Intraspecific interference among foraging blue crabs Callinectes sapidus : interactive effects of predator density and prey patch distribution

The interactive effects of predator density and prey distribution on the foraging behavior of an important estuarine predator were studied, at a fine temporal scale, using ultrasonic telemetry. The movement and agonistic activity of individual blue crabs Callinectes sapidus were monitored in large field enclosures, in which the density of crabs and the distribution of patches of bivalve prey Macoma balthica were varied. Agonism-related injury in blue crabs is common and may be quite costly. On a scale of days, blue crabs have been shown in previous studies to disperse, sometimes into prey-impoverished areas, in response to conspecific interference. On the scale of minutes to hours addressed in the present study, the density of predators and the distribution of their prey interacted to affect the foraging behavior and success of blue crabs. When only a single clam patch was available, blue crabs at high density interfered with each other's foraging by direct agonistic encounters, shown by an inverse correlation between agonistic activity and foraging success. Conversely, when prey were partitioned into 2 patches, blue crabs at high density apparently dispersed among the patches, thus minimizing direct agonistic clashes. Although crabs reduced the occurrence of agonistic encounters further than the 50 % attributable to their effectively halving their densities on each patch, they did not take refuge in the prey-impoverished areas between experimental patches for significant periods. Instead, they seemed to respond instantaneously to changing degree of risk by moving off the clam patch when another conspecific approached.