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Salmon Smolts Find Safety in Numbers

18 May 2016

CANADA - Using tags surgically implanted into thousands of juvenile salmon, UBC researchers have discovered that many fish die within the first few days of migration from their birthplace to the ocean.

"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, researcher and a PhD candidate in the faculty of forestry. "It's always been a mystery about what happens in between."

Researchers from the Pacific Salmon Ecology and Conservation Laboratory at UBC followed the migration of one of B.C.'s largest sockeye populations from Chilko Lake, in British Columbia's Cariboo region, to the ocean. Each spring, juvenile salmon known as smolts leave this central B.C. lake and migrate downstream through the Chilko, Chilcotin, and Fraser rivers and into the Salish Sea.

To follow the juvenile salmon, researchers implanted small electronic tags into the tiny 12-centimetre fish as they were leaving Chilko Lake. As the smolts made the 1,000-kilometre journey to the Pacific Ocean, acoustic receivers picked up the signals from the tags to monitor how many fish survived the migration.

More than 2,000 salmon were tracked over four years and researchers found that survival was poor in the clear and slow-moving Chilko River, where predators were feeding intensely on the smolts. Once in the murky and fast-flowing Fraser River, the salmon travelled day and night, covering up to 220 km per day, and experienced nearly 100 per cent survival. The researchers believe that in these waters, predators have difficulty finding and getting to the fish.

In a followup study, Furey found that juvenile salmon were more likely to survive the perilous days in the Chilko River if they traveled with lots of other fish. Survival was as low as 40 per cent for salmon that left the lake in small numbers compared to times of peak migration where survival rates increased to more than 90 per cent. The results confirm the old adage that there is safety in numbers and that salmon use this strategy, known as predator swamping, to avoid predators early in life.

"Our studies may help fisheries managers better understand why numbers of adults that return to spawn may be so low in some years," said Furey.

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