New study finds that sea lice from salmon farms can cause a 50% reduction in runs of wild Atlantic salmon

Thursday, 11th May 2017: The Board of Inland Fisheries Ireland welcomes a new study, published in the international journal Aquaculture Environment Interactions, which used 30 years of data from the Erriff river (National Salmonid Index Catchment) in the West of Ireland to evaluate the effect of sea lice from salmon aquaculture on wild Atlantic salmon. Juvenile salmon (called smolts) migrating to sea can become infected with sea lice from salmon farms and suffer increased mortality soon after leaving the coast. The study entitled “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 river, 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.

Sea lice from salmon farming have long been implicated in the collapse of sea trout stocks along the west coast of Ireland. However, this new study, authored by Dr Samuel Shephard and Dr Paddy Gargan from Inland Fisheries Ireland, is the first to clearly demonstrate significant losses of wild Atlantic salmon due to infestation with sea lice from salmon farms.

Dr Samuel Shephard stated that “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). Dr Paddy Gargan commented that increased mortality of wild salmon due to the impact of sea lice from salmon farming 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 critical therefore 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 that “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”.

For more information about Inland Fisheries Ireland, visit To view the full report, visit: .


For further information:

Órla Sheils
Communications Manager
Tel: (01) 8842673
E: This email address is being protected from spambots. You need JavaScript enabled to view it.

About Inland Fisheries Ireland

Inland Fisheries Ireland is a statutory body operating under the aegis of the Department of Communications, Climate Action and Environment (DCCAE) and was established under the Fisheries Act on 1st July 2010. Its principal function is the protection and conservation of the inland fisheries resource. Inland Fisheries Ireland promotes, supports, facilitates and advises the Minister on the conservation, protection, management, development and improvement of inland fisheries, including sea angling. Inland Fisheries Ireland also develops policy and national strategies relating to inland fisheries and sea angling and advises the Minister on same.