Passive acoustic monitoring of beaked whale densities in the Gulf of Mexico

Abstract

Beaked whales are deep diving elusive animals, difficult to census with conventional visual surveys. Methods are presented for the density estimation of beaked whales, using passive acoustic monitoring data collected at sites in the Gulf of Mexico (GOM) from the period during and following the Deepwater Horizon oil spill (2010–2013). Beaked whale species detected include: Gervais’ (Mesoplodon europaeus), Cuvier’s (Ziphius cavirostris), Blainville’s (Mesoplodon densirostris) and an unknown species of Mesoplodon sp. (designated as Beaked Whale Gulf — BWG). For Gervais’ and Cuvier’s beaked whales, we estimated weekly animal density using two methods, one based on the number of echolocation clicks, and another based on the detection of animal groups during 5 min time-bins. Density estimates derived from these two methods were in good general agreement. At two sites in the western GOM, Gervais’ beaked whales were present throughout the monitoring period, but Cuvier’s beaked whales were present only seasonally, with periods of low density during the summer and higher density in the winter. At an eastern GOM site, both Gervais’ and Cuvier’s beaked whales had a high density throughout the monitoring period.

Effects of duty-cycled passive acoustic recordings on detecting the presence of beaked whales in the northwest Atlantic

“…The results presented here demonstrate that frequent, short listening periods provide a more accurate assessment of daily presence than longer, less frequent periods, even when the overall amount of recording effort is lower. Unlike many other marine mammal species, which may be detected over long distances for hours at a time, beaked whale clicks are only detected over relatively short ranges and durations, while the animal is foraging in close proximity to the recorder (Hildebrand et al, 2015). Consequently, many beaked whale detections are likely to be missed if recordings are collected on a schedule where the cycle period duration greatly exceeds the average duration of detection events, which may be as short as a few minutes.…”

Using passive acoustic monitoring to document the distribution of beaked whale species in the western North Atlantic Ocean

“…Quantitative estimates of the probability of detecting each species at each recording site would require detailed information on the acoustic behavior of each species during foraging dives, including source levels and directionality of clicks as well as rates of click production and patterns of movement during dives (see Hildebrand et al 2015). For most beaked whale species this information does not exist, or is available only from a small number of individuals sampled at specific locations.…”

“…Assessment of populationlevel effects of anthropogenic disturbance is challenging for any cetacean species, and particularly problematic for beaked whales, due to the low encounter rates during visual surveys (Taylor et al 2007). We did not attempt to estimate species' abundance, although methods are being developed to use PAM data for that purpose (Marques et al 2009;Hildebrand et al 2015). Instead, we demonstrate the utility of PAM to estimate baseline levels of occurrence of beaked whale species across broad spatial scales and at high temporal resolutions, facilitating the detection of changes in distributions and habitat use over time.…”

“…In general, detection ranges for beaked whale clicks are expected to be fairly small due to the rapid attenuation of high frequency sound (Zimmer et al 2008;Küsel et al 2011). Hildebrand et al (2015) estimated that beaked whale clicks are detected with certainty only within a few hundred meters of a bottom-mounted HARP, with a maximum detection range of no more than 3.5 km for on-axis clicks directed at the hydrophone. While these ranges were found to be invariant across monitoring sites, our study included a broader range of depths and two instrument types with different sensitivities, which may result in greater variation in site-specific detection ranges.…”

Modelling the broadband propagation of marine mammal echolocation clicks for click-based population density estimates

“…However, options are limited when single sensors are deployed in a wide-baseline array such that they operate independently. In many cases (e.g., K€ usel et al, 2011;Harris, 2012;Hildebrand et al, 2015;Frasier et al, 2016), one must rely on an acoustical modeling approach, where one assumes a source level (or distribution of source levels), models signal propagation at the specified range, and then determines probability of detection using the passive sonar equation. In such cases, the reliability of the density estimation depends on the fidelity of the modeling of detection.…”