Effect of Photoperiods on the Behavior of Juvenile Atlantic Salmon (Salmo salarL.) in Vertical and Horizontal Light Gradients

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Abstract
The behavior of juvenile Atlantic salmon in vertical and horizontal light gradients was studied. Comparisons were made between the behavior of fish exposed to 16-hr photoperiods (long-photoperiod fish) and fish that had not been so conditioned (control fish). In vertical light gradients the long-photoperiod fish were very active and moved rapidly up and down the water-column. At low light intensities they tended to rise into the surface layers. Peak activity and the greatest numbers in the upper reaches of the water-column occurred at 0.04 ft-c (lowest intensity studied). The control fish, however, were relatively quiescent at all light intensities and settled to the bottom at low intensities. Peak activity and the greatest numbers in the upper reaches of the water-column occurred at 1.0 ft-c. In horizontal light gradients the control and long-photoperiod fish remained in areas of lowest light intensities. When allowed to remain in the light gradients for a short period of time, both groups showed a stronger avoidance of light sources of higher intensity but moved into the brighter areas with low intensity light sources. With prolonged stay in the light gradients, these trends were essentially reversed due to nipping and defence of territory. The long-photoperiod fish showed a stronger avoidance reaction than the control fish with the 100-watt light source, whereas the reactions of both groups were similar with the 7 1/2-watt and 300-watt light sources, when these fish were allowed to remain in the gradients for a short period of time. However, with prolonged stay in the gradients, nipping and defence of territory resulted in a stronger avoidance reaction by long-photoperiod fish to all light gradients. The differences in the behavior of control and long-photoperiod fish in vertical and horizontal light gradients is believed to be a result of exposure of the latter to controlled photo-periods.

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