DUSKY DOLPHIN (LAGENORHYNCHUS OBSCURUS) FORAGING IN TWO DIFFERENT
HABITATS: ACTIVE ACOUSTIC DETECTION OF DOLPHINS AND THEIR PREY
Kelly J. Benoit-Bird, Bernd Wursig, and Cynthia J. McFadden
ABSTRACT
Active-acoustic surveys were used to determine the distribution of dusky
dolphins and potential prey in two different New Zealand locations. During
seven survey days off Kaikoura Canyon, dusky dolphins were found within
the Deep-Scattering Layer (DSL) at 2000 when it rose to within 125 m of
the surface. As the DSL rose to 30 m at 0100, the observed depth of
dolphins decreased, presumably as the dolphins followed the vertical
migration of their prey. Acoustically identified subgroups of coordinated
animals ranged from one to five dolphins. Time, depth of layer, and layer
variance contributed significantly to predicting foraging dusky dolphin
subgroup size. In the much shallower and more enclosed Admiralty Bay,
dolphins noted at the surface as foraging were always detected with the
sonar, but were never observed in coordinated subgroups during the brief
(two-day) study there. In Admiralty Bay dolphin abundance was correlated
with mean volume scattering from potential prey in the water column; and
when volume scattering, an index of prey density, was low, dolphins were
rarely present. Ecological differences between the deep waters of Kaikoura
Canyon and the shallow nearshore waters of Admiralty Bay may result in
differences in how, when, and in what social groupings dusky dolphins
forage.
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