A single generation in captivity cuts reproductive success in the wild.
Breeding a species in captivity offers a strategy to save threatened populations, but a new study with steelhead trout shows that even a few generations of domestication can hinder the reproductive success of these animals in the wild.
Every year, fish hatcheries release more than 5 billion juvenile salmon and steelhead trout in the Pacific Northwest, primarily for harvest purposes. Recently, the practice has been tested as a means to supplement declining wild populations with captive-bred individuals.
But after time in captivity, these fish lose their ability to avoid predators and often differ in their mating behaviours. Researchers assumed that such changes required many generations to take hold — new research now shows that it can happen much more quickly.
Captive at heart
In 1991 the Oregon Department of Fish and Wildlife (ODFW) in Salem began supplementing wild steelhead trout populations in Oregon’s Hood River. Every year, wild steelhead are collected on the river, allowed to breed at the nearby Parkdale Hatchery, and their offspring are released a year later. When these fish are ready to spawn, they swim upriver where they are trapped at the Powerdale Dam, which forms a complete barrier across the river. There, project staff catalogue, measure and then sample DNA from almost every adult fish, before releasing them to spawn upstream or returning them below the dam.
During 3 years in the late 1990s, large numbers of captive-reared fish returned to the dam and were crossed with wild fish. By comparing reproductive outputs of fish with one wild parent to those with two wild parents, researchers led by Michael Blouin of Oregon State University in Corvallis estimated the consequences of just one generation of captive-rearing.
“What’s starkly clear,” says Blouin, “is that using hatchery fish to produce another generation of hatchery fish causes a very rapid decline in fitness.” Simply raising fish in captivity cuts their reproductive success by 15%. As the researchers report in this week’s Science 1, having one captive parent in addition to being raised in captivity will reduce reproductive success by an additional 45%, thanks to genetic effects.
Scientists working with steelhead agree that the new research could have a lasting impact on management decisions. Barry Berejikian, who runs a project to supplement fish numbers on the Hood Canal for the National Oceanic and Atmospheric Administration, says there may be ways to minimize the problem. His project, for instance, does not require artificial spawning. Eggs are collected from the wild, raised in captivity, and released again, without any mating between captive fish.
Even so, Berejikian questions whether Blouin’s results can be generalized to other captive-bred species. “I’m yet to be convinced that this is going to be the case for other salmon,” he says.
Kevin Goodson, a conservation coordinator at the ODFW, agrees that steelhead trout have a unique life history, and does not believe the study will put an immediate end to programs in other fish such as Chinook salmon. “We’ve got some situations where there aren’t a lot of options,” he says.
Blouin, however, says that his research adds to the weight of previous studies showing fitness losses in hatchery fish. And his findings may not be all bad news. If genetic changes appear so quickly when wild fish are brought into captivity, it may only take a few generations back in the wild for reproductive fitness to recover through natural selection. “What happens when you add the first generation of hatchery fish to a wild population?” he asks. “We don’t know yet.”