Foraging behavior of the Hawaiian spinner dolphin observed with active acoustics

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Abstract
The mesopelagic boundary community (MBC) in the Hawaiian Islands is an important food resource for many organisms, including spinner dolphins (Stenella longirostris). To understand the role of this community in the feeding ecology of spinner dolphins, spatial and temporal patterns in the density of the MBC and relative abundance of dolphins were characterized using acoustic sampling. A 200 kHz echosounder modified to sample directly into a computer was used to survey the leeward coast of Hawaii from 2100 until 0400. Acoustic data were mapped in ArcView to show the distribution of both the predator and its prey. Individual echoes were color-coded by target strength to allow the separation of spinner dolphins and mesopelagic organisms. Dolphin species identification from active acoustics was confirmed using a combination of passive acoustics and visual observations. The MBC migrated horizontally, raising questions about the accepted (Norris) model of spinner dolphin foraging in Hawaii. Observations of spinner dolphins suggest a more complex foraging pattern, compatible with patterns observed in their food resource. Spinner dolphin foraging patterns closely matched the spatial patterns of their prey both horizontally and vertically. Patches exploited by dolphins were also significantly different than other patches measured under similar conditions.

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