A REEVALUATION OF SWIMMING PERFORMANCE IN JUVENILE SALMONIDS RELATIVETO DOWNSTREAM MIGRATION

It has been hypothesized that downstream migration of juvenile salmoni ds is initiated by physiological changes that occur during smoltificat ion, which render the fish unable, or unwilling, to swim against curre nts that exceed 2 body lengths per second (BL.s(-1)). This decline in ability, coupled with the increase in flow rate generally associated w ith the spring run-off, is thought to result in passive downstream dis placement. To test this hypothesis, we measured holding ability of wil d Atlantic salmon (Salmo salar) parr and swimming ability of wild, mig rating Atlantic salmon smelts in the field under ambient environmental conditions. Atlantic salmon parr (fork length 4.8-13.1 cm) used their pectoral fins to anchor themselves for indefinite (i.e., > 200 min) p eriods in water speeds up to 0.86 m.s(-1). Atlantic salmon smelts (for k length 12.4-21.1 cm) swam indefinitely against currents up to 1.26 m -s(-1), maintained velocities as high as 1.64 m.s(-1) for short period s (2-10 min), and made short bursts at speeds up to 1.95 m.s(-1). Thes e findings indicate that absolute swimming performance is not impaired after smoltification and that wild Atlantic salmon smelts are capable of swimming at speeds much greater than 2 BL.s(-1) making it unlikely that they are involuntarily carried to the sea by river currents.