THE INFLUENCE OF HUNGER AND PREDATION RISK ON GROUP COHESION IN A PELAGIC FISH, WALLEYE POLLOCK THERAGRA-CHALCOGRAMMA

Variation in the intensity of schooling behavior in fishes suggests th at the benefits of aggregation are balanced by certain costs. We exami ned the proximity of group members to each other in juvenile walleye p ollock, Theragra chalcogramma, under different environmental condition s. Food availability, simulated by a gradient of six ration treatments , had a major influence on group cohesion, with increasing dispersion as food level decreased. Group cohesion also decreased at night relati ve to daytime levels. Small juveniles ((x) over bar = 53 mm TL) mainta ined on high rations were highly responsive to the potential threat of a predator, with groups becoming more cohesive and remaining so for u p to an hour after the initial threat. A chronic threat (continual pre sence of predators) resulted in tighter group cohesion than an acute t hreat (single simulated attack). Small juveniles maintained on low rat ions were less responsive to predation threats and recovered quickly, supporting the hypothesis that hunger induces risk-taking behavior. La rge juveniles ((x) over bar = 149 mm TL) did not change their degree o f aggregation in response to either type of predation threat. An overa ll plasticity in the degree of cohesiveness among group members indica tes that walleye pollock are capable of gradually modifying their scho oling behavior according to the environmental context.