Gene Pollution Threatens Wild Salmon. A Solution: Sterile Farmed Fish

Farmed Atlantic salmon don’t fare well in the wild. That’s a problem when they escape their pens and breed with vulnerable Atlantic salmon populations, spreading their genes. Scientists are now urging aquaculture companies to consider raising fish that won’t reproduce.

Written by Alastair Bland Published on Read time Approx. 5 minutes
Wild Atlantic salmon, returning upstream from the sea to spawn in fresh water.David Cheskin - PA Images/PA Images via Getty Images

Scientists are proving the viability of a new idea to protect wild Atlantic salmon, a species that once spawned in great numbers in rivers from Portugal to the Arctic, as far west as Greenland and Canada and as far south as New York. Today, the Atlantic salmon is endangered or threatened in much of its range, with populations depleted by dams, water pollution and overfishing.

The idea? Sterilizing their farmed relatives.

A new study published in Royal Society Open Science found that triploid farmed salmon – that is, fish modified to contain three sets of chromosomes rather than the usual two – are essentially incapable of reproducing. Making farmed fish into triploids involves a process of pressure shocking their eggs minutes after fertilization, explained David Murray, a marine scientist at the University of East Anglia in the United Kingdom, who led the research. Experts said the approach could be used by salmon-farming companies to alleviate genetic pollution of wild populations, which happens when escaped farmed salmon breed with native Atlantic salmon and weaken their ability to survive or reproduce.

Several decades ago, conservationists and scientists began to suspect that the rise of the salmon-farming industry had something to do with the fall of wild Atlantic salmon, especially when farmed salmon escaped from their nets into the ocean.

An aerial view of a salmon farm in Norway. (Artur Widak/NurPhoto via Getty Images)

In a 2000 experiment, researchers tagged both farmed and native Atlantic salmon and released them into the River Imsa in Norway. The farm-born fish were “inferior” in their new environment, according to the published results, and were less than a third as likely as the wild fish to succeed in breeding. But by competing for resources, their presence appeared to harm the productivity of the native salmon.

“Farmed salmon have gone through several generations of selection for traits like rapid growth and resistance to certain diseases that are desirable for commercial production but not necessarily for survival in the wild,” said Ian Fleming, a professor at the Memorial University of Newfoundland in Canada, who led that study nearly 20 years ago.

Those findings didn’t prove that escaped farmed fish would reduce the long-term success of wild salmon, but they did support the hypothesis. Much research in the years since has backed the same conclusion and shown how, over time, escaped farmed salmon that breed with wild salmon spread genes that diminish the genetic fitness of wild Atlantic salmon.

Interbreeding isn’t the only risk that farmed salmon pose to wild populations. For instance, fish farms also can be breeding sites for sea lice parasites in both the Atlantic and Pacific oceans. But there is mounting evidence that what is called “genetic introgression” is a significant problem in the Atlantic, and it has been widely documented in rivers in Norway, home of the world’s largest remaining wild salmon runs and also a center of the salmon-farming industry.

Sten Karlsson, a scientist with the Norwegian Institute for Nature Research (NINA), has studied how quickly wild populations are becoming adulterated by the genes of farm-born fish. In a paper published in 2016 in the ICES Journal of Marine Science, Karlsson and three coauthors reported “significant farmed genetic introgression” in 52 percent of the 147 Norwegian salmon rivers they studied. Another paper to which he contributed in 2017, published in Nature Ecology & Evolution, found that the interbreeding between farmed salmon and wild salmon had negative effects on “fitness-related traits” – most notably causing salmon to migrate from the sea into freshwater to spawn at a younger age. This means they were smaller, and because size is directly related to a salmon’s reproductive success, these genetically compromised fish would in theory face lower odds of producing offspring, according to the paper.

Norway is the world’s biggest exporter of farmed Atlantic salmon. (Artur Widak/NurPhoto via Getty Images)

In a 2017 paper in the ICES Journal of Marine Science, Torbjørn Forseth, a senior research scientist at NINA, concluded that genetic introgression from farmed to wild salmon is the largest current threat to the persistence of Atlantic salmon runs in Norway and other nations.

Today, as salmon farming booms and wild salmon numbers wane almost everywhere, the new research suggests one of the most effective ways to prevent the interbreeding of farmed and wild salmon is the use of triploid fish in commercial production.

“We show that female triploids are sterile and do not develop gonads,” Murray and his coauthors wrote in their study. And while males still produce sperm capable of fertilizing wild salmon eggs, the development and survival of the fertilized eggs was less than 1 percent compared with their two-chromosome counterparts. He said the Norwegian egg supplier AquaGen took an interest in the research, providing space and materials.

The researchers also looked at the nutritional value of triploid fillets. It turned out the triploid salmon flesh contained lower fat overall – not necessarily a desirable thing in terms of marketability – but the ratio of healthy omega-3 fatty acids as a portion of total fat content was higher.

“What this means is that if we can manage to get triploid fish to have the same amount of fat content as the diploids, then they should have significantly higher nutritional benefits to human beings,” Murray said. Improving the production of total body fat could require selecting new genetic strains of Atlantic salmon while also optimizing feed formulas and making them more sustainable, he noted.

Karlsson believes the salmon-farming industry should lose no time in incorporating triploid fish into their production systems in an effort to save wild salmon. He said it’s becoming increasingly clear from the mounting body of research that the salmon-farming industry could choose to make its fish almost entirely sterile.

“I think that there is an urgent need to stop or drastically reduce escapees and genetic introgression into wild salmon populations,” Karlsson said.

As the scientific evidence accumulates that escaped farmed salmon weaken the survival fitness of wild populations, a fresh look at fish-farming technologies will be in order, the researchers said.

“There was a time when we were asking, first of all, ‘Do salmon really escape?’ and then, ‘Do they really enter the rivers?’ and then, ‘Do they really interbreed with wild salmon, and if so, do they leave offspring, and if so, does it really change the wild populations?’” Karlsson said. “Now, the next really big question is whether the genetic introgression actually has a direct effect on the productivity of the wild salmon populations, and all the evidence and theoretical framework is pointing that way.”

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