Captive-bred steelhead trout used to boost declining fish populations are less fit than wild-bred trout, raising concerns over conservation programs for endangered species, says a new study.

A hatchery-born steelhead trout waiting for spawning in the South Santiam Hatchery in Oregon.
(Hitoshi Araki, University of Oregon)

Research published in Friday's edition of the journal Science shows that hatchery-raised steelhead trout dramatically and unexpectedly lose their ability to reproduce in the wild.

Oregon State University researchers found the reproductive success of trout, a species critical to many healthy aquatic ecosystems, drops by close to 40 per cent for every generation they spend in a hatchery.

"For fish to so quickly lose their ability to reproduce is stunning, it's just remarkable," Oregon State University zoologist Michael Blouin said in a release.

"We were not surprised at the type of effect but at the speed. We thought it would be more gradual. If it weren't our own data I would have difficulty believing the results."

The finding raises concerns over restocking programs around the world that rely on hatchery-raised fish to re-establish healthy numbers of top-predator fish and keep the aquatic food chain in balance.

Researchers studied 15 years of DNA data from about 15,000 wild and hatchery-bred fish in the Hood River in Oregon.

Genetic changes appear to be key, the scientists said, and probably stem from evolutionary pressures that quickly select for characteristics favoured in the safe world of the hatchery, but not in the comparatively hostile natural environment.

Evolutionary selection is rapid for certain characteristics in fish because of the huge numbers of eggs and smolts they produce and the relatively few fish that survive to adulthood. For every 10,000 eggs, roughly 100 adults survive.

Offspring that inherit traits favoured in hatcheries can be at a serious disadvantage in the wild where they face an uncertain food supply and hungry predators.

Even a few generations of domestication may have significant negative effects, and repeated use of captive-reared parents to supplement wild populations "should be carefully reconsidered," the scientists said in their report.

Both steelhead and salmon hatcheries get their brood stock and eggs from fish repeatedly bred in hatcheries.

"What happens to wild populations when they interbreed with hatchery fish still remains an open question," said Blouin. "But there is good reason to be worried."