#Salmon - Wild Atlantic salmon smolts migrating to sea from Irish rivers can become infected with sea lice from West Coast salmon farms and suffer increased mortality soon after leaving the coast, cutting their numbers by half.
That’s the conclusion of a new study published in international journal Aquaculture Environment Interactions, which used 30 years of data from the River Erriff in the West of Ireland to evaluate the effect of sea lice from salmon aquaculture on wild Atlantic salmon.
The study, titled ‘Quantifying the contribution of sea lice from aquaculture to declining annual returns in a wild Atlantic salmon population’, examined sea lice production from salmon farming in Killary Harbour and its effect on the return of wild salmon to the Erriff, at the head of the harbour, in the following year.
Results from this long-term study indicate that returns of wild adult salmon can be reduced by more than 50% in years following high lice levels on nearby salmon farms during the smolt out-migration.
Inland Fisheries Ireland says sea lice from salmon farming have long been implicated in the collapse of sea trout stocks along the West Coast.
But this latest study, authored by Dr Samuel Shephard and Dr Paddy Gargan from IFI, is “the first to clearly demonstrate significant losses of wild Atlantic salmon due to infestation with sea lice from salmon farms,” it adds.
Dr Shephard says: “There has been a lot of discussion as to the importance of the sea lice impact in the context of environmental variation and changing ocean conditions.
“We find that the predicted 50% reduction in 1SW salmon returns following a high lice year is greater than the average year-to-year variation attributable to environmental effects.”
Modelled lice impact levels and a fitted stock-recruitment relationship were used to estimate how annual returns of Erriff salmon might have looked over the last 30 years, in the absence of a serious impact of sea lice from aquaculture.
Results suggest that Erriff salmon returns could now be twice as large as without observed anthropogenic lice impacts, but would probably show a similar long-term decline.
The River Erriff is designated as a Special Area of Conservation (SAC) for Atlantic salmon under the European Union Habitats Directive (92/43/EEC). Salmon smolts from the Erriff can be followed via a new online tool, as previously reported on Afloat.ie.
Dr Gargan explained that increased mortality of wild salmon due to the impact of sea lice can result in salmon stocks not reaching spawning targets or not being at favourable conservation status as required under the EU Habitats Directive.
It is therefore critical that sea lice levels are maintained at a very low level on farmed salmon in spring – and where this has not been achieved, that farmed fish are harvested before the wild salmon smolt migration period.
The authors conclude: “Many Atlantic salmon populations are already under pressure from (possibly climate-mediated) reductions in marine survival. The addition of significant lice-related mortality during the coastal stage of smolt out-migration could be critical.”
