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Young salmon may leap to 'oust the louse'

August 09, 2018
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By Diane Mar-Nicolle

“Everyone who has gone fishing has wondered why fish jump,” says John Reynolds, SFU professor of marine ecology.

“Now, thanks to research by SFU undergraduate student Emma Atkinson, we have experimental proof that at least one of the reasons may be to remove external parasites.”

The study, published last month in the Journal of Fish Biology, stemmed from Atkinson's honours research project, which she recently completed in the Department of Biological Sciences.

Emma Atkinson building fish pens in Echo Bay, B.C.

According to Atkinson, previous ideas about why fish leap include getting over obstacles during their upstream migration as adults, catching food, and avoiding predators. However, Atkinson says these reasons may not apply to juvenile sockeye salmon because their diet is composed almost exclusively of zooplankton in the water and their tendency is to scatter rather than leap in response to predation threats.

“So why do these fish leap?” she wondered.

Atkinson hypothesized that leaping behavior in these fish could be their way of removing sea lice, a common marine parasite found on both wild and penned salmon off the coast of British Columbia.

“Sea lice can affect wild salmon in many ways,” she says, and “research led by Sean Godwin in our lab has shown that heavy sea-louse infestation is linked to reduced growth and feeding success for juvenile sockeye.”

Atkinson tested her hypothesis at Salmon Coast Field Station, a non-profit conservation organization located in Kwikwasut’inuxw Haxwa’mis territory in the Broughton Archipelago.

The research team building covers for the pens.

She and her team caught wild juvenile sockeye salmon (Oncorhynchus nerka) during their coastal migration away from the Fraser River. They held the fish in flow-through net pen enclosures, half of which were covered with netting to prevent leaping, while the other half were uncovered. After three-days, the team counted the lice on each fish.

“We found that on average, young sockeye allowed to leap had 22 per cent fewer sea lice at the end of the experiment than those prevented from leaping,” says Atkinson.

The researchers also estimated that it may take more than 50 leaps for a young salmon to dislodge a louse, which Atkinson acknowledges is a substantial amount of energy to expend.

“These costs may be offset by multiple potential benefits of parasite removal, but that’s for another study," she says.

Click here for the full study.