It is estimated that some two million Atlantic salmon escape each yearfrom fish farms in the North Atlantic, equivalent to about 50% of the total wild adult salmon in the sea. Escaped salmon can "enter rivers where they breed and interbreed with wild salmon, thereby potentially changing the genetic make-up of wild populations. The importance of such changes in the survival of the remaining wild populations of Atlantic salmon has been a matter of debate for the past decade but little empirical evidence has been available." A new paper in the journal Proceedings of the Royal Society of London B reports on a ten-year study which "examined, for the first time, multiple families of both first and second generation hybrids between wild and farm salmon in the freshwater and marine life history phases."

The study found that "farm Atlantic salmon have both genetic and competitive impacts on wild populations. As a result of domestication over several generations, genetic changes have reduced the capability of farm salmon to survive in the wild, especially during the marine phase. Overall farm salmon showed an estimated lifetime success of 2% of that of the native wild salmon, and results were consistent in three separate cohorts. Hybrids were intermediate in lifetime success between the wild and the farm (27% - 89% relative to wild). In the second generation hybrids, some 70% of the embryos died in the first few weeks as a result of outbreeding depression. This condition is caused by genetic incompatibilities between the parents but does not occur until the second generation when recombination of the parental genes has taken place. Taking our results alongside those available in the literature, it can be concluded that genetic changes leading to reduced survival in the wild is a feature of all domesticated salmon and trout and consequently hybrids between farm and wild fish also have reduced survival."

Furthermore, farmed salmon "have been selectively bred for faster growth. In this study, juvenile farm salmon and hybrids grew faster than wild juveniles and competitively displaced the wild fish from the river. The poorer survival of farm and hybrid salmon at sea means that they do not compensate for these displaced wild juveniles and so there is an overall reduction in returning adults.

"The combined impact of hybridization and competition means that, when a large number of farm salmon spawn in a river, the number of adult salmon returning to the river and the potential offspring production in the next generation are reduced. The degree of the impact will depend on various factors including relative numbers of wild and farm salmon, and juvenile habitat availability. As repeated escapes are now a common occurrence in some areas, a cumulative effect is produced generation upon generation, which could lead to extinction of endangered wild populations as a result of this 'extinction vortex'."

December 2003