Sea Trout Loss Linked to Salmon Farm Parasite

James Owen
for National Geographic News
October 22, 2002

Scientists have discovered a direct link between the explosion of sea lice in farmed fish populations and the decline of Scottish sea trout. The findings follow a two-year study in the western Highlands where wild sea trout share coastal waters with penned salmon.

Government researchers from Scotland's Fisheries Research Services monitored sea lice levels around the River Shieldaig in the county of Wester Ross. They found the amount of sea trout infested with immature lice rose from zero to 75 percent during the second year of the production cycle at local salmon farms. This massive increase coincided with large-scale infestations of two-year-old farmed fish by adult lice.

The biggest concentrations of immature lice were recorded close to the river mouth, where young trout (post-smolts) congregate before going to sea. To escape the parasites, many post-smolts abandoned their seaward migration and returned to freshwater—where the lice drop off—interrupting their life cycle.

Subsequent breeding behavior is likely to be disrupted in those trout that manage to survive infestation, scientists say.

Sea lice are natural parasites of wild salmonids (salmon and trout) in salt water, feeding on their mucus, skin, and blood.

Earlier studies suggested that unnaturally high levels of sea lice can devastate wild stocks. Researchers in Norway at the University of Bergen and the Institute of Marine Research found that up to 86 percent of young wild salmon in fjords containing fish farms die from lice infestations.

Growth Industry

In Scotland, salmon farming has spread rapidly in recent years, growing into a $468 million (£300 million) industry providing employment for some 6,000 people. Most of the 350 farms are in the northwest—once one of the world's top destinations for sea trout anglers.

But since the 1980s the sea trout—a migratory brown salmonid that feeds at sea and spawns in freshwater—has become increasingly elusive. Government figures show angler catches in the region have dropped 80 percent in the last 20 years.

"It's only where there are no fish farms around river mouths that wild stocks remain healthy," said Peter Cunningham, a biologist for the Wester Ross Fisheries Trust. "Our understanding from data we've collected is that sea lice infestations are the major problem faced by young sea trout."

Alan Jackson has worked as a fishing guide on Loch Maree in Wester Ross since the 1970s—before salmon farming began near the River Ewe, which connects Maree to the sea. In 1985, the year farming started, the average annual catch by anglers staying at the Loch Maree Hotel was just under 1,100 sea trout. By 1989 the figure had fallen 90 percent. Last year anglers barely caught a total of 50 fish.

"Catches of ten fish per boat, each weighing between six to ten pounds (2.7 to 4.5 kilograms), used to be common," said Jackson. "Now you consider yourself lucky to land a single sea trout, never mind its size."

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