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Eavesdropping on Hawaiian monk seals

Updated: Feb 21, 2023

Written by Kirby Parnell

As a first-year PhD student in the Marine Biology Graduate Program at UH Manoa, I am excited to begin my research on Hawaiian monk seal underwater acoustic communication. The blog below provides a brief introduction of the first stages of my research which aims to better understand Hawaiian monk seal vocal behavior. This project is in collaboration with NOAA’s Hawaiian Monk Seal Research Program (HMSRP) and Dr. Isabelle Charrier of the Acoustic Communications Laboratory. Enjoy!


Did you know that seals vocalize under water? That’s right! They produce all sorts of sounds to communicate with each other in their vast, underwater environment (listen to two of my all-time favorite seal sounds below in Videos 1 and 2). Specifically, adult male seals vocalize during the breeding season when competition for females and territories is the highest.* But for the endangered Hawaiian monk seal (Neomonachus schauinslandi) – with only 1,400 seals remaining – we know very little about their underwater communication. Sure, there’s the occasional Instagram or YouTube video of a monk seal vocalizing under water recorded by recreational divers,** but no one has ever recorded and described their underwater vocal behavior in the scientific literature.

Video 1. Bearded seal underwater "trill" vocalization (source:

Video 2. Weddell seal underwater vocalization (source:

The Question Master

Figure 1. Resting Hawaiian monk seal. PC: Mark Sullivan.

Our limited knowledge of sound production in this endangered species presents an abundance of questions to be answered. By first gaining a basic understanding of Hawaiian monk seal underwater vocal behavior, we can then begin evaluating the impacts of anthropogenic noise pollution on their communication system. Ultimately, through this research, we aim to inform conservation and management decisions to better protect this endangered and charismatic seal species (Fig. 1). Some of the questions framing my PhD research are:

1. What kind of sounds are the seals producing? What do they sound like? What frequencies are the vocalizations? How loud are these signals?

2. When are they producing these vocalizations? Daytime, nighttime, year-round, or only for a few short months?

3. What do these vocalizations mean? Are they to warn other seals of nearby predators? Are they to attract a potential mate? Or are they produced to compete with another seal?

4. How is man-made noise impacting their communication? Is underwater noise from boats, sonar, or oil and gas exploration masking these communication signals? Can the seals still hear each other vocalize with this constant man-made noise? Listen to these anthropogenic sounds here.

Where do I start?

As mentioned, nobody has ever recorded and described the underwater vocalizations of free-ranging (i.e., not in captivity) Hawaiian monk seals. BUT, many, many people have done so with other marine mammal species. So, I reached out to a few experts in the field of bioacoustics and passive acoustic monitoring to get the inside scoop on best methods for recording the voices of marine mammals. I also absorbed the knowledge of the HMSRP's summer field camp staff who dedicate half a year, each year, to camping on the isolated Northwestern Hawaiian Islands (NWHI) to keep tabs on the monk seal population. Together, their advice facilitated a strategy to record Hawaiian monk seal underwater vocalizations at French Frigate Shoals and Pearl and Hermes Reef in the Papahānaumokuākea Marine National Monument, where most of the Hawaiian monk seal population is found (Fig. 2).

Figure 2. Map of the Hawaiian archipelago and the estimated number of Hawaiian monk seals in these locations.

The Plan

The HMSRP field camp staff will deploy two underwater acoustic recorders (SoundTraps), one at each location mentioned above, for the duration of the summer field camp season. The SoundTraps will record underwater sounds continuously for approximately five months. Fingers crossed monk seals vocalize near our SoundTraps!

Design and Construction

I camped out at hardware stores and became best friends with employees during this phase. After explaining this project, employees happily led me to the proper equipment, like metal that won’t rust in saltwater. After purchasing all necessary supplies for building two sinkable SoundTrap mounts (e.g., two 80 lb. concrete bags, 10 eyebolts, 4 rebar steel pieces, etc…), we heaved-and-hoed the materials onto Coconut Island where my lab, the Marine Mammal Research Program (MMRP), is located. With the help of Jessie, the MMRP intern, and Drew, Hawaii Institute of Marine Biology’s Facilities Manager and best handy-man, we bent rebar, cut and drilled holes in PVC pipes, mixed and set a perfect concrete block, and ultimately created a SoundTrap mount MASTERPIECE equipped with an entanglement-proof line and float for easy deployment and retrieval (Fig. 3). This was an incredibly rewarding experience since I have limited handy-woman skills and the mount turned out perfect.

The SoundTrap underwater acoustic recorder and its concrete mount equipped with handles for carrying and lowering the unit, a line woven through PVC pipes, and a sub-surface float.
Figure 3. The underwater acoustic recording system.

Testing Phase

We made it - great! Now, how the heck do we lower almost 90 pounds of gear into the ocean, and how the heck do we get it out? To assess the feasibility, we first practiced lifting, lowering, and retrieving the gear in shallow water off Coconut Island. Now, 90 pounds is no walk in the park in air, but it’s actually fairly easy to lower and lift in the water. This worked out well (see Video 3), and we decided to practice again in realistic conditions mirroring what the HMSRP field camp team will experience (10-meter water depth, sandy substrate, deploying from a small vessel). This secondary deployment also allowed us to follow along our protocol and evaluate its clarity. With a four-person crew, including my boat-driving classmate Paolo, two deployment and retrieval personnel (Jessie and I), and Ella (MMRP Intern) leading us through the protocol, we successfully deployed and retrieved the unit… twice!

Video 3. Assessing the feasibility of deploying and retrieving the SoundTrap mount.

Another goal of the testing phase was to assess the sound created by the float. We wanted to know if the noise created by the surface float would mask the monk seals’ vocalizations, and sure enough, it did (Fig. 4). Therefore, we redeployed the unit with the float two feet below the water’s surface which decreased the noise tremendously. The testing phase was a great success, and we were able to perfect our protocol and determine the best depth for the float.

Figure 4. Spectrograms showing the difference between noise created by the float at the water’s surface (A) versus noise when the float was approximately 2 feet below the water’s surface (B). Cable strum, or noise from the line and float, is visible in panel A, whereas it is barely visible in panel B. Green boxes indicate cable strum.

Protocols. Protocols. Protocols.

Fortunately, I was taught by the greatest protocol-making scientists at UC Santa Cruz's Pinniped Lab that protocols are life. Protocols are reminders of what you did and how you did it and can serve as a template for the “Methods” section within a scientific research paper. Additionally, protocols provide peace of mind to the researcher that the task will be completed properly, especially when someone else is executing that task. Therefore, I have written three protocols for this project thus far, and you can bet that many more protocols will be created during the data analysis phase.

  1. How to create the SoundTrap 500HF concrete mount

  2. Practicing SoundTrap 500HF deployment and retrieval and ambient noise comparisons in Kaneohe Bay

  3. Recording HMS underwater vocalizations using the SoundTrap ST500 HF in the NWHI


Pandemic: Hahaha… Ha… Ha

Me: *begins working on Plan B*

Flexibility is one topic I am becoming increasingly familiar with day by day; a topic that was brought up when I interviewed for this position. There was no doubt in my advisors’ minds that things would go wrong, plans would certainly change, and that I would have to be flexible. Obviously, this pandemic has caused mayhem for all. My current mayhem is that data collection has been delayed and may be cancelled completely for this year. This project’s data collection phase relies on NOAA’s ship to transport the HMSRP field camp staff and our recording gear to the remote NWHI. At this time, all NOAA ships are under a stand down order until June 1st, which means the earliest that the field camp teams can depart Honolulu is June 30th, delaying our recording period by approximately three months. I am keeping my hopes up that the field camp teams’ hard work and preparations will pay off with a productive voyage to the NWHI where they can achieve data collection for my project, and most importantly, assess the health and status of the monk seal population.

This sound production project is a small piece of the Hawaiian monk seal biology puzzle that will provide a better understanding of their communication system in the years to come. I look forward to learning more about Hawaiian monk seals and sharing the results with you all!


*Van Opzeeland IC et al. (2010) Acoustic ecology of Antarctic pinnipeds. Mar Ecol Prog Ser 414:267–291



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