Harbour seals eating juveniles of salmon species most at risk

Harbour seals in the Strait of Georgia may be eating millions of juvenile sockeye, coho and chinook salmon as they transition from fresh to salt water, according to a new study that sheds new light on poor salmon survival rates.

Seals appear to be selectively consuming young fish — the species that are of the most conservation concern — just as they’re ready to begin adult life in the ocean, said supervising author Andrew Trites, director of the Marine Mammals Research Unit at the University of B.C.

“Seals have been seen lining themselves belly-up across the Puntledge River and gobbling down smolts as they come down,” he said. It’s the number of fish they eat that matters. A seal that requires two kilograms of food per day would require less than one adult coho to satisfy its needs, but the same seal would require about 100 juvenile coho to fill-up. And according to DNA analysis of their scat, that’s just what they’re doing.

“When you multiply it out — there are about 40,000 seals in the Georgia Strait — you see how significant that would be,” he said.

The study is part of the Salish Sea Marine Survival Program inquiry into the poor survival rates of B.C.’s most highly valued salmon species.

“Coho and chinook numbers have been down for a long time and it’s been a huge mystery as to why,” said Trites.

To help answer that question, the study’s lead author, Austen Thomas, developed a new way to determine exactly what the seals are eating and when.

DNA meta-barcoding as a diet analysis tool offers much higher resolution than conventional techniques, which rely almost entirely on identifying fish bones that survive the seals’ digestive systems.

According to an analysis of 1,200 samples, seals show a strong preference for adult chum and pink salmon in the fall, but don’t eat enough individual fish to make an impact on their numbers.

In spring, the opposite is true. The amount of chum and pink salmon in seal scat drops off and vast numbers of much-smaller juvenile coho, chinook and sockeye are evident.

The three species preferred by seals are all the more vulnerable to predation because they tend to linger in the strait for weeks before hitting the open ocean, said Trites.