Subsurface and nighttime behaviour of pantropical spotted dolphins in Hawai'i

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 night time behaviour of pantropical spotted dolphins were studied near the island s of Maui and Lana'i, Hawai'i, in 1999. Suction-cup-attached time-depth rec order/VHF-radio tags were deployed on six dolphins for a total of 29 h. Rat es of movements of tagged dolphins were substantially lower than reported i n pelagic waters. Average diving depths and durations were shallower and sh orter than reported for other similar-sized odontocetes but were similar to those reported in a study of pantropical spotted dolphins in the pelagic w aters of the eastern tropical Pacific. Dives (defined as >5 m deep) at nigh t 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 re sults, together with the series of deep dives recorded immediately after su nset, suggest that pantropical spotted dolphins around Hawai'i feed primari ly at night on organisms associated with the deep-scattering layer as it ri ses up to the surface after dark.