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The aim of thc prcscnt study is to test the extent to which dil'ferent interisities of continuous additional light affcct somatic growth and scxual maturation in Atlantic salmon postsmolts in sea cages. Postsmolts (Y 200 individuals) were randomly distributed among four cornmcrcial sizcd sca cages and exposed to either natural light or natural light + continuous additional liglit of low, medium or high intensity from January to Junc. In May the high light intensity group had a signitïcantly higher mean live body weight than thc othcr group" and at thc end of thc cxpcrimcnt in Junc, al1 groups dilkred in mean gutted weight. There was a positive logarithmic rcgrcssion bctwccn lcvcl of light intcnsity exposure during night and rriean gutted weight. A sinall proportion of fish niatured sexually (after 1.5 ycars in seawatcr) in thc natural light group, whereas no maturation was detected in the light groups. Thcse results suggcsts that to affcct growth and proportion or maturation in Atlantic salmon by additional light, there might he different threshold values of light intensity.

Positioning of sea cages at sites with high water current velocities expose the fish to a largely unknown environmental challenge. In this study we observed the swimming behaviour of Atlantic salmon (Salmo salar L.) at a commercial farm with tidal currents altering between low, moderate and high velocities. At high current velocities the salmon switched from the traditional circular polarized group structure, seen at low and moderate current velocities, to a group structure where all fish kept stations at fixed positions swimming against the current. This type of group behaviour has not been described in sea cages previously. The structural changes could be explained by a preferred swimming speed of salmon spatially restricted in a cage in combination with a behavioural plasticity of the fish.

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