When wild Atlantic salmon breed with escaped farmed salmon, their descendants grow faster and mature at a younger age, undermining the ability of the species to survive and reproduce in its natural environment.
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. By examining a scale from each fish and genotyping just over half of them, the team analysed what genetic ancestry with farmed fish means for their pace of development.
The biggest change came at an early stage of life when the fish in fresh water adapt themselves for saltwater before heading out to sea, a process known as smolting. On average, it happened much earlier for salmon descended from farmed fish.
Later in life, these salmon also matured more quickly and returned from the sea earlier to lay eggs. The net result: females descended from farmed salmon reached maturity 0.29 years younger and males 0.43 years younger than genetically wild ones.
This faster pace of life due to genetic contamination is bad news because it is linked to a whole suite of traits that make salmon less well adapted to their environment, such as increased boldness and aggression. Studies have found that the offspring of farmed salmon are less likely to survive as juveniles in the wild, in part because they are more susceptible to predators.
Bolstad says that as long as the flow of genes continues, “it will by all probability decline the population figures because it makes the population on average maladapted”.
The overall picture masks one striking finding, which is that the impact of farmed fish genes varies drastically between populations of salmon. For example, in those communities where natural selection had already produced extremely fast-growing fish, the introduction of farmed fish genes actually acted as a brake rather than an accelerator.
The differences suggest that conservation efforts to limit the impact of farmed fish genes should be directed at local rather than national levels, says David Murray at the University of East Anglia in Norwich, UK. “This is something we haven’t seen before regarding the impacts of farmed gene introgression and could only be determined from an experiment of this scale and scope,” he says.