Interactions between marine predators: dolphin food intake is related to number of sharks

Abstract

Dolphins and sharks feed at times on the same food; however, the influence of these interactions on the feeding success of either predator has not been measured. I employed underwater video to record bottlenose dolphins Tursiops truncatus and silky sharks Carcharhinus falciformis feeding on the same school of fish, and for the first time measured food intake of free-ranging dolphins. Regression analyses showed that dolphin food intake diminished as the number of feeding sharks increased, but was unrelated to the number of dolphins feeding, size of the prey clump or duration of feeding events. The number of dolphins increased at the beginning of a feeding event in the presence of sharks but not in their absence. This increase apparently provided a benefit to dolphins since the number of sharks feeding was negatively related to the number of dolphins feed- ing. Other studies have indicated that risk of shark predation influences dolphin group size and habi- tat use. This study indicates that interspecific contests over food influence dolphin food intake and perhaps also dolphin group size.