Odontocete Cetaceans Around the Main Hawaiian Islands: Habitat Use and Relative Abundance from Small-Boat Sighting Surveys

Knowledge of the distribution and relative abundance of odontocete cetaceans is important for helping to understand and potentially mitigate impacts of anthropogenic activities. We present small-boat survey and sighting data from 13 y (2000 through 2012) of field studies around the main Hawaiian Islands. We surveyed 84,758 km of trackline, with effort ranging from 3 to 11 y off each of the four different island areas. We had 2,018 sightings of odontocetes representing all 18 species known to exist in Hawai'i. Analyses indicated that sighting rates varied with depth for most species, with some found at their highest rates in shallow (< 1,000 m) water (e. g., common bottlenose and spinner dolphins), some in slope (500 to 2,500 m) water (e. g., dwarf sperm whales and short-finned pilot whales), and some in very deep (> 3,000 m) water (e. g., sperm whales, striped dolphins, Risso's dolphins, and rough-toothed dolphins). Most species (14 of 18) were recorded in all oceanographic seasons. Restricting effort data by depth indicates that in depths > 3,000 m, the most commonly encountered species are rough-toothed dolphins, pantropical spotted dolphins, striped dolphins, and sperm whales. In depths < 2,000 m, the most commonly sighted species were short-finned pilot whales, pantropical spotted dolphins, common bottlenose dolphins, and rough-toothed dolphins. Sighting rates for some species varied among islands, apparently unrelated to differences in effort in different areas. Off Kaua'i and Ni'ihau rough-toothed dolphins were seen more often than expected (25.9% of sightings) based on sighting rates of this species elsewhere in the islands, while pantropical spotted dolphins (3.9% of sightings) and short-finned pilot whales (6.5% of sightings) were seen less often than expected given that they are among the most common species off the other island areas. Such patterns are relevant to interpreting results of acoustic and aerial survey methods in which species identifications are inferred from classification methods or limited by brief sighting opportunities, respectively.