DOI: 10.5061/dryad.7wm37pvnp
title: Co-occurrence of beaked whale strandings and naval sonar in the Mariana Islands, Western Pacific

description: Mid-frequency active sonar (MFAS), used for antisubmarine warfare (ASW),
 has been associated with multiple beaked whale (BW) mass stranding events.
 Multinational naval ASW exercises have utilized MFAS offshore of the
 Mariana Archipelago semi-annually since 2006. We report BW and MFAS
 acoustic activity near the islands of Saipan and Tinian from March 2010
 through November 2014. Signals from Cuvier’s (Ziphius cavirostris) and
 Blainville’s beaked whales (Mesoplodon densirostris), and a third
 unidentified BW species were detected throughout the recording period.
 Both recorders documented MFAS on 21 August 2011 before two Cuvier’s
 beaked whales stranded on 22-23 August 2011. We compared the history of
 known naval operations and BW strandings from the Mariana Archipelago to
 consider potential threats to BW populations. Eight BW stranding events
 between June 2006 and January 2019 each included 1-3 animals. Half of
 these strandings occurred during, or within 6 days after naval activities,
 and this co-occurrence is highly significant. We highlight strandings of
 individual BWs can be associated with ASW, and emphasize the value of
 ongoing passive acoustic monitoring, especially for beaked whales that are
 difficult to visually detect at sea. We strongly recommend more visual
 monitoring efforts, at sea and along coastlines, for stranded cetaceans
 before, during, and after naval exercises.
description: Acoustic recordings were
 collected at a sampling rate of 200 kHz at two locations near the islands
 of Saipan and Tinian from 2010-2014 from High Frequency Acoustic Recording
 Packages (HARPs). All instruments were bottom-mounted and deployed to
 seafloor depths of 600-700 m for the “West HARP” location (15° 19.026’ N,
 145° 27.463’ E), and 1000 m at the “East HARP” location (15° 2.344’ N,
 145° 45.130’ E). From 2011-2013, the temporal coverage of recordings
 overlapped at both locations; however, recordings were not collected in
 all months for all years (see duty cycles in Figures 1 and 2). The
 hydrophone used was an omni-directional sensor (ITC-1042, International
 Transducer Corporation, Santa Barbara, CA), which had an approximately
 flat (±2 dB) hydrophone sensitivity from 10 Hz to 100 kHz of -200 dB re
 V/μPa. Each system contained a custom-built preamplifier board and
 bandpass filter. The calibrated system response was accounted for during
 analysis. The ability to assess the presence of beaked whales varies as a
 function of the recording schedule, the location, and the relative
 abundance and vocal activity of the beaked whale species of interest.
 Accordingly, the duty-cycled recording schedules used in this study may
 result in an underestimation of actual beaked whale presence, and the
 different recording schedules used throughout the study should be
 considered when evaluating relative abundance of acoustic
 activity. The
 acoustic activity of beaked whale signals was detected using a multistep
 detection process following methods described in Baumann-Pickering et al.,
 2013. All echolocation clicks were detected using a computer algorithm
 (Soldevilla et al., 2008). Click detections were then classified as
 Cuvier’s, Blainville’s, or “BWC” signal types based on the spectral and
 temporal characteristics of the species-specific descriptions provided by
 Baumann-Pickering et al., 2013. To identify mid-frequency active
 sonar events, the acoustic recordings were downsampled to a sampling rate
 of 10 kHz, and two analysts (AES and RH), trained to recognize MFAS
 signals, scanned long term spectral averages (LTSAs) over a frequency
 range of 10-5000 Hz to identify time periods with MFAS present. A “packet”
 was defined as a tightly spaced cluster of pulses or pings, which occurred
 within a 1 kHz band between 2.5-4.5 kHz, with a pause between signals of
 no more than 0.1 seconds. To inspect the packets more closely, the analyst
 scanned spectrograms (Hann window, DFT=1,000, 50% overlap) in a 20-second
 window to log the start time, and the lowest and highest frequencies of
 packet components. When the start time of one packet occurred within five
 seconds of the start of a previous packet, they were combined into a
 single packet. The
 acoustic energy of the MFAS was characterized based on MFAS packets. To
 minimize low-frequency ambient noise and focus on the energy band of MFAS,
 the data was filtered with a 10-pole Butterworth bandpass filter (2-4.95
 kHz). The duration of the sonar packet was defined as the interval over
 which 90% of the sound energy arrived at the receiver, with the start and
 end points of an event at the 5% and 95% levels of cumulative energy
 within a time window [5]. A ten second or three second window was used for
 sonar packets with multiple or single MFAS signals, respectively. Details
 of the signal level calculations are included in the electronic
 supplementary materials.
description: Acoustic recordings
 containing the detections of beaked whales and mid-frequency active sonar
 are available from the East HARP (03 July 2012 to 12 May 2013) and West
 HARP (25 June 2012 to 4 March 2013). The raw acoustic data for this study
 is only partially available through Dryad because the volume of the entire
 dataset (&gt;16 TB) exceeds the Dryad data limitations (10 GB) at the
 time of publication. To obtain a copy of the raw acoustic data, please
 contact Erin Oleson (erin.oleson@noaa.gov) at NOAA's Pacific Islands
 Fisheries Science Center. The dataset includes acoustic recordings of
 beaked whale detections collected at a sampling rate of 200 kHz, and
 mid-frequency active sonar detections downsampled to a sampling rate of 10
 kHz. Original acoustic data files had a maximum file size of 900 MB for
 recordings sampled at 200 kHz, and 1.57 MB for recordings downsampled to
 10 kHz. Occasionally, acoustic encounters of beaked whales or
 mid-frequency active sonar would continue beyond the end of a file.
 Detections may are extracted from the raw file containing the onset of the
 encounter. The WAV filename indicates the location of the recording
 ("Saipan" = East HARP, "Tinian" = West HARP),
 deployment number, decimation factor if present, and start time (UTC) of
 the file in the 'yymmdd_HHMMSS' format. For example, for MFAS
 detections, a file sampled at 10kHz (decimation factor=df20) from the East
 HARP beginning on September 21, 2012 00:31:53, has the filename:
 'Saipan03_df20_120921_003153.wav'.  A portion of the low-frequency data
 (&lt;5 kHz) collected in 2013 from the East HARP was not usable for
 detection of MFAS signals due to a hardware failure. This hardware failure
 did not affect the detectability of the higher frequency beaked whale
 signals.