An evaluation of the use of triploid Atlantic salmon (Salmo salar L.) in minimising the impact of escaped farmed salmon on wild populations

The impact of escaped farmed salmon on wild populations may have potentiall y negative genetic and ecological effects. There is widespread evidence tha t farmed salmon interact with wild salmon. The use of sterile fish in cultu re has been proposed as a means of eliminating genetic interaction and mini mising the ecological effect of farmed salmon. In this study, the migration behaviour of groups of triploid salmon were investigated through the contr olled release of microtagged triploid and diploid stocks on the western coa st of ireland. Mixed-sex and all-female stocks of ranched grilse origin wer e triploidised using hydrostatic pressure. Smolts were ranched from the hat chery of origin and two groups of post-smolts were released from cages in a marine site. The return of adult salmon from these experimental release gr oups to coastal and freshwater capture sites was monitored as part of the I rish national coded wire tag recovery programme. The return of triploid sal mon from each of the release groups, both to the coast and to fresh water, was significantly reduced compared to diploid salmon. The highest percentag e return to fresh water (2.25%) was in the ranched mixed-sex diploid group. In contrast, no salmon from the cage release groups returned to the hatche ry location on the Burrishoole river system and recoveries in other freshwa ter systems were low (< 0.01%). The return of a small number of hormonally deficient, sterile triploid female fish suggests that migration to fresh wa ter is not inextricably linked with reproduction. The substantially reduced return of hormonally competent triploid males to the coast and to fresh wa ter, indicates that other factors may have an effect on their marine surviv al. The reduced return of triploid salmon to the coast and to fresh water, together with their inability to produce viable offspring, demonstrates the potential for triploidy as a means of eliminating genetic interactions bet ween cultured and wild populations? and of reducing the ecological impact o f escaped farmed fish. (C) 2000 Elsevier Science B.V. All rights reserved.