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Every spring, hatcheries in Alaska release more than a billion year-old pink and chum salmon. The fish spend a year out at sea growing up, before they return to be caught by the state’s fishing fleet. At least, that’s how it’s supposed to work.
Of the roughly 1.8 billion fish released each year in the state, about 100 to 200 million make it back from the ocean. Of those that survive to adulthood, fishers catch almost 99 percent. Inevitably, though, some salmon evade the nets and make their way into local rivers and streams to spawn alongside their wild relatives.
There are concerns that these hatchery-raised fish might be negatively affecting wild salmon populations, either by disrupting their spawning, or by breeding with them and weakening the gene pool.
“Wild returns in Alaska are stable, but there is evidence from the lower 48 that hatchery fish have reduced spawning success,” says Kyle Shedd, a geneticist with the Alaska Department of Fish and Game.
The Alaskan hatchery program takes precautions to reduce the risk of strays, says William Smoker, a fisheries biologist at the University of Alaska Fairbanks. Hatcheries release fish in areas without large and important natural runs of salmon, and the releases are timed to try and keep them separate from the wild fish. Pink and chum salmon also have much shorter life cycles than other salmon—just two years from egg to adult—so the hatchery-reared fish spend less time in captivity than other farmed salmon and therefore may be less likely to suffer the negative effects of domestication. But there are still worries about the wild and farmed populations mixing.
New research by Shedd and colleagues adds some discouraging evidence that backs up those fears. Based on research conducted over the past two years, hatchery fish appear to have reduced spawning success compared to wild fish. This means that they could be weakening the whole population when they breed with wild fish.
It’s easy for scientists to identify the hatchery strays among the carcasses of spawning fish, says Shedd, because hatcheries adjust the water temperature in their tanks in a way that leaves a telltale mark on the fish’s tiny ear bones. For the research, Shedd and his team recorded the genetic fingerprint of identified hatchery strays, then checked the genes of the next generation to see how the strays’ spawning success compared with wild fish.
So far, Shedd only has data from one stream, but the initial results are worrying. Hatchery-raised fish produce significantly fewer offspring than wild ones. “The results are concerning,” Shedd says, but he adds that there is not yet enough data to allow policymakers to make decisions.
Shedd wants to extend the analysis to the next generation—the strays’ grandchildren—to find out whether the trend of low reproductive success is heritable, and so could continue to hurt future generations. He’s also hoping to tease out why hatchery-raised fish are less successful. Are the hatchery fish passing on bad genes, or is it due to something like the timing of their spawning run being suboptimal?
Smoker says that though these are only preliminary results, it is an important technical achievement to track the offspring of hatchery strays across generations. “This promises to bring a real wealth of information in the future,” Smoker says. “We’ll soon be at the point where we can talk with some real data about reproductive success.”