A re-evaluation of swimming performance in juvenile salmonids relative to downstream migration

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Abstract

It has been hypothesized that downstream migration of juvenile salmonids is initiated by physiological changes that occur during smoltification, which render the fish unable, or unwilling, to swim against currents that exceed 2 body lengths per second (BL ·s -1 ). This decline in ability, coupled with the increase in flow rate generally associated with the spring run-off, is thought to result in passive downstream displacement. To test this hypothesis, we measured holding ability of wild Atlantic salmon (Salmo salar) parr and swimming ability of wild, migrating Atlantic salmon smolts 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) periods in water speeds up to 0.86 m ·s -1 . Atlantic salmon smolts (fork 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 periods (2-10 min), and made short bursts at speeds up to 1.95 m ·s -1 . These findings indicate that absolute swimming performance is not impaired after smoltification and that wild Atlantic salmon smolts 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.

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