Contagious passage

Salmon fish farms spell trouble for wild populations

Published: Sunday 15 May 2005

Sea lice can kill the young sa (Credit: Alexandra Morton)salmon farms pose a massive threat of sea louse infection to wild populations of the fish, says a study conducted in a fjord in British Columbia, Canada (Proceedings of the Royal Society B, published online April 1, 2005). The researchers believe the results are also relevant to fish farms in the us, Ireland, Norway and Scotland.

Salmon spend all their lives in the sea but when they have to breed they miraculously choose the same locations in the rivers where they were born, reaching the spots after an arduous upstream journey. The scientists studied how the fish farm affected the wild fish as they went through the fjord back to the open sea.

Researchers studied more than 5,500 wild pink and chum salmon every 2 to 4 kilometres (km) on their 60-km route from their freshwater spawning grounds to the Pacific Ocean using new field techniques. The enormous data generated over months of painstaking work was combined with the state-of-the-art models of disease transfer.

The scientists found that near the salmon farm, parasite levels on wild fish were 73 times higher than normal, and remained raised for 30 km downstream even though the farm was only about 0.2 km long. Sea louse production from the farm was 30,000 times higher than natural, says team member Martin Krkosek from the University of Alberta.

Sea lice create open lesions on the bodies of the fish, thereby compromising their ability to maintain their salt-water balance. In large enough numbers, the parasites can suck nutrients from the fish faster than the fish can feed, literally eating them alive.

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