Salmon farms contribute to increase trout infestation with sea lice

The study, entitled 'Aquaculture and environmental drivers of salmon lice infestation and body condition in sea trout' was authored by Dr. Samuel Shephard and Dr. Paddy Gargan of Inland Fisheries Ireland alongside Craig MacIntyre of the Argyll Fisheries Trust.

Researchers examined sea lice levels over 25 years from more than 20,000 sea trout. The sea trout were sampled from 94 separate river and lake systems in Ireland and Scotland at varying distances from salmon farms.

The paper points out that sea trout captured closer to salmon farms had significantly higher levels of lice infestation, and were found to be of reduced weight.

Sea trout are known to remain for extended periods in near-coastal waters where the majority of salmon farms are located. This fish is therefore particularly vulnerable to sea lice impact, having the potential to encounter lice of farm origin throughout much of its marine life.

The impact of sea lice in farmed areas on sea trout has been substantial with increased mortality, reduced body condition and a changed migratory behaviour.

Heavily liced sea trout return to freshwater prematurely to rid themselves of lice and exhibit very poor marine growth and greatly reduced marine survival.

In fact, the study suggests, the most heavily lice infested sea trout die at sea.

Besides, the effect of the increased lice infestation was most evident in years of less rainfall, when a sea trout of average length (180 mm) caught within 10 kilometres of a farm could weigh up to 10g less than fish of similar length caught more than 40 kilometres from a farm.

The study covered the entire coasts of West Ireland and Scotland and accounted for variability in temperature and rainfall.

"While there had been some improvement in sea lice control in recent years, lice control on salmon farms was still not sufficient in certain west of Ireland bays during the spring migration period for sea trout to avoid heavy lice infestation and increased marine mortality," Dr. Gargan, Senior Research Officer at Inland Fisheries Ireland and one of the report authors, said.

"More effort is required to ensure lice levels on salmon farms are adequately controlled at this critical period when sea trout leave freshwater and enter the sea," the researcher concluded.