Responses to Light of Young Atlantic Salmon (Salmo salar) and Brook Trout (Salvelinus fontinalis)

Abstract

Responses of young Atlantic salmon (Salmo salar L.) and of brook trout (Salvelinus fontinalis (Mitchill)) to different levels of illumination and to overhanging and submerged cover were compared in the laboratory. At surface illuminations between 0.4 and 160 ft-c trout were either slightly photopositive or showed no preference for light or dark areas, while salmon were photopositive. At illuminations above 300 ft-c both species were photonegative when the light area had a smooth, unbroken substrate. Under these conditions starvation induced salmon into the light, but trout remained photonegative, although slightly less so than well-fed trout. With submerged cover (plants, broken flowerpots, etc.) in the light area, salmon were strongly photopositive, while trout were photonegative. Salmon were less photopositive, but still preferred the light area, with submerged cover in both dark and light areas, in most experiments trout maintained position at the edge of the shadow caused by overhead cover, looking out into the light area from the dark.The significance of these results to habitat segregation by the two species where they are sympatric in nature is discussed.