Investigating Odontocete Occurrence around Oahu and Maui Nui Using a Multimethod Approach

Abstract

The distribution of odontocetes is largely driven by bottom-up ecological processes that in turn influence foraging opportunities. Environmental variables such as bathymetry may help indicate productive foraging regions, and thus serve as useful tools when assessing dolphin spatial and temporal patterns. To understand the effects of bathymetry and diel patterns on odontocete distributions in an understudied region of the Hawaiian archipelago, the Maui Nui (Maui, Lana‘i, Kaho‘olawe, Moloka‘i) and Oahu Islands, three methodologies were utilized: passive acoustic monitoring techniques, directed vessel-based surveys, and citizen science observations. Acoustic results showed that high-frequency whistling dolphins, verified by a signal classifier to represent smaller odontocetes, tended to occur closer to deep waters and followed strong diel patterns of activity. In contrast, more rare low-frequency whistling dolphins, confirmed to be larger odontocetes, were not influenced by diel patterns. They also typically distributed further from deep waters, except in the Maui Nui region, where no relationship with bathymetry was found. This lack of a trend was likely driven by interspecific habitat differences, as visual sightings showed that false killer whales (Pseudorca crassidens) were observed in shallow waters of approximately 100 m while short-finned pilot whales (Globicephala macrorynchus) were found in deeper insular waters of approximately 650 m. Visual methods also indicated that, during the daytime, smaller species are typically found in shallow waters of 100 m to 300 m, which corroborates previous research on spinner dolphins (Stenella longirostris). When comparing the two visual approaches, depths of sightings were similar for the majority of species observed, reflecting the potential value of citizen science cetacean sighting projects. As a whole, this study provides insight into Hawaiian odonotocetes’ use of foraging and resting habitats relative to bathymetry around Maui Nui and Oahu, and demonstrates the value of a multi-method approach to reveal odontocete distributions.