PARENT SESSION
Oral Session # 63: Marine Ecology II: Communities, Barnacles, and Clams.
Presiding: RA Feagin
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 101.
Sound-scattering layers and the foraging ecology of pelagic marine mammals.
Benoit-Bird, Kelly*,1, 1 Hawaii Institute of Marine Biology, Kaneohe, HI
ABSTRACT- The foraging ecology of two pelagic predators, spinner dolphins in Hawaii and dusky dolphins in New Zealand, was examined with respect to the characteristics of the sound-scattering layers that serve as their prey. Active sonar surveys were used to simultaneously assess the distribution of dolphins and the density and distribution of their prey. Scattering layers in both Hawaii and New Zealand underwent a diel vertical migration. As a result of these migration patterns, the potential prey of dusky dolphins was within their depth range for longer each night than the prey of spinner dolphins. The vertical distribution of both dolphin species closely matched the diel vertical migrations of their prey over time. The mean depth of both dolphin species was within a few meters of the depth at which the scattering layers were most dense. The Hawaii scattering layer was much more heterogeneously distributed over horizontal space and reached much higher densities than the New Zealand layer. Groups of spinner dolphins were observed to actively aggregate patches of the layer while dusky dolphins were not, perhaps because of the relative homogeneity of the scattering layer in New Zealand. Spinner dolphins were found to exhibit very fixed foraging patterns; they were nearly always found in pairs that were part of very coordinated groups of up to 22 animals. Dusky dolphin group size ranged from 1 to 5 animals in each coordinated group. The size of these groups varied as a function of time of night, layer depth, and variance in the layer's density. I hypothesize that the differences in behavior observed between the two species can be accounted for by the differences in their degree of foraging specialization, their risk of immediate starvation, the temporal window of prey availability, and the predictability of their respective food resources.
Key words: Lagenorhynchus obscurus, acoustic sampling, Stenella longirostris, deep scattering layer