Sub-surface and night-time behaviour of pantropical spotted dolphins in Hawaii

Pantropical spotted dolphins (Stenella attenuata) are found in both pelagic waters and around oceanic islands. A variety of differences exist between populations in these types of areas, including average group sizes, extent of movements, and frequency of multi-species associations. Diving and nighttime behaviour of pantropical spotted dolphins were studied near the islands of Maui and Lana'i, Hawai'i, in 1999. Suction-cup-attached time-depth recorder/VHF-radio tags were deployed on six dolphins for a total of 29 h. Rates of movements of tagged dolphins were substantially lower than reported in pelagic waters. Average diving depths and durations were shallower and shorter than reported for other similar-sized odontocetes but were similar to those reported in a study of pantropical spotted dolphins in the pelagic waters of the eastern tropical Pacific. Dives (defined as >5 m deep) at night were deeper (mean = 57.0 m, SD = 23.5 m, n = 2 individuals, maximum depth 213 m) than during the day (mean = 12.8 m, SD = 2.1 m, n = 4 individuals, maximum depth 122 m), and swim velocity also increased after dark. These results, together with the series of deep dives recorded immediately after sunset, suggest that pantropical spotted dolphins around Hawai'i feed primarily at night on organisms associated with the deep-scattering layer as it rises up to the surface after dark.