Salmonid Fishes and the Estuarine Environment

Abstract

Salmonid fishes are typically anadromous, that is, they achieve the bulk of their growth in the sea, and migrate into rivers to reproduce. As juveniles, they may arrive in the estuary almost immediately after emergence from a gravel bed in the river, or after a few days, weeks, months, or years in fresh water, according to species and developmental opportunity. Thereafter, the length of estuarine residence depends on the estuary’s size, shape, and productivity; on water flow patterns and velocities; on salinity and temperature; and on the species and size of the salmon. It is supposed that estuaries offer salmonids three primary advantages: productive foraging, relative refuge from predators, and a physically intermediate environment for transition from fresh water to marine physiological control systems. The present paper finds the evidence for the foraging advantage strong, for the predator refuge equivocal, and for the physiological transition function applicable particularly to Pacific species migrant at the fry stage.