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Schematic spectrogram shows the grouping definition used by analysts for observed MFAS signals. “Packets” may contain multiple frequency-upsweeps, downsweeps and tonal pulses occurring with gaps of less than 0.1 seconds. A packet may actually consist of two packets that occur within 5 seconds of each other.

Major International Navy Training Operations in the Mariana Islands Range Complex from 2003 – 2019.

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 used 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 BWs (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 BWs 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 one to three animals. Half of these strandings occurred during, or within six 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 BWs 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.

Schematic spectrogram shows the grouping definition used by analysts for observed MFAS signals. “Packets” may contain multiple frequency-upsweeps, downsweeps and tonal pulses occurring with gaps of less than 0.1 seconds. A packet may actually consist of two packets that occur within 5 seconds of each other.

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 used 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 BWs (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 BWs 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 one to three animals. Half of these strandings occurred during, or within six 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 BWs 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.

Probability of n stranding events randomly occurring during or within 6 days after documented naval events from June 2006 through January 2019

Schematic spectrogram shows the grouping definition used by analysts for observed MFAS signals. “Packets” may contain multiple frequency-upsweeps, downsweeps and tonal pulses occurring with gaps of less than 0.1 seconds. A packet may actually consist of two packets that occur within 5 seconds of each other.