Migration Timing of Atlantic Salmon Smolts Relative to Environmental and Physiological Factors

Abstract

We determined the migration timing of fry-stocked smolts 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, Vermont. Smolt migration began in late April and early May when water temperature was 5°C, peak movements occurred in early and mid-May at temperatures exceeding 8°C, and migration was complete by early June. Within this seasonal window, significant differences in migration timing and gill Na⁺,K⁺-ATPase activity occurred among tributaries. In both years of the study, smolts tended to migrate earlier and exhibit greater gill Na⁺,K⁺-ATPase activity in the warmest tributary than in the coolest tributary. Smolt migration timing differed most among tributaries in mid-May when (1) water temperatures were more than 8°C, (2) smolts peaked in gill Na⁺,K⁺-ATPase activity, and (3) discharge peaked, stimulating smolt migration. Smolts captured after the migratory period had lower gill Na⁺,K⁺-ATPase activity than migrating smolts. Relating smolt physiology to migration was crucial for explaining complex interactions among water temperature, discharge, and smolt behavior during both the onset and cessation of migratory activity. Because the period between onset of migration and loss of smolt physiological characteristics may be brief, delays in downstream passage that may occur at dams must be minimized to maximize the successful recruitment of smolts to the marine environment.