The life cycle of wild salmon is being altered by breeding with farmed fish

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Genetic contamination is threatening the evolutionary fitness of Atlantic salmon. Wild fish are breeding with escapees from fish farm



22 December 2021

By Adam Vaughan

A salmon farm in Norway

Marius Dobilas/Shutterstock

When wild Atlantic salmon cross with farmed salmon, their offspring grow faster and mature earlier than the species’ natural population.

Aquaculture is expected to meet most of the world’s extra demand for fish in the coming decades. Fish farming can harm wild populations in various ways, from genetic contamination to disease, but most of our understanding of these dangers has been gleaned from experiments in laboratories and controlled settings.

To get a better idea of how the spread of farmed salmon’s genes is affecting wild fish, Geir Bolstad at the Norwegian Institute for Nature Research in Trondheim and his colleagues collected scales from 7000 adult salmon in 105 rivers in Norway, the world’s biggest producer of farmed fish. The team examined each fish’s scale and genotyped just over half to determine what their genetic ancestry means for their development.

The biggest change occurred in the early stages of life, when fish in freshwater adapt to saltwater and then head out to sea. This process is called smolting. It happens on average much sooner for salmon that descend from farmed fish.

Later in life, these salmon matured faster and returned from sea to lay eggs earlier. Females descended from farmed salmon reached adulthood at 0. 29 years younger and males 0. 43 years younger than genetically wild ones.

Genetic contamination can lead to a faster rate of life and a host of other traits that make salmon less adaptable to their environment. These include increased boldness, aggression, and aggressiveness. Research has shown that wild salmon offspring are less likely than their wild counterparts to survive as juveniles, partly because they are more vulnerable to predators.

Bolstad states that the flow of genetic material will continue to decline the population numbers because it makes the population averagely maladapted.

The overall picture hides a striking finding: the effect of farmed fish genetics varies greatly between salmon populations. In communities that had previously produced very fast-growing fish by natural selection, farmed fish genes acted more as a brake than an accelerator.

These differences indicate that conservation efforts to reduce the impact of farmed genetics should be directed at the local level, rather than at national levels, suggests David Murray, University of East Anglia, Norwich, UK. He says, “This is something that we haven’t seen before in terms of the effects of farmed gene introgression. It could only be determined through an experiment of such scale and scope.”

Journal reference: Science Advances, DOI: 10. 1126/sciadv.abj3397

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