Behavior of dusky dolphins foraging on the deep-scattering layer in Kaikoura Canyon, New Zealand

Abstract

Little is known of foraging habits of sound-scattering layer consumers. A 200-kHz echosounder was used to survey dusky dolphins and the sound-scattering layer in winter 2002, in Kaikoura Canyon, New Zealand. Visual observations of dolphin surfacings occurred 84% of the time that dolphins were acoustically detected, confirming identifications from the acoustic data. Dusky dolphins were within the layer at 2000 h (about 1.5 h after dusk), within 125 m of the surface. As the layer rose to within 30 m of the surface at 0100 h, the observed depth of dolphins decreased presumably as the dolphins followed the vertical migration of their prey. The mean depth of dolphins was within the scattering layer except when the top of the layer was deeper than 125 m. Dusky dolphins often forage within large groups. Acoustically identified subgroups of coordinated animals ranged from 1 to 5 dolphins. Subgroup size varied with time of night, minimum depth of the scattering layer, and the variance of the food resource. The largest subgroups occurred when the scattering layer was closest to the surface, and when the layer was most heterogeneous. Time, depth of layer, and layer variance contributed significantly to predicting foraging dusky dolphin subgroup size.