Effects of temperature, photoperiod, and season on the photobehaviour of juvenile Atlantic salmon (Salmo salar)
The selection of illuminated or shaded areas by juvenile Atlantic salmon (Salmo salar) was examined in relation to temperature and photoperiod. Water temperature had a strong effect on the photoresponse of salmon. Salmon acclimated to 14 °C and a 16-h photophase in summer selected illuminated areas when tested at their acclimation conditions, but rapidly became strongly photonegative while temperature fell to 7 °C. When tested under acclimation conditions of 7 °C and a 16-h photophase during summer, salmon selected shade, but still became distinctly photopositive as temperature increased to 14 °C. Under acclimation conditions of 14 °C and an 8-h photophase during summer, salmon selected illuminated areas, becoming photonegative as temperature fell. Thus, acclimation to photoperiod alone did not exert a clear effect on the photoresponse of summer salmon. If acclimated to 7 °C and an 8-h photophase in winter, salmon selected shade under acclimation conditions and remained photonegative also after temperature increased. Anomalously, after acclimation to 7 °C and an 8-h photophase in summer, salmon were photopositive at 7 °C, but became photonegative and selected shade upon temperature increase, indicating an endogenous seasonal difference not only in their response to thermal stimuli, but also in their susceptibility to acclimation regimes.