Seaward Migration of Hatchery-Reared Atlantic Salmon, Salmo salar, Smolts in the Penobscot River Estuary, Maine: Riverine Movements

Abstract

Forty-six hatchery-reared Atlantic salmon, Salmo salar, smolts tagged internally with ultrasonic telemetry transmitters were tracked in the Penobscot River estuary, Maine. Travel times of six smolts tracked from head of tide 35 km seaward to the river mouth ranged from 29.4 to 44.5 h (mean 37.1 h). During 1975 and 1976 onset of increased movement of smolts occurred when hatchery and river water temperatures rose past 5 °C. Full expression of migratory behavior occurred above 9 °C. Earlier increased movement in 1976 coincided with earlier warming trends. Water current was the main factor influencing routes and rates of smolt movement. Smolts usually moved in the same direction as current flow during ebb and flood tides. This resulted in net seaward progress and fastest ground speeds (mean 56.4 cm/s) on ebb tide, and net upriver progress and slowest ground speeds (mean 37.4 cm/s) on flood tide. Swimming depth measured during portions of six tracks averaged 2.4 m. Simultaneous tracking of 1- and 3-m drogues together with smolts during five tracks showed that seaward progress of smolts was due mainly to passive drift. During times of current reversal or upon reaching shallow water, smolts often stopped moving, milled about, or apparently swam against the current. No meaningful correlations were found between travel speed and any environmental factors other than water current, except for influence of water temperature on initiation of migration. This study substantiates previous indications that smolt migration through the estuary is effected relatively quickly. Key words: Atlantic salmon smolts, drift, estuaries, migration, telemetry