Can we identify escaped salmon by means of their DNA?

Escaped salmon are a problem for the fish-farming industry. Is it possible to identify the fish-farm from which salmon have escaped by testing a sample of their DNA? Scientists at the Institute of Marine Research in Bergen have been looking into the prospects of doing so.

Escapees are a major problem for fish farming, not only for the farmers who lose their fish, but also for stocks of wild salmon. This is because cultivated salmon have been bred to thrive in an artificial environment without predators, with plenty of food and without the need to migrate and orient over huge geographic distances. When these characteristics of farmed salmon are cross-bred into wild salmon stocks, we end up with salmon that are less well adapted to life in their natural environment, and such stocks suffer higher mortality rates.

For this reason, it has been suggested that natural DNA markers that are found in all salmon might be used to trace escapees back to the ongrowing farm from which they have escaped, so that future inspections could be concentrated on farms that tend to lose fish.

Breeding line identified with 95% certainty

In order to find out whether this is actually possible, scientists at the Institute of Marine Research carried out a pilot study. The first things that they found out were that there are major differences among breeding lines, and that they could identify the line that any given salmon comes from, to a certainty of better than 95%.

However, in the aquaculture industry, roe and fish are sorted, grouped and distributed from breeding populations to broodstock stations and on to hundreds of smolt farms, and then further on to hundreds more ongrowing farms. These logistical conditions mean that DNA profiles based on references from breeding plants or broodstocks would not be sufficient to identify the ongrowing farm from which an escaped fish had come.

Useful when many fish have escaped at the same time

If large numbers of escaped fish are discovered soon after they have escaped, however, DNA profiles can still identify the ongrowing farm that they came from, at least in some cases. In the pilot study, the scientists took samples from seven different deliveries of smolt to four ongrowing farms. With the aid of seven natural DNA markers it was possible to identify the fish farm from which seven out of ten “escapees” had come. This means that the method could be of interest in major escape episodes.

To the present day, there exists no professional monitoring system for aquaculture, in contrast to the monitoring programmes that are under way in Norway’s marine regions and in traditional fisheries.

Any method of identifying escaped fish would be dependent on a operational monitoring system capable of discovering escapees rapidly, sampling them and sending the samples for analysis.