Adult Return of Farmed Atlantic Salmon Escaped as Juveniles into Freshwater

The origin of Atlantic salmon Salmo salar adult returns from the ocean to the Magaguadavic River, New Brunswick, was determined for the 1996 smolt cohort by using scale examination of the freshwater–seawater transition zone and a discriminant function analysis based on scale measurements depicting the first year of growth in freshwater. Adult returns consisted of 57% farmed fish that escaped from sea cages, 34% wild fish that were the progeny of naturally spawned fish, and 9% farmed fish that were demonstrated to have escaped from hatcheries into the river as juveniles—the first report of farmed hatchery escapees into freshwater returning as adults. The analysis determined that 20% of returns previously thought to be wild were in fact farmed salmon that had escaped into freshwater from hatcheries. By comparison, using the same analysis, we determined that 51% of smolts from the same cohort migrating out to sea 1 and 2 years earlier were farmed escapees from hatcheries. The difference in proportion in the smolt run and adult return indicated that the return of farmed juvenile escapees from the ocean feeding grounds was considerably less than that of wild fish. For a single cohort, comparison of the counted number of returning adults with the estimated number of smolts partitioned by freshwater origin yielded a return rate of 0.45% for wild salmon and only 0.09% for the hatchery escapees. However, even with the benefit of a low return rate, the 20% incidence of farmed hatchery escapees in the returns allowed into the river to spawn presented a significant potential for interbreeding with wild salmon in the river.