Autumnal Habitat Shift of Juvenile Atlantic Salmon (Salmo salar) in a Small River

Autumnal changes in behavior and distribution of three age-classes of juvenile Atlantic salmon (Salmo salar) were determined during 3 yr in the Little Sevogle River of northeastern New Brunswick. In summer, salmon were always observed above the streambed, each holding a station over a single, unshaded stone. About 84% of the entire population occurred in the run habitat-type, 12% in riffles and 5% in pools. Underwater visual censuses showed the salmon to be continuously numerous in summer, but, as soon as water temperature fell to or below 10 °C in autumn, they disappeared from their stations and their visible population decreased by 92–98%. Thereafter, the salmon were found almost exclusively in sheltered substrate chambers beneath surface streambed stones. However, salmon distribution among runs, riffles, and pools (77, 18, and 5%, respectively) did not differ significantly from that in summer. Trapping, marking, and absolute population estimates indicated neither dwindling nor egress of the resident population. Thus, instead of suddenly leaving the river area or moving to another habitat-type at low temperatures, the salmon merely moved from their unsheltered summer stations to sheltered winter stations within the streambed. The results indicate the importance of winter habitat management in managing juvenile Atlantic salmon populations.