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Predators zoom in on lice-infested salmon

Parasite picked up near fish farms may harm wild juveniles in unexpected ways

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4:28pm, February 16, 2009
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CHICAGO — Young lice-infested wild salmon not only bear the burden of a parasite load, but they are also more likely to get snapped up by predators than their clean schoolmates, new research shows.

The research, presented February 15 at the meeting of the American Association for the Advancement of Science, adds to a growing body of evidence that aquaculture, which ideally would take pressure off wild fish stocks, may harm some wild populations in unexpected ways. Scientists are still untangling the web of interactions between farmed and wild fish — a web that includes parasites, antibiotics, feed fish and the humans who scarf down more than 9 million metric tons of farmed fish every year.

When juvenile, 1-inch–long pink and chum salmon swim down the rivers of the Pacific Northwest toward the open sea, many pass aquaculture pens that dot coastal inlets. Normally, there is little overlap of adult and juvenile habitats — the young and old fish travel in different circles — and most fish don’t pick up parasites such as sea lice until they are adults. But when the wild juveniles swim through fish farm territory, the sea lice that are prevalent in the close quarters of aquaculture pens can glom onto the juveniles.

Not only do the lice suck the lifeblood from the young fish, but the wounds are also an open door for harmful bacteria and viruses. Previous research suggests that juvenile mortality linked to lice-infested farms can be as high as 95 percent, says Martin Krkošek of the University of Washington in Seattle.

Now Krkošek reports that infested fish engage in riskier behaviors, making them more likely to become dinner for the 3- to 4-inch–long coho salmon smolts, a primary predator of the pink and chum juveniles.

Krkošek and his colleagues set up tanks with small schools of the juveniles, some of which were infected with sea lice. The scientists trained the fish to expect food in the exposed center of the tank, occasionally simulating a predator strike by having a fake bird dive down into the tank. Healthy fish quickly scattered, bolting for cover under the fake kelp in the tanks’ corners, but lice-burdened fish took longer to seek shelter, Krkošek says. Infested fish also were more inclined to swim in the exposed positions in the school, hanging toward the outside of the group and lagging behind their closest neighbors, making it easier for predators to see the fish and strike.

It isn't clear whether the selective removal of infested fish by predators dampens the negative effects of the lice by clearing out sick salmon or if this culling exacerbates mortality, says Krkošek.

Lice are just one of the ills of farmed salmon, Krkošek notes.

The high density of penned fish makes it easier for bacteria and viruses to spread, which often leads to heavy use of antibiotics on fish farms.

“When you crowd animals together, they tend to get sick,” he says.

Aquaculture is the new frontier of excessive antibiotic use, says Felipe Cabello of New York Medical College in Valhalla, who also presented at the conference. The practice fosters the growth of antibiotic-resistant bacteria, which threaten human health. Bacteria acquire resistance by picking up little circular bits of DNA called plasmids that can carry genes for resistance. He cited a recent study of the diversity of tetracycline-resistant bacteria in Chilean salmon farms that found that 40 percent of the resistance genes had never been described before.

Fish farming offers an opportunity to take pressure off of wild fish stocks and to feed the world’s growing population. But many scientists contend that farms should concentrate on shellfish and fin fish low on the food chain. Farming carnivores such as salmon doesn’t add up — more fish need to be extracted from the oceans to make the food for farmed salmon than is produced by the farms.

John Volpe of the University of Victoria in British Columbia, who also presented at the conference, is part of a research team that’s spearheading a consistent way of assessing the sustainability of aquaculture operations. The Global Aquaculture Performance Index evaluates a country’s fish farms using several parameters, such as water quality and the amount of disease and parasites. Currently the global production of farmed fish is growing, with the bulk of farmed species made up of fish that are high in the food chain and need to be fed other fish. "It's farming the tigers of the sea," Volpe says.

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