Written by Brijonnay Madrigal
False killer whales (FKW) (Pseudorca crassidens) and short-finned pilot whales (SFPW) (Globicephala macrorhynchus) are resident, toothed whale species to the Hawaiian archipelago. In Hawaiʻi, they are colloquially referred to as ‘Blackfish’ as they are largely dark gray in color, often appearing all black. Both FKW and SFPW exhibit complex social structure which translates into complexity in their communication systems. These two species generally produce three types of acoustic signals which serve different functions: clicks, whistles, and burst-pulse sounds. Short-duration, directional, broadband clicks are used in echolocation, and function in foraging and navigation. Whistles are narrow-band, frequency modulated signals that function in communication. Burst-pulse signals, including pulsed calls, are composed of a series of pulses produced in such rapid succession as to sound tonal and function in communication.
Data Collection
Five suction-cup attaching, multi-sensor Custom Animal Tracking Solutions (CATS) tags (Figure 1) were deployed on false killer whales (n = 2) and short-finned pilot whales (n = 3) in February and November 2023 through a collaboration between MMRP and the Pacific Whale Foundation. CATS tags were deployed off Maui (n = 1) and Lānaʻi (n = 4). One false killer whale tag was deployed on a Cluster 4 animal. Each tag was equipped with internal sensors (accelerometer, magnetometer, and gyroscope), hydrophones, and a video camera. HTI-96-min hydrophones were used to record sound at a sampling rate of 96 kHz. Check out this social media video and blog post created/written by MMRP PhD students, Jens Currie and Gussie Hollers respectively for a summary of our Lānaʻi 2023 field season.
Figure 1. A CATS tag deployed on a short-finned pilot whale off Lānaʻi, Hawaiʻi.
Audio Data in Summary
Overall, we currently have ~60 hours of audio from the CATS tags across both species. Deployment duration with audio ranged from ~3 hours to < 42 hours. A preliminary review of a ~ 8 hour deployment on a FKW showed that the tagged animal produced > 4,700 pulsed calls with the majority of calls containing simultaneously produced clicks (Video 1). This tag was deployed in February off Maui so if you listen closely to the audio in Video 1, you can hear humpback whale (Megaptera novaeangliae) chorusing in the background. Check out this awesome CATS tag video footage from the FKW's perspective as humpback whales come into view (Video 2)!
Video 1 Spectrogram of false killer whale calls. Many calls have simultaneously produced clicks.
Video 2. CATS tag footage of a false killer whale with humpback whales nearby.
Acoustic communication of false killer whales
Based on the fundamental frequency contour, pulsed calls can be categorized into discrete call types. There were ~ 8 call types produced by one tagged FKW (Figure 2). Previously, FKW whistles have primarily been recorded, with few accounts of pulsed calls produced by this species. Killer whales (Orcinus orca), the cousins of FKW, most commonly produce pulsed calls that differ across ecotypes and dialects have even been described for specific pods like the Southern resident killer whales (J, K, L pods). I am interested in not only describing the acoustic repertoire and diversity of pulsed call types produced by false killer whales but also exploring if individuals call types correspond with Main Hawaiian Island insular population clusters (e.g. Cluster 3) associated with different island regions. I am also interested in assessing if false killer whales produce individually distinct "signature whistle" like calls, which have been documented in bottlenose dolphins (Tursiops truncatus) and killer whales. These calls are highly stereotyped and contain information about the identify of the calling animal. Some of this individual identify information may be contained in the simultaneous click patterns we observe. In addition to diving into the vocal repertoire, we are interested in calculating call rates across the dive cycle and compare call production with different phases of the dive cycle (e.g. surface, descent, bottom, ascent) and behavioral state.
Figure 2. Spectrograms of false killer whale calls containing representative examples for a subsample of call types.
Foraging Ecology of Short-finned Pilot Whales
Video footage from the CATS tags can provide an incredible window into the world of deep diving SFPW which feed on the mesopelagic boundary community. They are a highly social species and often travel in subgroups (Video 3). The specific CATS tags we have deployed on pilot whales are deep-rated tags specifically designed for withstanding depths of >800 meters. When the animal dives down below the photic zone (~180-200m), a light on the tag is activated which allows us to gain a better view into the animal's foraging behavior at depth (Video 4). In this footage, you can see squid and other prey items flowing past the head of the animal indicating an attempted prey capture. When we look at the audio, we can hear this behavior and also see it in the spectrograms (Figure 3). Since dolphins use echolocation as a scanning mechanism to locate prey, as inter-click-interval (ICI) between clicks decreases, clicks are produced more rapidly, indicating the animal is honing in on prey, in what bioacousticians refer to as a "terminal buzz".
Although we have video footage from the CATS tags to provide insight into the behavior of these animals, often times the video will stop recording before the hydrophones stop recording. For example, we had a SFPW tag continue to record audio for 42 hours, long after the video had stopped recording. Therefore, acoustics can be an excellent tool to fill in the gaps especially for these deep diving species, like pilot whales, that rely on echolocation for foraging and navigating in their environment at depth! From the acoustic data, we can calculate feeding rates by quantifying prey capture attempts (a.k.a. presence of terminal buzzes). By quantifying foraging effort, we can add this to the bioenergetic budget model of these animals to understand how much energy they need to survive. We can also potentially explore the spatial perception of pilot whales to their prey by calculating the prey distance from the tagged animal based on the echolocation clicks.
Video 3. CATS tag footage of a short-finned pilot whale group swimming together near the surface.
Video 4. CATS tag footage of a short-finned pilot whale foraging at depth. A light on the tag turns on at ~180-200 m depth so we can observe what is happening at depth below the photic zone to see the perspective of the animal. At 00:07 you can see squid/prey passing the head of the animal indicating the animal is feeding.
Figure 3. Spectrogram of short-finned pilot whale regular clicks. A terminal buzz (yellow bracket) indicates a prey capture attempt in the audio which coincides with the video.
Upcoming Data Processing and Analysis
We will run an automated click detector to extract times for each click produced by the tagged animal and measure inter-click interval (ICI) to locate the presence of terminal buzzes which indicate prey capture attempts. Calls produced by the tagged animal will be identified and annotated through manual inspection of spectrograms. Features of the call fundamental frequency contour will be extracted (e.g. begin frequency, end frequency, duration). Calls will be categorized based on their extracted contour spectral and temporal features using both experienced observers and Random Forest classification algorithms. A special thanks to all my interns currently helping me with the huge data processing stage!
Project Significance
This MMRP and Pacific Whale Foundation joint-project, marked the first time CATS tags have been deployed on toothed whales in Hawai'i. Very few acoustic tags have been deployed on these species, so this project will provide valuable insight into the acoustic behavior of these highly vocal species. FKW are considered a species of high concern in Hawaiʻi with the Main Hawaiian Island Insular population listed as endangered under the Endangered Species Act. The findings from this work may aid in species identification and population classification efforts led by the PIFSC Cetacean Research Program and help inform management and conservation efforts in Hawai'i.