Riverine and Estuarine Migratory Behavior of Coho Salmon (Oncorhynchus kisutch) Smolts

Abstract

Using radio and ultrasonic transmitters, we tracked coho salmon (Oncorhynchus kisutch) smolts in the Chehalis River and Grays Harbor estuary, Washington, to document patterns of seaward migration. In spite of dramatic differences in the two habitats, the fish behaved similarly in both systems. Migratory progress was saltatory, characterized by movement in the direction of the current and extended periods of holding in areas of low current velocity. Consequently, smolts were displaced rapidly downstream by swift, unidirectional river currents but were retained in the estuary by relatively low-velocity, reversing tidal currents. Smolts did not use tidal currents to move seaward and appeared to swim most rapidly against strong ebbing currents. Estimated mean total metabolic cost incurred by migrating smolts was 0.027 cal∙s⁻¹∙smolt⁻¹ (1 cal = 4.184 J). Estimated daily caloric expenditure by smolts in 1988 was twice that of smolts tracked in 1989, apparently due to faster water velocities in 1988. The observation that coho salmon migration through estuaries is slower than riverine migration suggests that a period of estuarine residence may be necessary for them to adjust their osmoregulatory capability, orient for their return migration, feed, or reduce their vulnerability to predators.