Friday, August 10, 2007
Exhibit Halls 1 and 2, San Jose McEnery Convention Center
We investigated the role of dusky dolphins (Lagenorhynchus obscurus) in the feeding ecology of apex predators from Aug. to Nov. 2005 and May to Aug. 2006 in Admiralty Bay, New Zealand. During 335 dusky dolphin feeding bouts (52 observed underwater), we collected data on numbers of predators, dolphin behavior, and prey ball parameters. In spring, when dolphins herded prey balls into stationary aggregations at the surface, they increased prey accessibility by decreasing depth of prey balls and decreasing mobility. About one-half of prey balls (51%) ascended during the course of feeding bouts; 35% stayed at a constant depth and 14% descended. Dusky dolphins played a role in causing prey balls to ascend, as they swam under prey balls the majority (59%) of the time during feeding. Dolphins also appeared to play a role in stabilizing prey balls, as 24% of prey balls were mobile during feeding before they became stationary; 17% of prey balls that became mobile during feeding bouts subsequently became stationary again. Finally, in winter and spring, other predators appeared to use dolphins to locate prey more easily. Dusky dolphins potentially play an important role in the foraging ecology of apex predators: for example, Australasian gannets (Morus serrator) fed with dolphins for 40% of feeding observations, and shearwaters (Puffinus sp.) fed with dolphins for 24% of observations. Gulls (Larus sp.), terns (Hydroprogne caspia and Sterna striata), fur seals (Arctocephalus forsteri), and spiny dogfish (Squalus acanthias) also frequently fed with dusky dolphins.
