Possible influence of salmon farming on long-term resident behaviour of wild saithe (Pollachius virens L.)

Abstract

The culture of Atlantic salmon is one of the most developed aquaculture industries in the world. The production from smolt to market size usually takes place in sea cages in open waters, and these structures tend to attract wild fish, as they do for other farmed species. For salmon farming, saithe (Pollachius virens) is one of the most-frequently observed species around sea cages. An important question is whether the large concentration of salmon farms in some areas might alter the natural behaviour and migration pattern of wild saithe. We tagged 62 wild saithe with acoustic tags and followed their movements for up to 2 years in an area in Southwestern Norway with many salmon farms. Furthermore, nearly 2000 saithe were tagged with external T-bar tags to study migration beyond the study area. The recaptures of the T-bar tagged saithe from offshore areas suggest that the offshore migration routes of saithe are similar to published results from before salmon farming became significant in the area. However, a large proportion of the saithe population appears to remain in the release area and was observed at the salmon farms for much of the time. We conclude that the aquaculture industry is influencing the local saithe distribution. Large-scale population effects are more difficult to prove, but it is possible that the dynamic relationship between the coastal and oceanic phases has been altered.