西洋鲑捕捞量的影响。分析结果显示，野生大西洋鲑捕捞量的下降与每平方公里成年雌性鱼虱数量的增加之间存在显著相关性。该效应在挪威西海岸的PA4区域最为显著，该区域隶属于政府的“红绿灯”管理体系，洄游的鲑鱼幼鲑在此区域会面临极高的鱼虱致死风险。我们的模型预测，在部分生产区域内，当总鱼虱负荷超过政府设定的每养殖鲑鱼平均0.1只成年雌性鱼虱的限值时，野生鲑鱼捕捞量将低于平均水平。此外，研究结果表明，当鲑鱼养殖场突破该限值时，捕捞量低于平均水平的风险将提升约47%（风险比估计值为1.47，95%置信区间（Confidence Interval，简称CI）[1.10, 1.96]）。

研究发现与应用启示：本研究进一步丰富了现有证据链，证实水产养殖与野生鲑鱼种群所提供的生态系统服务之间存在负相关关系，对水产养殖管理具有重要指导意义。其一，研究表明养殖场鱼虱会直接影响依赖健康野生鲑鱼种群的当地居民、权利主体及利益相关群体，例如土地所有者与垂钓爱好者。其二，研究显示，将鱼虱限值设定为大于0.1只/养殖鱼，可能不足以避免部分区域出现捕捞量不达标的情况。此外，在允许养殖鲑鱼生物量进一步增长的背景下，为整个生产区域设定绝对鱼虱限值，或许比为单条养殖鲑鱼设定平均鱼虱限值更为有效。本研究结果为养殖场及区域尺度下设定合理的鱼虱限值提供了科学依据。

研究方法
本研究使用的数据涵盖挪威境内鲑鱼养殖场生产区域内的鲑疮痂鱼虱（Lepeophtheirus salmonis，简称L. salmonis）总数量，以及挪威野生大西洋鲑的捕捞统计数据。
1. 本研究借助挪威境内鲑鱼养殖场的全面鱼虱监测数据与大西洋鲑休闲河流捕捞数据，评估不同生产区域（PAs）内养殖场鱼虱感染对野生鲑鱼捕捞量的影响。
2. 我们估算了每一年对应生产区域内的总鱼虱数量。为便于不同生产区域间鱼虱相对水平的比较，我们将生产区域面积纳入考量，计算出每平方公里的雌性鱼虱总数量，将其定义为养殖场鱼虱负荷。
3. 挪威官方的鲑鱼捕捞统计数据记录了通过鱼竿垂钓捕获的成年大西洋鲑名义河流捕捞量。我们将每个生产区域内所有河流的各海冬龄组别（Sea-winter class，简称SW）捕捞量进行汇总，随后在所研究的时段内对汇总值进行标准化处理（减去均值后除以标准差（Standard Deviation，简称SD））。该标准化变量为服从正态分布的连续型变量，取值可正可负，分别代表对应生产区域内某一SW组别的捕捞量高于或低于平均水平的年份。