Salmon are paying the price for the resurgence of an endangered killer whale population off the US west coast.

Populations of h arbour seals, sea lions and some fish-eating killer whales have been recovering along the US's Northeast Pacific Ocean in recent decades.

But that boom has come with a trade-off as they are 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 US west coast ate dramatically more chinook salmon, from 6,100 metric tons to 15,200 metric tons, according to a study published on 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 Centre, 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, Mr 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 authors estimated how much salmon in different life stages four marine mammals ate based on a number of assumptions, including their weight, diet and size.

The species included California sea lions, Steller sea lions, harbour seals and fish-eating killer whales.

Puget Sound orcas, also known as southern resident killer whales, face greater challenges than their orca counterparts farther north because they have a narrower menu of fish stocks and fewer available fish compared with what they need, Mr Kaplan said.

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