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 predominantly at night time, suggesting that light level is the controlling factor 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 incident light levels. Five experimental redds and two controls were run over a period of 3 years. Combining the results gave a significant negative correlation between the numbers of salmon alevins moving away from redds on light nights and light level. This behaviour was interpreted as a negative photoresponse since the frequency of alevin movement away from the redd was dependent on the light intensity. Two possible explanations as to why alevins may react in this way to light level were considered. Firstly, the behaviour could have evolved to ensure that alevins only move away from the redd when they are least likely to be caught by a predator or secondly, retinal developmental differences between alevins could have led to the observed behaviour.