MEPS 167:215-226 (1998)  -  doi:10.3354/meps167215

Research was undertaken to examine the influence of light intensity on the shoaling behavior, activity and anti-predator behavior of juvenile walleye pollock Theragra chalcogramma. Under a 12 h light/12 h dark photoperiod, juveniles displayed a diurnal shoaling and activity pattern, characterized by fish swimming in cohesive groups during the day, with a cessation of shoaling and decreased swimming speeds at night. Prior studies of schooling fishes have demonstrated distinct light thresholds below which schooling abruptly ceases. To see if this threshold effect occurs in a predominantly shoaling species, like juvenile walleye pollock, another experiment was undertaken in which illumination was lowered by orders of magnitude, giving fish 20 min to adapt to each light intensity. Juvenile walleye pollock were not characterized by a distinct light threshold for shoaling; groups gradually dispersed as light levels decreased and gradually recoalesced as light levels increased. At light levels below 2.8 x 10^-6 µE s^-1 m^-2, juvenile walleye pollock were so dispersed as to no longer constitute a shoal. Exposure to simulated predation risk had differing effects upon fish behavior under light and dark conditions. Brief exposure to a model predator in the dark caused fish to swim faster, for 5 or 6 min, than fish which had been similarly startled in the light. Chronic exposure to a living predator produced similar results; fish tended to swim slower when a predator was present in the light, but faster when a predator was present in the dark. In the light, shoaling and/or schooling provide protection against predators. But in the dark, with fish unable to see one another, increased prey activity resulting from predator disturbance may lead to accelerated dispersal of prey shoals. Thus, perceived predation risk may have different effects upon the spatial distribution of juvenile walleye pollock under light and dark conditions. This has implications for survival, as fish which have become widely scattered during the darkness may take longer to reform shoals at dawn, resulting in greater predation risk.

Schooling · Spatial distribution · Twilight hypothesis · Predator-prey · Swimming speed · Illumination