Description of sounds associated with Sowerby's beaked whales (Mesoplodon bidens) in the western North Atlantic Ocean.

Several groups of Sowerby's beaked whales (Mesoplodon bidens) were encountered on July 4, 2011, during a shipboard cetacean survey conducted off the eastern seaboard of the United States. Acoustic recordings were collected using a three-element towed hydrophone array. Many echolocation clicks were recorded during the encounter, but no tonal sounds were detected. A total of 2969 echolocation clicks were included in analyses of frequency and temporal characteristics. A Gaussian mixture model with four mixtures was fitted to the histogram of peak frequencies; four subsets of clicks were designated. The majority of clicks (n = 2048) contained a median peak frequency of 33 kHz, while the others contained a median peak frequency of 25 kHz (n = 324), 51 kHz (n = 304), or 67 kHz (n = 293). Most clicks did not contain a clear frequency-modulated upsweep, though some clicks exhibited a slight sweep from 30-36 kHz. Seven burst pulses were detected in the encounter, two of which were of high enough quality for detailed analysis. The acoustic characteristics of Sowerby's beaked whales have not previously been described; the current study will facilitate incorporation of these data into passive acoustic monitoring programs in the North Atlantic Ocean.