Hatchery-raised fish became genetically distinct from their wild counterparts in a single generation, a new study of Oregon steelhead trout found.

The study, published in the journal Nature Communications, tracked the offspring of Hood River hatchery steelhead and compared their genetic profile to that of wild steelhead in the same river system.

What they found were differences in 723 genes -- differences that emerged in just a single generation.

It's no surprise that the hatchery fish differed from wild fish, even in such a short period, said Michael Blouin, a study coauthor and professor of integrative biology in the Oregon State University's College of Science, since the differences between the two have been studied before. Previous research has shown, for example, that hatchery steelhead have less reproductive success than wild fish when they return from the ocean to freshwater spawning grounds.

"But to actually be able to see this at the DNA level is really quite extraordinary," he said, as is the degree to which the DNA differed. (Though the study examined steelhead, Blouin said their findings likely had implications for other salmonids, including salmon, raised in hatcheries.)

He knew, from previous work, that there were genetic differences, but he was expecting to see possibly half a dozen genes affected, he said.

"I didn't expect to see hundreds of them," he said.

Why is there such a quick and big genetic change?

One emerging hypothesis is that hatchery fish develop in a way that allows them to thrive in crowded conditions, Blouin said. The genes where differences were found were dominated by those related to immunity and wound repair, he said. That suggests hatchery fish that make it out into the wild are those that are best at fighting off the fish crowds in the surroundings where they grow, he said.

"You take 50,000 of them and you crowd them in a concrete box, they start biting each other," he said.

But the adaptation that allows hatchery fish to endure box-like crowding is not so valuable in the open waters of the Hood River or the wide ocean, he said.

"Whatever advantage these fish are gaining in the hatchery they're losing in the wild," he said.

The findings have implications for fishery management, an arena sometimes marked by sharp conflicts between supporters of hatcheries, who defend the facilities as necessary, and hatchery opponents, who fear natural stocks will become weakened by interbreeding with hatchery fish.

The study's purpose is not to revisit that sometimes-bitter debate, Blouin said.

Rather, he said, "The reason we did this is to try to find out what's going on in the hatchery, and can we fix it?"

Blouin and his colleagues are continuing to examine the question, with some related studies already underway.