Genetic impact on two wild brown trout (Salmo trutta) populations after release of non-indigenous hatchery spawners

A genetically marked hatchery strain of brown trout (Salmo trutta) was employed to study the genetic impact from non-indigenous hatchery fish on wild stocks. The hatchery spawners were released in autumn 1989 into the spawning localities of two wild trout stocks in River Oyreselv, Norway The F-1 generation was sampled and genotyped at the 0+, 1+, and 2+ stages. Juveniles carrying the genetic markers were found in both localities, proving that the introduced spawners had spawned among themselves and with the wild stocks. The genetic contribution from the hatchery fish was estimated at 19.2 and 16.3% at the 0+ stage in the two wild stocks. Estimates of survival rates of 0+ trout revealed that survival was nearly three times higher in wild trout than in hybrids of wild and introduced trout, possibly because of a difference between introduced and wild stocks in size of eggs and alevins. The frequency of the marker alleles in the F-1 generation declined during the 2-year observation period.