Migration timing of Atlantic salmon smolts relative to environmental and physiological factors

We determined the migration timing of fry-stocked smelts of Atlantic salmon Salmo salar, relative to environmental and physiological factors, by using net weirs and counting fences in three tributaries of the West River, Verm ont. Smelt migration began in late April and early May when water temperatu re was 5 degrees C, peak movements occurred in early and mid-May at tempera tures exceeding 8 degrees C, and migration was complete by early June. With in this seasonal window, significant differences in migration timing and gi ll Na+,K+-ATPase activity occurred among tributaries. In both years of the study, smelts tended to migrate earlier and exhibit greater gill Na+,K+-ATP ase activity in the warmest tributary than in the coolest tributary. Smelt migration timing differed most among tributaries in mid-May when (1) water temperatures were more than 8 degrees C, (2) smelts peaked in gill Na+,K+-A TPase activity, and (3) discharge peaked, stimulating smelt migration. Smel ts captured after the migratory period had lower gill Na+,K+-ATPase activit y than migrating smolts. Relating smelt physiology to migration was crucial for explaining complex interactions among water temperature, discharge, an d smelt behavior during both the onset and cessation of migratory activity. Because the period between onset of migration and loss of smelt physiologi cal characteristics may be brief, delays in downstream passage that may occ ur at dams must be minimized to maximize the successful recruitment of smel ts to the marine environment.