Evidence for sea lice-induced marine mortality of Atlantic salmon (Salmo salar) in western Ireland from experimental releases of ranched smolts treated with emamectin benzoate

Abstract

Sea trout (Salmo trutta) stock collapses in coastal areas of western Ireland subject to salmon aquaculture were contemporaneous with high abundances of larval sea lice (Lepeophtheirus salmonis) on juvenile sea trout. Whereas sea trout remain in near-shore waters throughout their marine migration, Atlantic salmon (Salmo salar) smolts typically move quickly offshore into oceanic waters. It might therefore be predicted that salmon smolts would be less vulnerable to coastal stressors and less likely to be negatively affected by infestations of sea lice early in their marine phase. Groups of microtagged, hatchery-reared Atlantic salmon smolts were fed either untreated pellets or pellets incorporating the in-feed sea louse treatment SLICE (emamectin benzoate) prior to eight experimental releases in three marine locations over a 3-year period. In total, 74 324 smolts were released and analysis of tag recaptures from returning adult salmon showed that emamectin-treated smolts experienced increased survivorship and were 1.8 times more likely to return compared with control fish. These results suggest that sea lice-induced mortality on adult Atlantic salmon returns in Ireland can be significant, and that sea lice larvae emanating from farmed salmon may influence individual survivorship and population conservation status of wild salmon in these river systems.Résumé : Les effondrements des stocks de truites de mer (Salmo trutta) dans les régions côtières de l'ouest de l'Irlande affectées par l'aquaculture de saumons coïncident dans le temps avec les fortes abondances de larves de poux de mer (Lepeophtheirus salmonis) sur les jeunes truites de mer. Alors que les truites de mer demeurent dans les eaux près des côtes tout au cours de leur migration en mer, les saumoneaux des saumons atlantiques (Salmo salar) typiquement s'éloignent rapidement des côtes vers les eaux océaniques. On pourrait alors prédire que les saumoneaux du saumon sont moins vulnérables aux facteurs de stress sur les côtes et moins susceptibles d'être affectés négativement par les infestations de poux de mer tôt durant leur phase en mer. Nous avons nourris des groupes de saumoneaux du saumon atlantique élevés en pisciculture et porteurs de micro-étiquettes ou bien de granules non traitées ou alors de granules contenant le traitement SLICE (benzoate d'émamectine) par l'alimentation contre les poux de mer avant huit libérations expérimentales dans trois sites marins sur une période de 3 ans. Un total de 74 324 saumoneaux ont été relâchés; l'analyse des retours d'étiquettes chez les saumons adultes qui reviennent montre que les saumons traités à l'emamectine connaissent une survie plus élevée et sont 1,8 fois plus susceptibles de revenir que les poissons témoins. Nos résultats laissent croire que les effets de la mortalité causée par les poux de mer sur les retours des saumons atlantiques en Irlande peuvent être significatifs et que les larves de poux de mer provenant des élevages de saumons peuvent influencer la survie individuelle et le statut de conservation d...

Evidence of salmon lice-induced mortality of anadromous brown trout (Salmo trutta) in the Hardangerfjord, Norway

“…To the best of our knowledge, this is the first study to present evidence of induced mortality due to salmon lice infection at the population level in anadromous brown trout. Several studies have quantified mortality in Atlantic salmon caused by salmon lice by comparing the growth and survival of groups with prophylactic chemical treatment with untreated controls (Skilbrei & Wennevik 2006;Hvidsten et al 2007;Gargan et al 2012). Gargan et al (2012) compared treated and untreated groups of salmon in eight experimental releases from three locations in western Ireland and found that treated groups were 1.8 times as likely to return as controls.…”

“…Such studies require accurate information about return rates of smolt cohorts, and preferably also about regional infection levels. In Atlantic salmon, comparisons of return rates of smolt groups protected against salmon lice by various chemicals with untreated controls have shown that treated groups often have higher survival rates, suggesting a population-regulating effect of salmon lice (Skilbrei & Wennevik 2006;Jackson et al 2011;Gargan et al 2012). For anadromous brown trout, however, this is less well documented.…”

Quantifying the contribution of sea lice from aquaculture to declining annual returns in a wild Atlantic salmon population

“…Sea lice present during the spring smolt outmigration through Killary Harbour could have salmon farm and/or wild salmon sources. Gargan et al (2012) found that the number of wild salmonids was very low during this period, and that sea lice abundance on local farmed salmon was 3 to 4 orders of magnitude greater than the estimate for wild salmonids. A study on the production of sea lice larvae from farmed and wild salmon and its relation to the infestation of wild sea trout found that farmed salmon contributed 95% of the total production of L. salmonis nauplii in the mid-west Irish coast region (Tully & Whelan 1993).…”

Can the river location within a fjord explain the density of Atlantic salmon and sea trout?

“…Krkošek et al 2007). One of the mechanisms that couples increased mortality in wild salmon populations with fish farming is the increased spread and production of the salmon louse, the parasitic copepod (Lepeophtheirus salmonis), which can infect and kill salmon smolt during migration (Jackson et al 2011;Gargan et al 2012). The Hardangerfjord is one of the fjords with the highest density of salmon farms in the world (Skaala et al 2014).…”