VANCOUVER — A bountiful population of harbour seals is a prime suspect in the decline of coho and chinook in the Strait of Georgia, according to a new study.
“This is really the first study of its kind,” said Ben Nelson, a PhD student in the Institute for the Oceans and Fisheries at the University of British Columbia. “Harbour seals are having a pretty important impact on the productivity and marine survival of these two very important species of salmon.”
The population of harbour seals has grown steadily with federal protection, from fewer than 5,000 in 1970 to about 40,000 in 2008 in the Strait of Georgia — a period that corresponds with marked declines in coho and chinook.
“In the 1970s, you could take 60 to 70 per cent of the population sustainably every year and there’d still be plenty of fish coming back to spawn — and that just doesn’t happen any more,” Nelson said in an interview.
The study conducted bone and DNA studies over three years on more than 1,000 samples of harbour seal scat at four sites — the Cowichan River estuary, Comox Harbour, the lower Fraser River, and the Belle Chain Islets in the Gulf Islands.
Although up to only about five per cent of the seals’ diet was juvenile chinook and coho, that five per cent adds up. If you apply the same rate of consumption across the entire seal population, the dewy-eyed predators could be responsible for about 55 per cent of natural mortality of juvenile coho and 45 per cent of chinook.
Although more research is needed to make a tight connection, the study’s general results are clear. “Seal predation on young fish in the strait is significant.”
The study has not yet been peer-reviewed and published in a scientific journal. “It’s been met with surprise,” Nelson said of the initial response.
The smolts are most heavily eaten as they emerge from freshwater streams into the salty ocean, in April and May for coho and in July for chinook, he said, a time when the fish measure about 100 to 200 millimetres.
Smolt abundance is determined, in part, by known releases by hatcheries and surveys by the federal fisheries department. The study assumes that seals can catch a hatchery smolt as easily as a wild smolt.
The question is what, if anything, is to be done about it?
Nelson said hatcheries might want to alter the timing of the release of juvenile salmon. Humans could simply start catching fewer coho and chinook. Log booms could be removed from river mouths where they provide easy haulouts or breeding sites for seals where juvenile salmon enter the ocean.
A total of 52 harbour seals were killed in 1997 and 1998 to benefit chinook salmon on the Puntledge River on Vancouver Island. But Nelson warned that the removal of one species can have unintended consequences for a marine ecosystem.
The continued recovery of mammal-eating transient killer whales could also reduce the seal population and benefit juvenile salmon — along with southern resident killer whales, which depend in large part on adult chinook.
While other studies have looked at the impact of seals on adult salmon returning to their home streams, this study looked at seals’ impact on juveniles in the strait, Nelson said, noting that the first three or four months at sea are critical for the survival of salmon.
In a separate but related UBC study, 40,000 juvenile coho salmon were tagged before their release from Big Qualicum Hatchery on Vancouver Island. At the same time researchers glued radio-frequency identification “beanies” to the heads of 20 seals along with packs onto the seals’ backs to measure location, depth, and acceleration in three dimensions. A record is made every time a seal eats one of the tagged coho.
The Vancouver-based Pacific Salmon Foundation has provided UBC with almost $500,000 toward a five-year project.
During 1879-1914 and 1962-1968, harbour seal populations in B.C. were depleted during commercial harvests for their pelts, according to the federal fisheries department. Bounty payments offered for predator control from 1914 to 1964 maintained populations below natural levels.
Legislation in Canada and the U.S. in the early 1970s made it illegal to kill marine mammals without a permit. The federal government in 2008 estimated the B.C. population of seals at about 105,000.
