The tool Norwegian scientists have developed will provide important knowledge in the debate about escaped farmed salmon.
In order to be able to use the tool on salmon from all breeding programmes and all Norwegian wild salmon stocks, the scientists have hunted for gene variants which occur with a different frequency in all farmed salmon compared to wild salmon. They have been successful with this task.
“We will use this tool to quantify the interbreeding of farmed salmon and wild salmon in a large number of salmon rivers in Norway,” says Sten Karlsson, who was the project manager from Nofima.
The degree of genetic changes in wild populations from interbreeding with escaped farmed salmon is best seen through changes in the gene composition of a wild stock over time, so consequently the measurements will be repeated over several years.
The scientists analysed more than 4000 genetic markers from four strains of farmed salmon from three breeding companies (12 in all) as well as historical DNA samples from 13 wild salmon stocks. Historical samples dating back to before the farmed salmon’s time are used to rule out that the wild salmon was affected by escaped farmed salmon.
From these the scientists found 60 genetic markers that may be used to distinguish a wild salmon from a farmed salmon, regardless of which river or breeding programme an individual salmon originates from. The work to develop this tool was expensive and laborious. However, with the 60 markers, the genomes of many salmon can now be analysed rapidly, efficiently and relatively cheaply.
Initially, around 1970, the farmed salmon was from a genetic perspective the same as a wild salmon. After approximately nine generations of breeding, the farmed salmon has a genetic composition that is more favourable for aquaculture. However, since the breeding stock originates from several Norwegian rivers, the farmed salmon only has genes from wild salmon.
On a global basis there are many species that are farmed parallel with wild stocks of the same species.
“The knowledge about this tool and the approach we have taken may be used to develop a similar tool for other species, for instance cod,” says Karlsson.
In several countries escaped farmed salmon are regarded as a major threat to some wild salmon stocks. However, to what extent farmed salmon interbreed with wild salmon, and how this affects the wild salmon populations remain unanswered questions
On the one hand the studies indicate that productivity in the wild salmon stocks is lower in rivers with large numbers of escaped farmed salmon. However, on the other hand, there are several factors that are believed to reduce the actual crossing of farmed salmon to wild salmon. Examples of this include the low adaptability of farmed salmon to conditions in the wild, poor capacity of reproduction and low survival of offspring. How this variation of genes develops over time and which important properties in wild salmon are changed with the counterproductive effect of natural selection remains an open question.
This newly developed tool will provide important answers about the degree of interbreeding between escaped farmed salmon and wild salmon, but it does not provide certain information about how this influences the properties of the salmon in the river. In order to understand this, it is important to study the link between genes and traits. Consequently, Sten Karlsson believes the tool should be improved in the long term:
“The genetic information needs to gradually be seen in connection with physical conditions in the river. It is also important to carry out follow-up studies in which the degree to which farmed salmon has crossed into wild salmon stocks is seen in connection with any changes in the productivity in the wild salmon stocks.”
Nevertheless, as a tool for determining the degree of gene variation, it provides a solid basis of discussion in the debate about escaped farmed salmon.