THE EFFECT OF LIGHT-INTENSITY ON THE NIGHTLY MOVEMENTS OF JUVENILE ATLANTIC SALMON ALEVINS AWAY FROM THE REDD

Atlantic salmon (Salmo salar) alevins move away from the redd predomin antly at night time, suggesting that light level is the controlling fa ctor releasing activity. However, the exact relationship between light level and alevin activity is unknown, so a series of artificial redds was used to monitor alevin movements under various night time inciden t light levels. Five experimental redds and two controls were run over a period of 3 years. Combining the results gave a significant negativ e correlation between the numbers of salmon alevins moving away from r edds on light nights and light level. This behaviour was interpreted a s a negative photoresponse since the frequency of alevin movement away from the redd was dependent on the light intensity. Two possible expl anations as to why alevins may react in this way to light level were c onsidered. Firstly, the behaviour could have evolved to ensure that al evins only move away from the redd when they are least likely to be ca ught by a predator or secondly, retinal developmental differences betw een alevins could have led to the observed behaviour.