Geographic differences in Blainville's beaked whale (Mesoplodon densirostris) echolocation clicks

Understanding the distribution of cetacean species and their populations over space and time is relevant to conservation, management, and mitigation goals. Geographic differences in Blainville's beaked whale (Mesoplodon densirostris) frequency-modulated (FM) echolocation pulses are investigated as a potential tool for population-level discrimination. Blainville's beaked whales have a cosmopolitan distribution from temperate to tropical waters. They have been previously known to produce species-specific FM pulses with a steep energy onset at around 25 kHz, a small energy peak at 22 kHz, a peak frequency of 30 to 34 kHz, and an inter-click interval of 280 ms. We have identified several FM signals at recording sites across the North Pacific and North Atlantic which have spectral shapes and temporal characteristics resembling Blainville's beaked whale FM pulses, and are not attributable to other known beaked whale species or signal types. Quantification of the variability in spectral shapes and inter-click intervals measured within and between encounters was achieved through weighted network clustering. Spectral averages obtained from the primary clusters identified at each site exhibited frequency shifts of up to 6 kHz relative to previously described Blainville's FM pulses, and occurred instead of the "usual" Blainville's beaked whale FM pulse type at any given site. We identified a latitudinal cline, with higher peak frequencies occurring in lower latitudes. The observed variability may have several possible origins. Body size has been shown to influence signal frequency, with lower frequencies being produced by larger animals. In turn, larger animals tend to be found in higher latitudes for some species, but this has not been investigated in beaked whales. Furthermore, prey size may shape the frequency content of echolocation signals and larger prey items may occur in higher latitudes, resulting in lower echolocation frequencies of their predators. The observed differences in echolocation signal frequency may be a first indication of acoustic delineation between population-level boundaries of Blainville's beaked whales that have not been identified previously.

Suggested Citation:

Baumann-Pickering, S., Trickey, J.S., Oleson, E.M. (2018) Geographic differences in Blainville's beaked whale (Mesoplodon densirostris) echolocation clicks, Marine Physical Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, MPL Technical Memorandum #630 under Cooperative Ecosystems Study Unit Cooperative Agreement N62473-17-2-0014 for U.S. Navy, U.S. Pacific Fleet, Pearl Harbor, HI.