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Farmed Escaped Salmon Change Wild Salmon Stocks

Farmed escaped salmon affect both the genetic composition of wild salmon and their geographic population structure. However, no direct relationship has been established between the frequency of escaped salmon in a given river and the extent of the genetic impact on that specific population.

These findings come from a study of over 3,000 salmon collected from 21 distinct populations in Norway during the period from1970 to 2010. Results have been recently published in the PLoS ONE science journal.

- Each year hundreds of thousands of salmon escape from commercial farms, but until now the extent to which the genetic composition of wild salmon is affected by this has not been known. Escapees enter rivers, outnumbers wild fish in some years, and have been documented to spawn. The proportion of escaped to wild salmon varies widely in time and space, says Øystein Skaala, a geneticist at Norway’s Institute of Marine Research who helped to conduct the study.

Major Changes in Four of 21 Rivers

- Results clearly indicate a reduction in genetic differences among salmon populations over time.  In six of the 21 populations surveyed, distinct genetic changes were observed over time.  In four of these rivers (Opo, Vosso and Lone in Hordaland and Western Jakobselv in Finnmark) the changes were highly significant. Possible reasons for this were investigated; it was concluded that the differences are due to cross-breeding between escaped and wild salmon, says Skaala.

Unpredictable Effects

- Considering the numbers of escapees observed in Norway in the past 20-30 years, and the high proportions of them that may enter into some wild populations, one surprising result from the study was the inability to detect genetic changes in 15 of the populations surveyed. In some of these rivers, such as the river Etne in Hordaland, there have been large numbers of escaped fish over time, says Skaala.

This suggests that in many places escapees have a reduced likelihood to genetically mix with the native populations, and therefore, the effects of escapement are unpredictable. The authors of the PloS ONE paper concluded that density of the native population may play a key role in whether escapees manage to genetically introgress with the wild population. Where there is a robust wild population, it is likely that the escapees don’t manage to interbreed as effectively as in rivers where there is less spawning competition.

- The study concludes that there have been significant changes in four of the 21 investigated populations as a result of cross-breeding between farmed escapees wild salmon over a thirty-year period. The study also shows that much of the primordial geographic distribution in populations of wild salmon remains intact, says Skaala.

Provided Materials

- One prerequisite to conduct this study was to obtain a substantial number of scales collected from wild salmon stocks which had not yet been significantly affected by farmed escpaees. Without these scales it would not have been possible to conduct this study. Accordingly, a sincere thanks is extended to helpful colleagues in other departments, particularly those at the Norwegian Directorate for Nature Management, for having provided these materials, says Skaala.

The Article

Kevin A. Glover, María Quintela, Vidar Wennevik, François Besnier, Anne G. E. Sørvik, Øystein Skaala. 2012. Three Decades of Farmed Escapees in the Wild: A Spatio-Temporal Analysis of Atlantic Salmon Population Genetic Structure throughout Norway. PLoS ONE. 2012.