A popular way to manage endangered Pacific salmon may do more harm than good. Many salmon recovery plans supplement wild populations with hatchery-raised fish. But new research shows that hatchery salmon can readily adapt to captivity and transfer these traits to wild populations.
“As the population adapts to the hatchery, its survival rate in the wild goes down,” says Michael Ford of the National Marine Fisheries Service’s Northwest Fisheries Science Center in Washington, who presents this work in the June issue of Conservation Biology.
Previous studies have shown that hatchery-raised Pacific salmon can adapt to captivity. For instance, compared to wild fish, hatchery coho salmon don’t compete as well for mates, and hatchery steelhead are not as good at avoiding predators. However, there have been few studies of whether hatchery salmon affect wild populations genetically, and some people have suggested that the former could completely substitute for the latter.
To help determine how hatchery salmon affect wild populations, Ford used a genetic model that accounted for differences between captive and wild environments. He found that hatchery salmon can develop “captive traits” (i.e., those that are optimal in captivity) and introduce them to wild populations, which decreases the survival of salmon in the wild. The greater the difference between the two environments, the greater the impact of hatchery fish on wild populations. “When the captive environment is very different from the wild, within 20 generations the population can lose much of its ability to survive in the wild,” says Ford. “In other words, the population adapts to the hatchery and becomes dependent on the hatchery for its survival.”
Ford found that there are no easy fixes: the wild population shifts toward “captive traits” even if managers keep adding wild-caught salmon to the hatchery population. To help counteract this shift, Ford suggests reducing the salmons’ adaptation to hatchery conditions by using “natural” methods of breeding and rearing. This is being studied by other researchers at the Northwest Fisheries Science Center.
In addition, there might be an even better way to help counteract the shift toward captive traits. When Ford extended the model to account for reproductive rate and other demographic factors, he found that hatchery-supplemented wild populations develop captive traits faster when their habitat is limited. This suggests that “conserving or restoring a population’s habitat may be the most effective method of preventing [trait] change during supportive breeding,” says Ford.
This research also has implications for the impact of aquaculture on conservation. Ford found that even at low escape rates, aquaculture-raised fish could decrease the survival of wild populations over time. This is most likely to be a problem in areas with intensive aquaculture such as Norway, where salmon that escape from fish farms comprise more than half of the wild populations in some areas.
Ford, M.J. 2002. Selection in captivity during supportive breeding may reduce fitness in the wild. Conservation Biology 16:815-825.