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 between cultured and wild populations, and of reducing the ecological impact of escaped farmed fish. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Triploid; Atlantic salmon; Migration; Escapes
1. Introduction
In recent years, concerns have been expressed about the genetic and ecological Ž
. consequences of interaction between wild and cultured Atlantic salmon Salmo salar L.
Ž .
Hindar et al., 1991; Saunders, 1991; Heggberget et al., 1993; Ferguson et al., 1995 . The escape of cultured salmon and subsequent interaction with wild salmon in rivers
Ž has been widely reported Gausen and Moen, 1991; Webb et al., 1991; Carr et al.,
. 1997 . Evidence of genetic changes in wild salmon populations has been demonstrated
in recent studies, where adult farmed salmon are known to have escaped and interbred Ž
. with wild salmon in adjacent rivers Crozier, 1993; Clifford et al., 1998 . The ecological
and behavioural interactions between cultured and wild salmon were reviewed by Ž
. Jonsson 1997 , highlighting differences in migration behaviour and reproductive suc-
cess. Commercially farmed salmon are frequently derived from non-indigenous sources
that have been selectively bred for production traits, such as rapid growth and late Ž
. maturity Gjedrem et al., 1991; Friars et al., 1996; Gjoen and Bentsen, 1997 . Escapes of
commercial stocks are considered to present a potential genetic threat to wild popula- tions, particularly to small indigenous populations in local rivers, where interbreeding
could result in significant genetic change. Where adaptive differences exist between native and cultured salmon, interbreeding may lead to an overall reduction in survival
Ž .
and recruitment in the native population Verspoor, 1997 . Genetic changes can also occur indirectly through competition, which may cause a reduction in the effective
Ž population size, resulting in inbreeding and loss of genetic variability Hindar et al.,
. 1991 .
The use of sterile fish in culture has been proposed as a measure to minimise the Ž
genetic impact of cultured salmon on wild populations Hindar et al., 1991; Anon., .
1994; Hansen and Youngson, 1998 . Few published data are available on the behaviour of sterile fish, but a study on the migratory behaviour of hormone-sterilised coho salmon
Ž .
Oncorhynchus kisutch Walbaum found that they remained in the marine environment Ž
. Solar et al., 1986 .
The most effective method of the sterilisation of Atlantic salmon is high pressure Ž
. induction of triploidy in newly fertilised eggs Johnstone et al., 1991; Johnstone 1998 .
Ž .
In diploid fish, the cell nucleus contains two sets of chromosomes 2N . However, in Ž
. triploid fish, a third set of chromosomes 3N is retained in the cell nucleus resulting in
functionally sterile fish. Although there are many similarities between triploid and diploid fish, there are basic differences which are addressed in a review by Benfey
Ž .
1999 .
The objective of this study was to evaluate the potential use of triploid salmon in minimising the genetic and ecological impact of farmed salmon on wild populations.
The return migration to Irish coastal waters and to fresh water of triploid salmon was monitored through a controlled release programme. Triploid cultured salmon and diploid
controls, which had been microtagged, were released from freshwater and marine release sites. The retrieval of tag data was made possible by the Irish national coded wire tag
recovery programme, which was initiated in 1980 to estimate the marine survival of Irish salmon stocks and exploitation rates by high seas fisheries and home water
Ž .
commercial and recreational fisheries Browne, 1982 .
2. Materials and methods
