Hawaiian Monk Seal Karaoke Gone Wrong
Written by Kirby Parnell
In early May, I traveled to Kauai to deploy an underwater acoustic recorder, known as a SoundTrap. The goal was to record the underwater vocalizations of Hawaiian monk seals offshore the tiny, uninhabited island of Lehua Rock (Fig. 1). Lehua Rock is a monk seal hotspot where adult male seals are seen and heard vocalizing often by recreational divers (Video 1). With the help of Captain Tara Leota and Adam King at Kauai Searider Adventures, we completed the 17-mile crossing from Kauai to Lehua Rock where we successfully deployed the SoundTrap at a popular dive site known as Vertical Awareness. The SoundTrap was programmed to passively record sounds as it rested peacefully on the rocks below the surface (Fig. 2).
Figure 1. Google Earth image of Lehua Rock (red mark) in relation to Niʻihau and Kauai. The yellow star indicates where the SoundTrap was deployed (left). A photograph of Lehua Rock taken from the boat (right).
Figure 2. The SoundTrap resting on the seafloor at the dive site Vertical Awareness where Hawaiian monk seals are often observed vocalizing under water.
Video 1. An adult Hawaiian monk seal vocalizing under water at Lehua Rock. Notice its throat expanding while it produces sound. (Video credit: Jonathan Bird)
Little did I know, monk seals REALLY love karaoke, but they haven’t quite figured out the appropriate distance from their mouths to the hydrophone (AKA: underwater microphone which is the most sensitive part of the equipment) (Fig. 3). These singing seals repetitively touched the hydrophone, bit it, and yelled at it – all things that any public speaker or performer would avoid doing with a microphone. A seal was even observed rubbing its back on the SoundTrap like a bear would do to a tree.
Figure 3. The SoundTrap prior to its deployment. The shiny metal cylinder stores the batteries and the recording equipment. The hydrophone (black rubber tip) is the piece that listens and records sounds. The SoundTrap was attached to the concrete blocks via hose clamps, and the equipment was lowered into the ocean by the white rope.
As you can guess, this type of antagonistic behavior towards the hydrophone did not end well for our SoundTrap. While leading a dive tour, Captain Clay at Bubbles Below, noticed that the hydrophone was missing and was nowhere to be found. Of course, I thought the worst of the situation: I was nervous that a monk seal had ingested the 4” long hydrophone, that the SoundTrap had leaked, and that all the data was destroyed. Miraculously, none of that happened. Captain Clay found the hydrophone near the SoundTrap in a crevice in the reef, the SoundTrap had a teeny, tiny bit of water in the connection point (which is fixable for a steep price) (Fig. 4), and the acoustic data was safe! The SoundTrap recorded for six days before the hydrophone was damaged and removed by a seal. These six days of data show some of the cleanest and loudest Hawaiian monk seal vocalizations that I’ve seen (Video 2), as well as the acoustic signature of a monk seal torturing the hydrophone (Videos 3 and 4).
Figure 4. Corrosion at the hydrophone connection point. Luckily, only one prong hole was corroded. The SoundTrap is currently being repaired by the manufacturer, Ocean Instruments.
Video 2. Recording of a Hawaiian monk seal underwater vocal bout shown as a spectrogram (read more about "spectrograms" below). The high-frequency clicks throughout the recording are snapping shrimp.
Video 3. Spectrogram progression of a Hawaiian monk seal vocalizing towards the SoundTrap and touching/biting the hydrophone. The seal's vocalizations are below 1 kHz. The red and yellow signals that span the entire frequency range (0-24 kHz) indicate that the seal is touching or biting the hydrophone. The dark blue color at the end of the video signifies that the hydrophone is damaged or has been removed from the SoundTrap.
Video 4. Recording of a Hawaiian monk seal interacting with the hydrophone.
What are Spectrograms?
Acoustic data is commonly analyzed as spectrograms (Video 2) which is a way to visualize sound. Spectrograms show the frequency or pitch of the sound on the y-axis, time on the x-axis, and the amplitude or loudness of the sound is displayed by its color with red being really loud and blue being quiet. Viewing this acoustic data as spectrograms allowed me to determine exactly when the hydrophone stopped working. Being able to “see” and hear sound at the same time is one of my favorite things about this research. Not only can I hear what an endangered species is saying underwater, in this case, I can also infer the seal’s behavior and know that it was very interested in our equipment.
Lessons learned from this experience:
Lehua Rock is a great location to record the underwater vocalizations of Hawaiian monk seals. This data set confirms underwater vocalizations at Lehua Rock. Vocalizations were detected in all six days of recordings. We have plenty of video evidence from divers, and now, acoustic recordings that validate Lehua Rock as a vocal hot spot for Hawaiian monk seals.
The hydrophone must be protected – In future deployments, the hydrophone will be surrounded by a metal housing so that seals cannot touch it.
The SoundTrap should be less conspicuous. Hawaiian monk seals are curious creatures. When they see something new and interesting, they often approach and investigate the item. As these seals were probably very familiar with their environment, our shiny SoundTrap most likely caught their attention. Hopefully we can avoid seal-SoundTrap interactions by camouflaging the equipment and making it less reflective next deployment.
Although this deployment did not go exactly as planned, we gathered more information on Hawaiian monk seal acoustic communication, and I learned valuable lessons that I’ll apply at the next Hawaiian monk seal karaoke bar.
I would also like to acknowledge Jessie Hoffman, Jamie Thomton, Aaron Swink, Captains Tara and Adam, and Captain Clay for their interest, encouragement, and assistance with the project, as well as the many divers that contributed videos of seals vocalizing at Lehua Rock.
More photos from the excursion to Lehua Rock
Our trusty vessel "Hoʻokahi" that safely got us where we needed to go.
"Seagret" was our mascot for the trip. We discovered this baby egret floating near Lehua Rock.
Humpback whales spotted on the ride to Lehua Rock. This is most likely a mom, calf (middle), and escort. Read more about our research on humpback whales here.
Dorsal fin of a Hawaiian spinner dolphin off the coast of Kauai. Read more about our research with spinner dolphins here.