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Parasites Leap Across Salmon

US - Sea lice on salmon, jump ship when their host gets nabbed by a bigger fish--and they then latch onto the predator, according to research published in the journal Science.

This previously unknown escape tactic, described in a study published online this week in Biology Letters, may mean bad news for an entire food web.

Due to large-scale fish farming, sea lice have spun out of control in the waters of British Columbia.

The closely crowded fish cages are hotbeds for the parasites, which eat skin, muscle, and blood and frequently jump to migrating schools of wild, juvenile salmon. As a result, wild salmon infestation rates have skyrocketed and driven some populations nearly extinct (Science, 14 December 2007, p. 1711).

A team led by Brendan Connors, a behavioural ecologist at Simon Fraser University in Burnaby, Canada, has now shown that the sea lice's knack for dispersal goes even further. They enclosed unparasitized predators - coho salmon and cutthroat trout - with two juvenile salmon, one infested and one not.

After the predators had eaten one of the salmon, the researchers examined the predators for sea lice.

The predators that ate a salmon that wasn't infested remained free of sea lice. However, predators that ate parasitized salmon became infested 70 per cent of the time, with the sea lice making daring escapes onto their new host that were visible to the naked eye.

"It's absolutely amazing," says Connors.

"They literally do a backflip off the fish they were on and land right between the eyes of the predator."

Many more male sea lice escaped their host than females - a finding that puzzles Frédéric Thomas, a parasitologist with the French national research agency in Montpellier, France.

A situation in which males escape death but females don't is an evolutionary dead end, he says.

Fisheries biologist Alan Pike of the University of Aberdeen in the UK notes that these sea lice live only on salmonid fish, which include salmon and trout.

If a sea louse were to escape being eaten only to end up on the wrong species of predator, it might not be able to survive. Pike wonders what sea lice would do in that tough situation.

Connors says the sea louse's lifesaving leap suggests that fish farms may infest not just juvenile wild salmon but ultimately the predators that eat them.

"We hadn't even thought about these sea lice being transferred up the food web," he says.