Salmon Swamp Their Predators to Survive a Perilous Migration

May 17, 2016 | Erica Tennenhouse

Juvenile salmon (smolts) swimming in Chilo lake
Photo credit: Nathan Furey

There’s safety in numbers.

Migration is fraught with risk for juvenile salmon. Fish-eating fish lurking below and avian predators hovering above all anxiously await the opportunity to binge on unsuspecting smolts as they make their way to the open ocean.

Researchers have been aware that survival of smolts migrating from their birthplace of Chilko Lake in British Columbia to the Pacific Ocean is extremely low.

"We knew that on average 10 to 40 million smolts leave Chilko Lake every year and only about 1.5 million return as adults two years later," said Nathan Furey, a doctoral candidate at the University of British Columbia in Canada and co-author of two recent studies on salmon migration, in a press release. "It's always been a mystery about what happens in between."

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Furey, along with a team of researchers, recently found out just how hazardous the initial leg of the journey is for these migrants. By implanting small electronic tags into the smolts before they departed Chilko Lake and monitoring the signals emitted from the tags, the researchers were able to track survival of the fish over the course of their migration.

Roughly a third of the smolts were found to be dead within 48 hours of commencing migration. The calm, clear waters of the first river they traverse, the Chilko, offer the smolts little concealment from watchful predators.

Those who manage to exit the Chilko River in one piece fare much better in the murky, fast-flowing Fraser River, which boasted a nearly 100 percent survival rate. These findings were published in the journal Ecological Applications.

But the smolts do not rely purely on luck to get out of the Chilko alive. A follow-up study published in the Journal of Animal Ecology revealed that smolts traveling with lots of other fish were roughly twice as likely to survive the most dangerous portion of the voyage than those moving in small numbers.

This survival strategy is a simple game of probabilities. Any given predator can only eat a certain number of prey, so by swamping these predators with high smolt densities, each individual smolt lowers his or her chances of being picked off.

Though many animals are likely to find safety in numbers, this predator swamping strategy is hard to study in the wild. According to the authors, their research provides “a rare example of density-dependent swamping effectively reducing predation risk of individual migrants.”

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