Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., smolts down a release ladder. 1. Environmental effects on migratory activity

Abstract

The effects of light intensity, water temperature and river spate conditions on the rate of migration of hatchery‐reared Atlantic salmon smolts down a release ladder were examined. Low light intensity and high day time water temperatures raised smolt migration rates: water temperature had littleor no effect at night. Thediel patternofdown‐ladder movement wasdetermined by these two environmental stimuli. Smolts demonstrated a threshold response to both light and water temperature: a fall in light intensity, or increase in water temperature, below or above their respective thresholds elicited no further response. As fish progressed through the ladder their movement became increasingly nocturnal, and most entry into the estuary occurred at night. Heavy rainhll caused rapid migration of smolts through the ladder. As ladder discharge remained constant, smolts must have been responding to some other change in the water conditions. Smolts responded to spate conditions irrespective of water temperature and light intensity, and they responded to light intensity irrespective of water temperature. A hierarchy of environmental cues, responsible for triggering migratory behaviour on a day to day basis during the smolt run, of spate> light intensity > water temperature is therefore suggested.