Foraging and agonistic activity co-occur in free-ranging blue crabs (Callinectes sapidus):: observation of animals by ultrasonic telemetry

To define the temporal and spatial patterns of agonism and foraging activity in blue crabs (Callinectes sapidus), we monitored five free-ranging animals in the Rhode River subestuary of the central Chesapeake Bay by ultrasonic telemetry during the summers of 1991-93, The interdependence between the two activities was of special interest. High crab densities have been associated with more frequent aggressive interactions and decreased foraging success in previous laboratory studies. High crab population density is correlated with increased frequency of aggression-related injury (autotomy) and cannibalism in the field. Consequently, we predicted that as crabs aggregate to clam patches during feeding periods in the field, the level of aggressive interactions would increase. In early trials, we collected data on location and agonistic activity (the stereotypical spreading of the chelae in the 'meral spread' threat display) of crabs moving freely in the estuary by using single-channel telemetry transmitters. With subsequent technological advancements, we received simultaneous data on agonism and feeding. Crabs exhibited a diel pattern of agonism with peaks in threat display occurring in mornings and sometimes in evenings. Crabs fitted with single-channel telemetry transmitters were observed interacting aggressively most often at times previously identified as feeding periods, although the highest levels of agonism came slightly later than periods associated with the highest levels of feeding. Simultaneous telemetry of the two behaviors indicated that periods of increased agonism and feeding overlapped. Feeding activity tended to wane as threat activity increased, consistent with the hypothesis that aggressive interference impairs foraging. (C) 1999 Elsevier Science BN. All rights reserved.