Vertical distribution of Atlantic salmon ( Salmo solar L.) in net pens: trade-off between surface light avoidance and food attraction

Abstract

The vertical distribution of Atlantic salmon ( Salmo salar L.) reared in marine net pens of 7 m depth was studied by means of an echo integration system in relation to temporal and environmental factors. Great variations in local densities were found. In summer up to 80% of the fish could be localized within a 1-m depth interval, whereas in winter the fish were more evenly spread out in the pen. High light levels resulted in deeper fish distribution. The fish usually descended at dawn and ascended at dusk and swam deeper in summer than winter, and there was a negative correlation between light level and fish density at the surface in summer. Feeding also influenced fish distribution most strongly in summer, with rapid upwards swimming when feeding started and a gradual descent during the course of feeding. The fish stayed generally deeper in a feeding regime with infrequent (twice daily) than frequent (every 10 min) feeding. The vertical distribution could largely be explained by a trade-off between light-induced surface avoidance and food attraction. Avoidance of the pen bottom and excessively high densities also seemed to play a role. The implications of the findings regarding choice of pen size, selection of feeding regimes and development of automatic feeding systems are discussed.