Signals from the deep: Spatial and temporal acoustic occurrence of beaked whales off western Ireland

Abstract

Little is known of the spatio-temporal occurrence of beaked whales off western Ireland, limiting the ability of Regulators to implement appropriate management and conservation measures. To address this knowledge gap, static acoustic monitoring was carried out using eight fixed bottom-mounted autonomous acoustic recorders: four from May to December 2015 on Ireland’s northern slope and four from March to November 2016 on the western and southern slopes. Recorders ran for 205 to 230 days, resulting in 4.09 TB of data sampled at 250 kHz which could capture beaked whale acoustic signals. Zero-crossing-based automated detectors identified beaked whale clicks. A sample of detections was manually validated to evaluate and optimize detector performance. Analysis confirmed the occurrence of Sowerby’s and Cuvier’s beaked whales and Northern bottlenose whales. Northern bottlenose whale clicks occurred in late summer and autumn, but were too few to allow further analysis. Cuvier’s and Sowerby’s clicks occurred at all stations throughout the monitoring period. There was a significant effect of month and station (latitude) on the mean daily number of click detections for both species. Cuvier’s clicks were more abundant at lower latitudes while Sowerby’s were greater at higher latitudes, particularly in the spring, suggesting a spatial segregation between species, possibly driven by prey preference. Cuvier’s occurrence increased in late autumn 2015 off northwest Porcupine Bank, a region of higher relative occurrence for each species. Seismic airgun shots, with daily sound exposure levels as high as 175 dB re 1 μPa2·s, did not appear to impact the mean daily number of Cuvier’s or Sowerby’s beaked whale click detections. This work provides insight into the significance of Irish waters for beaked whales and highlights the importance of using acoustics for beaked whale monitoring.

Click characteristics of northern bottlenose whales (Hyperoodon ampullatus) and Sowerby's beaked whales (Mesoplodon bidens) off eastern Canada

“…The northern bottlenose whale clicks described in this study are consistent with clicks previously found in bottommounted recordings from the Gully (Martin and Moors-Murphy, 2013;Stanistreet et al, 2017) and Ireland (Kowarski et al, 2018). We found this click type in recordings made in the presence of northern bottlenose whales, not only in the Gully and other submarine canyons along the Scotian Shelf, but also off Newfoundland.…”

“…Other factors that can influence the variation in acoustic parameters are the specific environmental context in which they were recorded, such as the whale's range and orientation relative to the hydrophone, propagation effects, or differences in recording systems. However, the click parameters reported here are consistent with northern bottlenose clicks found in other large acoustic datasets from the Scotian Shelf and Ireland, which include both towed array and bottom-mounted recordings, suggesting that the frequency characteristics we describe are not unique to the environmental context or recorder characteristics of this study (Stanistreet et al, 2017;Kowarski et al, 2018). While we focused on describing the general characteristics of clicks produced by two beaked whale species, it is also important to consider intraspecific variation.…”

“…These on-axis clicks were characterized as having an FM upsweep structure with a center frequency of 43 6 7 kHz and an inter-click interval of 306 6 118 ms. This description is limited in sample size and not detailed enough for a full comparison, but it is not consistent with the northern bottlenose whales clicks typically recorded in the western North Atlantic (Moors-Murphy, 2012; Martin and Moors-Murphy, 2013;Stanistreet et al, 2017) or in the eastern north Atlantic, off Ireland (Kowarski et al, 2018).…”

“…Temporal and spectral characteristics of the FM clicks, such as the inter-click interval (ICI), peak frequency, center frequency, bandwidth and upsweep appear distinct between beaked whale species (Baumann-Pickering et al, 2013a). Detailed characterization of beaked whale acoustic signals allows species-specific classification and has informed the development of automated beaked whale click detectors resulting in significant improvements in the interpretation of PAM data for beaked whale monitoring (Dawson et al, 1998;Johnson et al, 2004;Zimmer et al, 2005;Johnson et al, 2006;Tyack et al, 2006;Gillespie et al, 2009;Baumann-Pickering et al, 2010;Rankin et al, 2011;Baumann-Pickering et al, 2013a;Baumann-Pickering et al, 2013b;Yack et al, 2013, Stanistreet et al, 2017, Kowarski et al, 2018.…”

“…In this study Cholewiak et al (2013) identified four different FM click types based on median peak frequencies, at 26, 33, 51, and 67 kHz. However, only one of these click types, with a 65-70 kHz peak frequency, has been found in other acoustic datasets, and therefore automated detectors, PAM programs, and analysts looking at data from bottom-mounted recorders have primarily relied on this high-frequency click type to detect and identify Sowerby's beaked whale clicks (Martin, 2016;Stanistreet et al, 2017, Kowarski et al, 2018.…”