Temperature-related loss of smolt characteristics in Atlantic salmon (Salmo salar) in the wild

Atlantic salmon (Salmo salar) that had previously been released as fry in t ributaries of the Connecticut River were captured from 1993 to 1997 during their normal spring smolt migration 198 km from the mouth of the river. Smo lts had peak levels of,gill Na+,K+-ATPase activity and salinity tolerance e arly in migration (early May), indicating physiological readiness to enter seawater. Significant decreases in gill Na+,K+-ATPase activity (29-66%) and salinity tolerance were seen in smolts at the end of the migratory period (late May and early June). Reduced gill Na+,K+-ATPase activity occurred ear lier in warm years and was directly related to the degree-days during migra tion (r(2) = 0.75). Reduced gill Na+,K+-ATPase activity was found at the en d of migration in warmer, southern rivers (Connecticut River and Penobscot River, Maine) but not in northern rivers (Catamaran Brook, New Brunswick, a nd Conne River, Newfoundland). Both hatchery- and stream-reared fish held i n the laboratory exhibited a more rapid loss of physiological smolt charact eristics when held at higher temperature. The results indicate that late mi grants in southern rivers lose physiological smolt characteristics due to h igh temperatures during spring migration. Delays in migration, such as thos e that occur at darns, may have negative impacts on smolt survival in warme r rivers.