FROM Norway to Canada, farmed Atlantic salmon are escaping in such large numbers that they are聽a聽threat to the local salmon. This could be prevented by making the farmed fish sterile. But there is a catch: the sterile fish聽are less nutritious.
Salmon farming has satisfied soaring demand without the need to catch ever more wild salmon. But it has also created an entirely new threat.
Farmed salmon have been transformed into a domesticated animal by selective breeding. They eat more, grow faster and have less fear of predators. This means they are less likely to survive in the wild and so weaken wild salmon when they interbreed with them.
Advertisement
Norway is tackling the problem in several ways, from trying to reduce the number of escapees to having snorkellers spear farmed salmon that turn up in rivers before they can interbreed. But some experimental farms there are already testing the ultimate solution: making farmed salmon聽sterile.
One way to do this is to briefly pressurise fertilised eggs, which makes them retain an extra set of chromosomes. The resulting fish are triploid: they have three sets of chromosomes.
This method is already used worldwide to sterilise trout for release in rivers. Now David Murray at the University of East Anglia, UK, has confirmed that triploid salmon are also effectively sterile. The females didn鈥檛 develop gonads and while the males did produce sperm, less than 1 per cent of the eggs they fertilised survived to the 5-week stage, when the experiment stopped (Royal Society Open Science, DOI: 10.1098/rsos.180493).
鈥淎 quarter of the wild salmon in Norway have 10聽per cent or more DNA from farmed salmon鈥�
The method doesn鈥檛 need to be聽100 per cent reliable because a聽little interbreeding doesn鈥檛 matter: any disadvantageous gene variants will soon be eliminated by evolution. The聽problem comes when so many escapees interbreed that they introduce 鈥渂ad鈥� genes faster than they can be weeded out.
鈥淚f this is happening year after year after year, and there are enough of them, you are swamping the local population,鈥� says Kevin Glover at the University of Bergen in Norway.
A quarter of the wild salmon in Norway have 10 per cent or more DNA from farmed salmon. The problem could be even worse in other countries such as Scotland, but no one is checking.
Nor do we know just how much of a threat this 鈥済ene pollution鈥� is, because wild salmon populations are declining for many other reasons, such as dams blocking their way to spawning grounds.
鈥淲e don鈥檛 have evidence of wild populations collapsing because of [interbreeding],鈥� says Glover. 鈥淭hat does not mean it has not occurred.鈥�
While triploid farmed salmon could eliminate this threat to wild salmon, Murray鈥檚 work reveals another problem. The triploid salmon have a lower fat content and thus less beneficial omega-3s. The omega-3 content of farmed salmon is already falling because of efforts to make it more sustainable by relying less on wild-caught fish as feed.
And while triploid fish are doing fine on some experimental fish farms in Norway, says Glover, in others they are not. Unlike triploid trout, triploid salmon appear to be less resilient. It might be necessary to breed a strain of salmon that copes better with triploidy and has a higher fat content, says Murray.
Others are working on ways of making salmon sterile by genetic modification but it won鈥檛 be easy聽to get regulatory approval. 鈥淭riploidy is the method that is聽ready right now, that can be implemented much more easily,鈥� says Murray.
This article appeared in print under the headline 鈥淪terile farmed fish may help protect wild salmon鈥�
