Using multipath reflections to obtain dive depths of beaked whales from a towed hydrophone array

Abstract

Beaked whales are deep divers, emitting echolocation clicks while at depth. Little is known about the dive behavior of most species; however, passive acoustic data collected with towed hydrophone arrays can provide depth information using multipath reflections of clicks coupled with a two-dimensional localization of the individual. Data were collected during a shipboard survey in the western North Atlantic Ocean using a towed linear hydrophone array. Beaked whale tracks were classified as either Cuvier's (Ziphius cavirostris) or Gervais'/True's (Mesoplodon europaeus/Mesoplodon mirus). Weighted species average depths and weighted species standard deviations were 1158 m ± 287 m for Cuvier's (n = 24), and 870 m ± 151 m for Gervais'/True's (n = 15). Depth uncertainties ranged from 3% to 142% of the average depth. Slant ranges were corrected for depth to provide average horizontal perpendicular distance estimates. The average horizontal perpendicular distance distribution exhibited fewer detections in the first bin than the second. This is the first report of dive depths for Gervais'/True's beaked whales and use of this method to obtain depths for beaked whales using a towed linear array.

A description of echolocation clicks recorded in the presence of True's beaked whale (Mesoplodon mirus)

“…It had a non-linear frequency response across the signal of interest, with an increase in sensitivity of approximately 6 dB from 30 to 70 kHz, which was corrected for during the analysis using custom-written scripts in the program MATLAB (R2017a; MathWorks, Inc., Natick, MA). For further details on array design, see DeAngelis et al (2017).…”

Sperm whale acoustic abundance and dive behaviour in the western North Atlantic

“…The array was towed 300 m behind the vessel at approximately 5–10 m depth while the vessel was in waters more than 100 m deep and underway at speeds of 16–20 km/h. For more details see DeAngelis et al 31 , with the only change being that two APC hydrophones and one Reson hydrophone in the aft section were replaced with HTI-96-Min hydrophones (High Tech, Inc., Long Beach, MS). The HTI’s had a flat frequency response from 1 to 30 kHz (− 167 dB re V/uPa ± 1.5 dB).…”

“…Using the methods established by DeAngelis et al 31 and custom Matlab R2021a (MathWorks Inc., Natick, NA) scripts, the multipath arrival of clicks and surface reflected echoes were used to mathematically convert the linear array into a 2D planar array and estimate 3D localizations. Using the .wav clips exported from PAMPal, the time delay between the click and the corresponding surface reflected echo, known as the slant delay, was measured via autocorrelation.…”

“…Alternatively, 3D localization using passive acoustic towed array data yields information from a larger proportion of the population and thus can be used to draw broader conclusions about dive behaviour and foraging ecology. DeAngelis et al 31 combined 2D TMA localizations with the time difference between the arrival of a direct click and corresponding surface reflected echo on a towed hydrophone element to estimate the depth of echolocating beaked whales. As sperm whales also echolocate at depths greater than most towed hydrophone array depths, this analysis applied the same method to their usual clicks.…”

Data_Sheet_3.docx

“…When it is necessary to know the range, bearing, location, or depth of a vocalizing marine mammal (e.g., for real-time tracking or estimating abundance of individuals), an array of time-synchronized hydrophones is required (Thode, 2004;von Benda-Beckmann et al, 2010;Gillespie et al, 2013;DeAngelis et al, 2017). PAM arrays may be towed behind a vessel that is underway, but in this case, flow noise can obstruct lowfrequency sounds from being heard.…”