Effective beam pattern of the Blainville's beaked whale (Mesoplodon densirostris) and implications for passive acoustic monitoring

Abstract

The presence of beaked whales in mass-strandings coincident with navy maneuvers has prompted the development of methods to detect these cryptic animals. Blainville's beaked whales, Mesoplodon densirostris, produce distinctive echolocation clicks during long foraging dives making passive acoustic detection a possibility. However, performance of passive acoustic monitoring depends upon the source level, beam pattern, and clicking behavior of the whales. In this study, clicks recorded from Digital acoustic Tags (DTags) attached to four M. densirostris were linked to simultaneous recordings from an 82-hydrophone bottom-mounted array to derive the source level and beam pattern of the clicks, as steps towards estimating their detectability. The mean estimated on-axis apparent source level for the four whales was 201 dB rms97 . The mean 3 dB beamwidth and directivity index, estimated from sequences of clicks directed towards the far-field hydrophones, were 13 and 23 dB, respectively. While searching for prey, Blainville's beaked whales scan their heads horizontally at a mean rate of 3.6 /s over an angular range of some þ/À10 . Thus, while the DI indicates a narrow beam, the area of ensonification over a complete foraging dive is large given the combined effects of body and head movements associated with foraging.

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

“…This could reflect a greater number of individuals present, behavioral differences in dive depths and movement patterns of foraging animals, and/or acoustic characteristics of echolocation signals, such as frequency content, source level, and beam width (Shaffer et al, 2013;Tyack et al, 2006a;Zimmer et al, 2008). The foraging and acoustic behavior of Cuvier's and Blainville's beaked whales have been reasonably well-studied (e.g., Baird et al, 2006;Johnson et al, 2008;Tyack et al, 2006b), but there is little to no information available for many other beaked whale species, including Gervais' and Sowerby's beaked whales.…”

Review of Underwater and In-Air Sounds Emitted by Australian and Antarctic Marine Mammals

“…During foraging, FM echolocation clicks are produced in long trains, including speed-up click trains (0.2-0.4-s ICI, 15-60 s/train), terminating in buzzes (400 clicks/buzz, 3 s/buzz) indicating a prey-capture attempt. Source levels are 200-220 dB re 1 µPa pp @ 1 m [399][400][401]403,405,406]. Blainville's beaked whales mostly do not echolocate during shallow dives or during deep descents and ascents, clicking vividly only at deep foraging depths.…”

“…These animals embark on deep foraging dives of up to 45-min duration to 1250 m depth followed by a sequence of short and shallow resting dives [397], hence spending little time at the surface where they could otherwise be easily observed. Blainville's beaked whales have been recorded off the Canary Islands [398][399][400][401], in the Bahamas [402,403], off Hawaii [13,404] and off the Northern Mariana Islands [13].…”

Single-click beam patterns suggest dynamic changes to the field of view of echolocating Atlantic spotted dolphins (Stenella frontalis) in the wild

“…Given the high biosonar update rate (typically 1-100 Hz) compared with swim speed (1-5 m s −1 ), it is likely to be more efficient for an echolocating toothed whale to scan a narrow beam gradually through an environment than it is to swim a greater distance with a shorter but wider biosonar. Blainville's beaked whales depend on significant head-scanning movements of up to ±10 deg at rates of 4 deg s −1 when searching for prey patches in the deep ocean (Madsen et al, 2013a;Shaffer et al, 2013), demonstrating how a narrow beam can be sequentially scanned through the environment to search a greater volume of water. However, a narrow beam can be a significant disadvantage when approaching and capturing prey items as rapid escape behaviours at close range might take the prey outside of the acoustic field of view of the approaching predator.…”