Genetic Analysis of Atlantic Salmon Natural Reproduction In Hobart Stream, Maine

To increase natural reproduction of Atlantic salmon in Maine rivers, and to evaluate the potential of hatchery adults to successfully spawn in the natural environment, gravid adult Atlantic salmon from Craig Brook National Fish Hatchery have been stocked prior to spawning. In general, these adult hatchery salmon are returned to their population of origin; however Hobart Stream has been experimentally stocked with adults from multiple hatchery broodstocks.  Prior to this study, no Atlantic salmon or natural reproduction by Atlantic salmon had been detected in Hobart Stream.  We used microsatellite loci to genotype all stocked hatchery adults and sampled juveniles, and genetic parentage analysis to assess the reproductive ability of the hatchery adults and determine mate choice patterns for hatchery adults in the wild. We assessed natural reproduction for two release years (2006 and 2007), through captures of parr and smolts in 2008, 2009, and 2010.  In 2006, adults from three different broodstock populations were stocked into Hobart Stream, and in 2007, adults from two different broodstock populations were stocked.  Of the 90 juvenile Atlantic salmon sampled and genotyped, 84 were assigned to parental spawning pairs, indicating that hatchery-raised Atlantic salmon can successfully reproduce in streams and produce offspring that survive through the juvenile life stages.  Based on assignment of sampled juveniles to parental broodstock, observed spawning pair combinations significantly deviated from expected proportions based on numbers stocked, indicating either non-random selection of mates or differential survival of offspring occurred, both for the 2006 release year (X²=40.36, p<0.001) and 2007 release year (X²=9.17 p=0.03).  Determination of reproductive capabilities of hatchery reared Atlantic salmon, and their mating strategies following stocking in the wild will provide important information to guide restoration activities and stocking practices of hatchery Atlantic salmon.