Parasite was fatal to wild juvenile fish swimming past coastal farms
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Parasite was fatal to wild juvenile fish swimming past coastal farms

By Martin Mittelstaedt
The Globe and Mail
Tuesday 03 October 2006

A team of researchers has found that sea lice from a group of fish farms along the British Columbia coast infected and then killed up to 95 per cent of the wild juvenile pink and chum salmon swimming past the pens.

The dramatic finding, to be published this week in the on-line edition of the US journal Proceedings of the National Academy of Sciences, is likely to further intensify the debate about the impact of Canada's growing aquaculture business on the health of the country's remaining wild salmon stocks.

The aquaculture industry has long played down the threat posed to wild fish by parasites and diseases from fish farming, but the study's lead author is worried the high mortality he observed indicates that wild B.C. salmon will be at risk if steps are not taken to protect them.

"This is one of the last places in the world where we have natural, thriving ecosystems that depend on wild salmon, and unbeknownst to most Canadians, we're trading it off right now for these fish farms," said Martin Krkosek, a PhD student at the University of Alberta's Centre for Mathematical Biology.

Previous studies have demonstrated that sea lice can spread from fish farms to wild salmon. But the latest research demonstrates the high mortality of infected fish and the threat this poses to the long-term survival of wild stocks.

In the study, the researchers monitored thousands of juvenile salmon in 2004 and 2005 for the presence of sea lice as they migrated past four fish farms in the Tribune Channel area near the northeast corner of Vancouver Island, on their journey to the open ocean.

Although sea lice are not harmful to humans and are a kind of parasite that does not normally harm adult fish, they can be lethal to young salmon that have not developed protective scales and sufficient size to withstand their ill effects. The lice, which can grow to be more than a centimetre across, feed on the skin and mucus of fish, creating open lesions that undermine a young fish's ability to survive in the ocean's harsh saltwater environment.

"We often worry about wildlife making humans sick, but here is a case where humans are making wildlife sick," said Mark Lewis, a co-author of the study and a mathematician and biologist at the University of Alberta.

The research found that the number of juvenile salmon killed by the sea lice increased over the migration season, rising from a low of 9 per cent early in spring when parasite populations were low to the 95 per cent figure in late spring when the area was teaming with lice.

The results were based in part on counting the sea lice on more than 14,000 juvenile salmon, fish that were only a few months old and only a few centimetres long.

The Aquaculture Association of Canada, a trade group based in St. Andrews, N.B., that represents the industry on the Pacific and Atlantic coasts, has disputed previous scientific findings that link illnesses in wild fish to farms.

Association president Chris Hendry said in an interview that "there is some uncertainty" about this link. The group has contended in the past that the transfer of sea lice from farmed to wild salmon is unlikely to be of a sufficient scale to have an impact on populations.

But Mr. Krkosek said that based on his research, only about 5 per cent of the sea-lice infections in young salmon were caused by transmission from other wild fish, and the rest were attributed to fish swimming past the farms.

The researchers believe fish farms undermine a key natural defence the salmon have evolved to protect their young from sea lice.

While salmon are young, they travel down the rivers they were born in and swim to the open ocean in a migration route that does not cross paths with adult salmon likely to have the parasite, reducing the chances of transmission.

But in many coastal waterways, farms containing adult fish have been placed directly in the path of migrating young salmon, causing a breakdown in natural protection.

The study was funded mainly by the federal government, although it did receive some money from a B.C. fishing group, a tourist association and the David Suzuki Foundation.

Foundation spokesman Jay Ritchlin said that to prevent transmission of parasites and diseases to wild fish, the aquaculture industry has to stop raising fish in open net pens, and needs to convert to enclosures that block the movement of pathogens.

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