(Reuters) - Activists in Washington state plan to launch a protest flotilla on Saturday in the Pacific Ocean’s Puget Sound over the accidental release of tens of thousands of farm-raised Atlantic salmon that they say threaten dwindling stocks of wild fish.

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The afternoon protest, which seeks to shut down farms that raise the non-native salmon in underwater pens, is expected to draw dozens of boats, kayaks and canoes on a route along the San Juan Islands where the spill happened.

Last month, a damaged pen operated by Cooke Aquaculture, a global seafood corporation based in Canada, accidentally allowed the salmon to escape.

That raised fears they would compete with wild fish for food, prey on the young, and expose them to disease. Protesters said native fish like Chinook salmon and steelhead trout were already struggling before the spill.

“Our wild salmon are in trouble and we can’t afford to have an industry in our state that only adds to their problems,” Kurt Beardslee, head of activist group Wild Fish Conservancy and organizer of the protest, said in a telephone interview.

Cooke, which is privately owned, said in an email to Reuters that it “apologizes humbly for this accident,” which is under investigation by state regulators. The company pledged to review operations of other fish pens it has in Washington.

Native American tribes said they were told by Cooke and by state officials to “go fish,” said Brian Cladoosby, chairman of the Swinomish Indian Tribal Community. The invitation for anyone to catch as many of the fugitive fish as possible was, at best, ineffectual, he said.

Swinomish rights to fish in the Puget Sound are guaranteed by historic treaties and their livelihoods depend on commercial fisheries.

“Many native salmon populations in the Pacific Northwest are already endangered or threatened. Putting this many Atlantic salmon, an invasive species, into the ecosystem cannot be good for it,” Cladoosby said.

Atlantic salmon are seen as particularly suitable for farming, due to their quick growth and disease resistance.

Company spokesman Chuck Brown said there was no evidence Atlantic salmon posed a threat to wild Pacific salmon stocks. He added that Cooke had offered to fund a two-year study examining the impact of the farmed fish.

Cooke said it quickly took steps to contain the breach and gained emergency permits from government officials to recapture escapees.

Findings from a state investigation are expected to be released in November, said Cori Simmons, spokeswoman for the state Department of Natural Resources.