( BBC ) - Animals bred in captivity to help conservation programmes can quickly become less fit for survival in the wild, research suggests.
US scientists found steelhead trout reared in hatcheries were much less good at reproducing than wild fish.
Writing in the journal Science, they say the use of captive breeding needs careful re-consideration.
For some animals, such as amphibians, captive breeding is being used more and more as wild habitats disappear.
"This study proves with no doubt that wild fish and hatchery fish are not the same, despite their appearances," said Michael Blouin of Oregon State University in Corvallis, US, who oversaw the research.
Steelhead populations in rivers along the US west coast are listed as threatened or endangered by the US Fish and Wildlife Service. Captive breeding and release is one of the measures being used to safeguard numbers of a fish that is much prized by anglers.
The Oregon State team had previously shown that first-generation captive-reared steelhead were just as successful reproductively as their wild-reared relatives, producing just as many young.
In the new study, they compared the success of fish hatched from two captive-bred parents with those possessing one captive-bred and one wild parent.
The results were startling, with the first group about 40% less successful than the second.
"For fish to so quickly lose their ability to reproduce is stunning, it's just remarkable," said Professor Blouin. "If it weren't our own data, I would have difficulty believing the results."
Hatchery programmes for steelhead and other salmonids (species within the salmon family) release more than five billion juvenile fish into Pacific waters each year. So if captive breeding does result in fish markedly less fit and less able to reproduce in the wild, the implications could be significant.