The impact of hatcheries on salmonids 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.

The findings, published in Proceedings of the National Academy of Sciences, show a speed of evolution and natural selection that surprised researchers.

They confirmed that a primary impact of hatcheries is a change in fish genetics, as opposed to a temporary environmental effect.

“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. “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.”

Hatcheries are efficient at producing fish for harvest, the researchers said, but this and other studies continue to raise concerns about the genetic impacts that hatchery fish may have when they interbreed with wild salmon and steelhead, and whether or not they will help wild salmonid runs to recover.

These findings were based on a 19-year genetic analysis of steelhead in Oregon’s Hood River. It examined why hatchery fish struggle to reproduce in wild river conditions, a fact that has been made clear in previous research. Some of the possible causes explored were environmental effects of captive rearing, inbreeding among close relatives, and unintentional “domestication selection,” or the ability of some fish to adapt to the unique hatchery environment.