Vertical sonar beam width and scanning behavior of wild belugas (Delphinapterus leucas) in West Greenland

Abstract

Echolocation signals of wild beluga whales (Delphinapterus leucas) were recorded in 2013 using a vertical, linear 16-hydrophone array at two locations in the pack ice of Baffin Bay, West Greenland. Individual whales were localized for 4:42 minutes of 1:04 hours of recordings. Clicks centered on the recording equipment (i.e. on-axis clicks) were isolated to calculate sonar parameters. We report the first sonar beam estimate of in situ recordings of wild belugas with an average -3 dB asymmetrical vertical beam width of 5.4°, showing a wider ventral beam. This narrow beam width is consistent with estimates from captive belugas; however, our results indicate that beluga sonar beams may not be symmetrical and may differ in wild and captive contexts. The mean apparent source level for on-axis clicks was 212 dB pp re 1 μPa and whales were shown to vertically scan the array from 120 meters distance. Our findings support the hypothesis that highly directional sonar beams and high source levels are an evolutionary adaptation for Arctic odontocetes to reduce unwanted surface echoes from sea ice (i.e., acoustic clutter) and effectively navigate through leads in the pack ice (e.g., find breathing holes). These results provide the first baseline beluga sonar metrics from free-ranging animals using a hydrophone array and are important for acoustic programs throughout the Arctic, particularly for acoustic classification between belugas and narwhals (Monodon monoceros).

Acoustic differentiation and classification of wild belugas and narwhals using echolocation clicks

“…Only one species was observed and recorded at each sampling location, so there were no occurrences in which both narwhals and belugas were present. Since the detection range for echolocation is < 1 km 39 and our search radius was > 5 km, we assume our data are single-species recordings.…”

“…No high pass filter was applied, and the hydrophones served as a low pass filter (150 kHz, 1 pole). The clipping level was at 206 dB pp re 1 µPa at 100 kHz 39 , 41 . To safeguard against potential file corruption and facilitate data post-processing, whale recordings were partitioned, loss-less, in 5-s-long sound files.…”

“…Individual echolocation clicks were detected using the open source passive acoustic analysis software PAMGuard (v. 2.01.03f) 60 . Based on localization analyses using the same data 39 , 41 , the highest amplitude signals were recorded on hydrophones 10 and 11 (positioned at 12 and 13 m deep, respectively) and thus provided the highest quality recordings. For species differentiation analyses and classification models, only data from clicks recorded on hydrophone 10 (12 m depth) were used to avoid pseudo-replication.…”

“…Thorough analyses of echolocation clicks have been conducted separately on belugas 21 , 36 – 40 and narwhals 25 , 27 , 41 , 42 , but no comparative study with data from the same region exists. The broadband signals of beluga and narwhal echolocation extend frequencies between 2–150 kHz, but high-frequency clicks have been reported for both species with energies up to 200 kHz 25 , 39 . Measurements of beluga click peak frequencies in captive and wild environments vary widely between 40–120 kHz 16 , 36 – 39 .…”

“…The broadband signals of beluga and narwhal echolocation extend frequencies between 2–150 kHz, but high-frequency clicks have been reported for both species with energies up to 200 kHz 25 , 39 . Measurements of beluga click peak frequencies in captive and wild environments vary widely between 40–120 kHz 16 , 36 – 39 . Unlike belugas, narwhals have not been held in captivity where controlled experiments can occur, and as a result, narwhal phonation is poorly described compared to belugas and other delphinids.…”