More than 31,000 Atlantic salmon raised in a Michigan fish hatchery had to be killed after failing to recover from disease, officials said Tuesday.

The decision followed an unsuccessful 28-day treatment period at the Harrietta hatchery in Wexford County.

It was "gut-wrenching for staff," even if the fish were just a fraction of the millions raised in hatcheries each year, said Ed Eisch, assistant chief in the fisheries division at the Department of Natural Resources.

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The fish, around 6 inches long, were loaded into a truck Monday, euthanized with carbon dioxide and buried in a pit, Eisch said Tuesday.

The salmon, sick with a bacterial kidney disease, were treated with medicated feed.

"We kind of suspected when we went into the treatment that it might not be effective," Eisch told The Associated Press.

The unhealthy fish would have posed a risk to other fish if they had been released into Michigan waters, he said.

The disease likely came from brown trout at the hatchery.

"We think there some latent bacteria in the brown trout, and they were releasing the bacteria, enough that the Atlantics picked it up and got sick from it," Eisch said.

Scientists at Michigan State University plan to try to develop a vaccine to protect fish from future outbreaks, he said.