Abundance and distribution patterns of Hawaiian odontocetes: focus on Oahu

Abstract

This dissertation is an assessment of the status of odontocetes in Hawaiian waters focussing on Oʻahu. The work builds on available literature, and on data collected by the author and by others in Hawaiian waters. Abundance and distribution patterns of odontocetes were derived from stranding and aerial survey data. A stranding network operated by the National Marine Fisheries Service, Pacific Area Office collected 187 stranding reports throughout the main Hawaiian Islands between 1937 and 2002. These reports included 16 odontocete species. Number of stranding reports increased over time and was highest on Oʻahu. Strandings occurred throughout the year. The difference in number of strandings per month was not significant. Fifteen of the 16 species reported in the stranding record for the main Hawaiian Islands. They were also reported by aerial survey studies of the area between 1993 and 1998. Only 7 of the species reported were detected during aerial transects around Oʻahu between 1998 and 2000. Based on the stranding record, Kogia sp., melon-headed whales, striped dolphins and dwarfkiller whale appear to be more common than suggested by aerial surveys. Conversely, pilot whales and bottlenose dolphins were more common, according to aerial surveys, than predicted by the stranding data. Aerial surveys of waters between 0 and 500m around the Island of Oʻahu showed that the most abundant species by frequency of occurrence was the pilot whale (30% of sightings), followed by the spinner (16%) and bottlenose dolphin (14%). Because of small sample size, abundance estimates for odontocetes have a high level of uncertainty. The unavailability of a correction factor for g(0)<1, and the reduced visibility below the aircraft further reduced accuracy and increased the inherent underestimation in the data. The most abundant species according to distance sampling estimates were spotted dolphins, pilot whales, false killer whales and spinner dolphins. A natural factor shaping the ecology of odontocete populations is predation pressure both by other odontocetes and, more frequently, by sharks. An account of predation by a tiger shark on a spotted dolphin near Penguin Banks is used as an example of the potential mechanisms of predation by sharks on odontocetes.