Long-term changes in migration timing of adult Atlantic salmon (Salmo salar) at the southern edge of the species distribution

The Connecticut River historically represented the southernmost extent of the North American range of Atlantic salmon (Salmo salar), 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). Recent work has shown differences in age structure between donor and derivative populations. Here we focus on a related life-history trait, the timing of the adult migration. We examined 23 years of migration timing data collected at two capture locations in the Connecticut River drainage. We found that both dates of first capture and median capture dates have shifted significantly earlier by about 0.5 days·year–1. To conclude whether this is a consequence of local adaptation or a coast-wide effect, we also quantified changes in migration timing of more northerly stocks (in Maine and Canada). We found that the changes in migration timing were not unique to the Connecticut River stock and instead observed coherent patterns in the shift towards earlier peak migration dates across systems. These consistent shifts are correlated with long-term changes in temperature and flow and may represent a response to global climate change.