Photo: Contributed

Harbour seals, sea lions and some fish-eating killer whales have been rebounding along the Northeast Pacific Ocean in recent decades. But that boom has come with a trade-off: They're devouring more of the salmon prized by a unique but fragile population of endangered orcas.

Competition with other marine mammals for the same food may be a bigger problem than fishing, at least in recent years, for southern resident killer whales that spend time in Washington state's Puget Sound, a new study suggests.

Researchers used models to estimate that from 1975 to 2015, marine mammals along the U.S. West Coast ate dramatically more chinook salmon - from 6,100 metric tons to 15,200 metric tons, according to a study published Monday in the journal Scientific Reports.

In the same period, salmon caught by commercial and recreational fishing from Northern California to Alaska declined from 16,400 to 9,600 metric tons.

"This really quantifies yet another pressure on recovering the salmon population," said co-author Isaac Kaplan, a research fishery biologist with the Northwest Fisheries Science Center, part of NOAA Fisheries. Other threats to salmon include habitat damage, dams and pollution.

The emphasis typically has been on managing how fishing affects salmon. But this study brings the rest of the ecosystem, including predators, into the picture, Kaplan said.

Researchers have known marine mammals gorge on salmon in certain hotspots, including the Columbia River between Oregon and Washington. But the predators may be eating even more in the ocean than thought.

The study found southern resident killer whales, which increased from 292 to 644, ate the most salmon in terms of biomass, or weight, while harbour seals ate the greatest numbers of salmon, mostly juvenile fish.

The whales have struggled due to lack of food, pollution and impacts from boats since they were listed as endangered in Canada in 2003 and 2005 in the U.S. There are now just 76, down from a high of 140 decades ago.