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Salmon Farm Sea Lice Halve Wild Atlantic Salmon Runs Says New Study

13th May 2017
Salmon Farm Sea Lice Halve Wild Atlantic Salmon Runs Says New Study Photo: IFI

#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.”

The full report can be found HERE.

Published in Marine Wildlife
MacDara Conroy

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MacDara Conroy

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MacDara Conroy is a contributor covering all things on the water, from boating and wildlife to science and business

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Marine Wildlife Around Ireland One of the greatest memories of any day spent boating around the Irish coast is an encounter with marine wildlife.  It's a thrill for young and old to witness seabirds, seals, dolphins and whales right there in their own habitat. As boaters fortunate enough to have experienced it will testify even spotting a distant dorsal fin can be the highlight of any day afloat.  Was that a porpoise? Was it a whale? No matter how brief the glimpse it's a privilege to share the seas with Irish marine wildlife.

Thanks to the location of our beautiful little island, perched in the North Atlantic Ocean there appears to be no shortage of marine life to observe.

From whales to dolphins, seals, sharks and other ocean animals this page documents the most interesting accounts of marine wildlife around our shores. We're keen to receive your observations, your photos, links and youtube clips.

Boaters have a unique perspective and all those who go afloat, from inshore kayaking to offshore yacht racing that what they encounter can be of real value to specialist organisations such as the Irish Whale and Dolphin Group (IWDG) who compile a list of sightings and strandings. The IWDG knowledge base has increased over the past 21 years thanks in part at least to the observations of sailors, anglers, kayakers and boaters.

Thanks to the IWDG work we now know we share the seas with dozens of species who also call Ireland home. Here's the current list: Atlantic white-sided dolphin, beluga whale, blue whale, bottlenose dolphin, common dolphin, Cuvier's beaked whale, false killer whale, fin whale, Gervais' beaked whale, harbour porpoise, humpback whale, killer whale, minke whale, northern bottlenose whale, northern right whale, pilot whale, pygmy sperm whale, Risso's dolphin, sei whale, Sowerby's beaked whale, sperm whale, striped dolphin, True's beaked whale and white-beaked dolphin.

But as impressive as the species list is the IWDG believe there are still gaps in our knowledge. Next time you are out on the ocean waves keep a sharp look out!

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