Temporal and spatial trends in acoustic detections of delphinid species around Niihau, Hawaii

Abstract

Passive acoustic monitoring using seafloor-mounted recorders allows cetacean occurrence to be examined over time and space. Four ecological acoustic recorders (EARs) were moored around the Hawaiian island of Niihau in summer/fall (July—November) 2011, and winter/spring (January—May) 2012. Delphinid whistle “detections” (a proxy for schools) were identified and characterized. Whistles were identified to species using a random forest classifier trained with whistles recorded from seven species (Globicephala macrorhynchus, Pseudorca crassidens, Stenella attenuata, S. coeruleoalba, S. longirostris, Steno bredanensis, and Tursiops truncatus) in the tropical Pacific Ocean. The highest number of detections per day occurred during summer/fall at all sites. All species except for G. macrorhynchus were detected at every site during both deployments. No single species dominated the detections at any site, with the exception of Stenella longirostris at the Pueo Point site during summer/fall (53% of detections). Pseudorca crassidens, a species of particular management/conservation interest due to small population sizes, were detected most frequently (18% of detections) at the Niihau NW site during summer/fall and least frequently (7% of detections) at the Pueo Point site during summer/fall. Understanding trends in species composition provides insight into how species use different habitats and aids in management efforts.