Effect of Photoperiods on the Reactions of Juvenile Atlantic Salmon (Salmo salarL.) to Light Stimuli

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Abstract
The reactions to light stimuli of juvenile Atlantic salmon exposed to photoperiods (light-exposed fish) and those exposed to control conditions (control fish) were compared. The light-exposed fish showed more activity and reacted more readily to stimuli than the control fish. Both the control and light-exposed fish exhibited a negative phototaxis at all light intensities except the very lowest where a slight positive phototaxis was obvious. The intensity of an intermittent light stimulus had very little effect on the intensity of the reaction in the light-exposed fish, while the control fish showed an increase in the intensity of the reaction with an increase in the intensity of the light stimulus. The control fish showed an increase in the reaction to the intermittent light stimulus the longer the stimulus was applied at the higher levels of stimulation, while the light-exposed fish showed this behavior at the lower levels of stimulation. The reactions to continuous light were similar to the reactions to intermittent light stimuli, but the light-exposed fish reacted more quickly to continuous light than the control fish. These differences in behavior are attributed to the increased activity and sensitivity of the light-exposed fish, resulting from their exposure to photoperiods.