New research conducted in Ireland into inter-breeding between wild Atlantic and farm salmon suggests the mixing of salmon types could be a significant threat to the wild species.
Domesticated salmon, hybrids and their progeny were weaker and less likely to survive at all stages of the salmon life cycle in the wild. However, the farm salmon were also more likely to outdo the wild salmon for space and food as they are more aggressive and larger and took up more space on the riverbeds.
Deliberately introducing farm salmon or trout for re-stocking purposes, as is often the case in Scotland, was also found to threaten the wild populations to an even greater extent.
The ten-year study of two generations of salmon was jointly directed by Dr Philip McGinnity, of the Marine Institute, and Prof Andy Ferguson, of the school of biology and biochemistry at Queen's University, Belfast. It was carried out on the Burrishoole Catchment in Co Mayo, a centre for salmon research for over 50 years.
The findings were published yesterday in the scientific journal Royal Society Proceedings. The wild and farmed salmon were studied in fresh water and marine phases. These included a control group involving wild salmon parents, a group where the father was wild and the mother farmed, and vice versa, and a fourth group where both parents were farm salmon.
Farm salmon have been selectively bred for faster growth and the study showed that juvenile farm salmon and hybrids grew faster than wild juveniles and displaced the wild fish from the river. Fewer of these "survivors" returned from the sea after migrating from the rivers and so there was an overall reduction in returning adults. Over a number of generations this could lead to the extinction of wild populations in areas where escapees were common occurrences.
It is thought some two million salmon escape each year from fish cages in the north Atlantic - this represents about 50 per cent of the total number of wild adult salmon in the sea. However, in Ireland escapees are low and represent less than 1 per cent of total commercial catches, according to the Marine Institute.
It was quickly established that farmed salmon escapees and wild salmon do in fact inter-breed, Dr McGinnity said.
The research was only possible because of the emergence of DNA profiling. They had set out to answer a simple question - was the progeny of escaped farm salmon any different - and he believed they had answered this question. The wild was stronger and the farmed survived less well in the wild.
One of the major findings was that deliberate stocking of large numbers of cultivated or non-native pure farm juveniles to supplement wild populations was likely to be more damaging than accidental escapes. These juveniles would outcompete the wild salmon juveniles. The study therefore had significant implications for salmon and trout stocking policies in Ireland and internationally.
The results of the study have been widely welcomed by representatives of both the wild and farm sectors. Dr Kjetil Hindar, of the Norwegian Institute for Nature Research and one of the world's leading fish geneticists, said a long-term study of the fitness of cultured salmon in the wild was unique and would be of major importance for aquaculture.
Dr Peter Heffernan, chief executive officer of the Marine Institute, said the study proved Ireland had the capability to undertake world class marine research.