Ocean migration and recaptures of tagged, triploid, mixed-sex and all-female Atlantic salmon (Salmo salar L.) released from rivers in Ireland

Fiftyfour thousand diploid and triploid Atlantic salmon were tagged with coded-wire micro-tags and released to the sea as smolts from two freshwater sites in Ireland in 1996 and 1997. Over 36,000 were mixed-sex groups in which the triploids (MS3N) were treated batches and the diploids (MS2N) were untreated batches of a single group of fertilized eggs at each site and year class. Over 17,000 were all-females, the triploids (AF3N) being treated batches and the diploids (AF2N) untreated batches of a single group of fertilized eggs at each site and year class. Adult tagged fish were subsequently recovered in the coastal fishery and in freshwater traps and angling fisheries in the rivers of release during 1997, 1998 and 1999. Recoveries from migration ranged from 0.08 to 9.79%. Diploid recoveries were within the normal ranges for salmon ranched from these sites, being between 0.64 and 1.82% at one site and between 3.85 and 9.79% at the other. Triploid recoveries, which occurred in the coastal fishery and in freshwater, ranged from 0.08 to 0.40% at the first site and from 0.98 to 2.05% at the other. Overall, triploid recoveries were between 12 and 24% of their diploid siblings within each release group. There were two peaks of recapture in the coastal fishery, the first in mid-June and the second in mid to late July. All-female diploids appeared to enter the fishery in advance of mixed-sex diploids. Triploids were the latest to arrive. There was no evidence of increased straying in triploids relative to diploids and the mean lengths, weights and condition factors were not significantly different between the ploidy groups. Triploidisation therefore reduced the home water harvest of treated salmon to less than 25% but did not completely eliminate triploid recaptures. The way in which triploidisation may influence the return of salmon from the oceanic feeding grounds is discussed.