Longterm changes in migration timing of adult Atlantic salmon at the southern edge of the species distribution: local adaptation or a coast-wide response to environmental change?

No abstracts are to be cited without prior reference to the author.

The Connecticut River historically represented the southernmost extent of the North American range of Atlantic salmon, but the native population was extirpated 200 years ago by dam construction. An extensive restoration effort has relied upon stock transfers from more northerly rivers, especially the Penobscot River (Maine). Long-term success depends on the donor stocks adapting to the new river. Recent genetic work has shown differences between donor and derivative populations but life history traits have not been quantitatively compared. Here we focus on the timing of the adult migration. Based on data from other salmon stocks we expect that many native salmon migrated in March/April compared to May/June in more northerly stocks. We examined 23 years of migration timing data collected at two capture locations in the Connecticut drainage and found that both date of first capture and median capture date shifted significantly earlier by about 0.5 days/year. In order to conclude whether this is an adaptive change or a coast-wide effect we also quantified changes in migration timing of more northerly stocks (in Maine and Canada). We found a latitudinal pattern in the average return dates as well as coherent patterns in the shift towards earlier peak migration dates across systems. We suggest that these consistent shifts may be related to coast-wide changes in temperature and flow, and may represent a response to global climate change.