An Acoustic Survey of Beaked Whales and Kogia spp. in the Mariana Archipelago Using Drifting Recorders

Abstract

The distribution, abundance, and habitat of cryptic cetacean species such as beaked whales and dwarf/pygmy sperm whales (Kogia spp.) are challenging to study due to their long dive times and/or very limited surface behavior. Even less is known in minimally studied and remote regions, including the Mariana Archipelago and parts of the broader western Pacific. In 2018, we deployed a network of eight Drifting Acoustic Spar Buoy Recorders (DASBRs) on the west side of the Mariana Archipelago with the goal of examining the distribution and habitat of beaked whales and Kogia spp. in this region using passive acoustic monitoring. Concurrently, conductivity-temperature-depth (CTD) data were collected within the drift area and combined with satellite oceanographic data to build Ensemble Random Forest Models to identify specific oceanographic features that determine the distribution of these species. DASBRs deployed at locations ranging from 13°N to 18°N generally drifted from east to west between the Mariana Archipelago and the West Mariana Ridge. Spectral and temporal characteristics of echolocation signals were used to identify the presence of beaked whales and Kogia spp. species. This dataset contained frequency modulated (FM) pulses characteristic of Longman’s (Indopacetus pacificus), Cuvier’s (Ziphius cavirostris), and Blainville’s (Mesoplodon densirostris) beaked whales, as well as the unidentified beaked whale FM pulse known as the “BWC,” along with narrow-band high frequency clicks from Kogia spp. The detection rate was substantially higher for all species on the five tracks in the region north of 15.5°N than for those drifts occurring farther south. Species distribution models suggest that differences in the oceanographic characteristics between the northern and southern regions may impact foraging opportunities, possibly explaining the specific ecological niche for these species within this water mass. This is the first study of the distribution of cryptic cetacean species within the wider Mariana Archipelago region. We demonstrate that autonomous drifting acoustic recorders, combined with environmental sampling and remote satellite data are a powerful tool for studying the habitat dependent distribution of cryptic cetacean species.

Geographic differences in Blainville's beaked whale (Mesoplodon densirostris) echolocation clicks

“…Knowledge of beaked whale ecology is still very limited but has been substantially improved by the application of acoustic monitoring in pelagic and remote regions (e.g. Barlow, Cheeseman, & Trickey, 2021; Barlow, Fregosi, et al, 2021; Baumann‐Pickering et al, 2014; McCullough et al, 2021; Simonis et al, 2020). The ability to use echolocation FM pulse spectral structure to determine population association for individual and groups of Blainville's beaked whales would represent a significant step in detailing the ecology of this cryptic species.…”

Beaked whale dive cycle estimation based on acoustic data from drifting recording systems

“…Here we estimate dive cycle durations from beaked whale echolocation pulses recorded on Drifting Acoustic Spar Buoy Recorders (DASBRs) from multiple studies in 2016–2021 (Barlow & Schorr 2018; Barlow et al, 2022; Keating et al, 2018; McCullough, Olsen, et al, 2021; McCullough, Wren, et al, 2021; Simonis et al, 2020; Yano et al, 2020). Study sites included the California Current (off the U.S. West Coast and west of northern Baja California, Mexico) and pelagic waters around islands of the central and western Pacific (main Hawaiian Islands and Mariana Archipelago).…”

“…Potential pulses were confirmed or rejected based on the waveform, frequency spectrum, and Wigner representations. Fuller descriptions of the data processing chain are given in Simonis et al (2020), McCullough, Olsen, et al, (2021), and McCullough, Wren, et al, 2021.…”

Geographic distribution of the Cross Seamount beaked whale based on acoustic detections

“…A unique FM pulse type was first recorded near Cross Seamount in the Pacific Ocean south of Hawaii, with a linear frequency upsweep from 35 to 100 kHz, duration of 932 μs, and mean IPI of 0.14 s (McDonald et al 2009). This FM pulse type, called the BWC signal (Baumann‐Pickering et al, 2013), has been recorded in a number of tropical locations throughout the central and western Pacific Ocean (Baumann‐Pickering et al, 2014) and elsewhere in Hawaii (Manzano‐Roth et al, 2023), the Mariana Archipelago (McCullough et al, 2021b), and off Baja California, Mexico (Simonis et al, 2018), but has never been definitively linked to a known species. Its acoustic features are distinctive from other beaked whale signals due to the pulse's longer duration, shorter IPI, and extremely wide bandwidth.…”