A 19-year analysis of fish raised in an Oregon hatchery found dramatic genetic changes can occur in a single generation, with the domesticated fish responding rapidly to artificial evolutionary pressures found in a crowded, human-regulated tank.

But out in the wide-open world of riverine predators and ocean migrations, these same hatchery-bred smolts faltered when compared to their wild-raised brethen.

“The impact of hatcheries on salmon is so profound that in just one generation traits are selected that allow fish to survive and prosper in the hatchery environment, at the cost of their ability to thrive and reproduce in a wild environment,” wrote a team of four scientists in a new paper published this week in the Proceedings of the National Academy of Sciences.

The paper — which tracked a run of Hood River steelhead for 19 years — confirmed that hatchery life had the power to trigger genetic shifts that were then passed onto the next generation. A release about the study called it “evolution at warp speed.”

"We've known for some time that hatchery-born fish are less successful at survival and reproduction in the wild," said Michael Blouin, a professor of zoology at Oregon State University, in this story about the research. "However, until now, it wasn't clear why. What this study shows is that intense evolutionary pressures in the hatchery rapidly select for fish that excel there, at the expense of their reproductive success in the wild."

"We expected to see some of these changes after multiple generations," added lead author Mark Christie, in the same story. "To see these changes happen in a single generation was amazing. Evolutionary change doesn't always take thousands of years."

Which traits were being favored by hatchery life wasn’t clear, the scientists said. But they speculated that growing up indoors inside giant tanks might hard-wire a tolerance for “extreme crowding” into the fish — and that trait might work against fish attempting to reproduce under real-world conditions.

“If research can determine exactly what aspect of hatchery operations is selecting for fish with less fitness in the wild, it could be possible to make changes that would help address the problem,” the scientists explained in this story.

Evolution at 'warp speed'

In the study, scientists found that offspring that had adapted best to the tank world were about three times worse reproducing in the wild than fish that hadn’t made the shift out of a hatchery.

“First-generation hatchery fish had nearly double the lifetime reproductive success (measured as the number of returning adult offspring) when spawned in captivity compared with wild fish spawned under identical conditions, which is a clear demonstration of adaptation to captivity,” the authors explained in the study.

The findings suggested a dramatic "tradeoff" had taken place rapidly, the authors said.

“Those with the greatest fitness in a captive environment produced offspring that performed the worst in the wild,” they wrote. “Specifically, captive-born individuals with five … or more returning siblings … averaged 0.62 returning offspring in the wild, whereas captive-born individuals with less than five siblings averaged 2.05 returning offspring in the wild.

“These results demonstrate that a single generation in captivity can result in a substantial response to selection on traits that are beneficial in captivity but severely maladaptive in the wild.”