Analysis of bearings of vocalizing marine mammals in relation to passive acoustic density estimation

Abstract

The use of passive acoustic monitoring in population density estimation of marine mammals is a current area of interest, providing an efficient and cost-effective alternative to visual surveys. One challenge that arises with this method is uncertainty in the distribution of individuals. With large arrays where instruments are placed randomly with respect to the animals, it is often assumed that animals are uniformly distributed with respect to the instruments; however with sparse arrays this assumption is likely violated and could lead to bias in the density estimates. Distribution can be better determined through consideration of the horizontal azimuths or bearings of vocalizing animals. This paper presents bearing estimates of fin whales around Wake Island in the Equatorial Pacific Ocean, using ambient recordings from the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) hydrophones at this location. Bearings were calculated for calls detected automatically. Multiple automatic detectors were assessed for optimal performance. Spectrogram correlation was found to produce the best results and bearings were calculated on calls detected with this method. The bearings were calculated using time delay information from the cross-correlation of received signals. Seasonal variation in animal distribution is also discussed. [This work was supported by the Office of Naval Research.]