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Secret life of seals: Exploring the underwater acoustic behavior of Hawaiian monk seals with biologging tags

Written by Kirby Parnell

I am excited to share that last week, in collaboration with NOAA’s Hawaiian Monk Seal Research Program (HMSRP) and Pacific Islands Regional Office Protected Resources Division, we successfully instrumented two Hawaiian monk seals with Customized Animal Tracking Solutions (CATS) tags! This company creates multi-sensor biologging devices so that scientists can track the fine-scale movements and behaviors of wild animals including sharks, whales, crocodiles, and penguins. The CATS Cams that we deployed on monk seals include a suite of sensors: a triaxial accelerometer to measure acceleration, a triaxial magnetometer to measure the direction the seal is facing, a gyroscope to measure its rotational velocity, a pressure sensor to measure depth, a GPS, a video camera, and a hydrophone to record underwater sounds. Think of this as a seal FitBit and GoPro combined into one device!

Figure 1. CATS Cam and VHF tag attached to the back of a seal via glue. NMFS permit 22677.

With these simultaneous data streams, we hope to answer questions about seal behavior while at sea — something that is extremely difficult to do because seals spend 2/3 of their time in the ocean. Using the data collected from CATS cams, we aim to better understand monk seal underwater acoustic behavior and soundscape (acoustic environment), and to assess their behavioral responses to man-made sounds.

How do CATS Cams help us explore these topics?


Hydrophone Data

The hydrophone within the CATS Cam records seal vocalizations and other sounds produced near the seal including boats, snapping shrimp, and even fish! Analysis of the acoustic data can determine the vocal repertoire of wild monk seals and the frequency, amplitude, and temporal characteristics of their calls. Currently, we have only described the vocal behavior of two seals in human-care. The CATS Cam project combined with my passive acoustic monitoring project will result in the first descriptions of the underwater vocal behavior for free-ranging Hawaiian monk seals!

Figure 2. Spectrogram showing free-ranging Hawaiian monk seal vocalizations recorded at Lehua Rock. You can hear seal calls and snapping shrimp in the video.

Hydrophone + GPS Data

The hydrophone and GPS data can be used to describe the soundscape of aquatic habitats that the tagged seals frequent. This includes determining locations with high levels of man-made noise, as well as locations with increased vocal activity that may be ecologically important for Hawaiian monk seals (e.g. for breeding, socializing, or foraging). Using this information, we can evaluate spatial and temporal patterns in vocal activity with respect to man-made noise.

Hydrophone + Video Data

By pairing the video footage with the audio data, we can identify behaviors associated with vocalizations to infer function of sound production. For example, is the seal vocalizing while it’s foraging to scare prey out of the reef, or while it’s interacting with other seals? The function of Hawaiian monk seal vocalizations still remains a mystery, but for other seal species, underwater calls are likely used for reproductive purposes (e.g. mate attraction, territory defense).

Figure 3. Video showing a monk seal “whooping” as it approaches another seal. Video was taken with a Crittercam in 2013. NMFS permit 10137.

Hydrophone + Accelerometry Data

We are interested in testing the ability of the triaxial accelerometer to pick up the low frequencies of monk seal vocalizations. If possible, we can determine if the tagged seal is vocalizing, or if the calls are produced by another seal nearby. If successful, this method can be utilized to contextualize vocal behaviors and quantify calling rates for individual seals. You can read more about this method here.


Hydrophone + Accelerometry + Depth Data

Pairing the acoustic, depth, and movement data enables an assessment of fine-scale changes in diving, foraging, and resting, as well as changes in vocalizations (e.g. cessation of call, increases in call amplitude) when a seal encounters man-made noise.  Ultimately, these data streams will shed light on the effects of noise exposure on Hawaiian monk seal behavior and communication.

Amazing, right?! There are so many questions that can be answered using multi-sensor biologging instruments!


Tagging Expedition


Using monk seal sighting data from the public and our partners, we identified beaches on O’ahu where adult male seals were seen hauled out the week of our tagging efforts. We then “scouted” these beaches for our candidate seals and boy did we get lucky! On our third day of effort there were FIVE seals on the beach. Our first attempt was unsuccessful as the seal continuously “crocodile rolled” — however, we were still able to give him a new flipper tag so that he can be easily identified in the future (RG28 or “Leftie” now has tag G44). We had another stroke of luck as one of the five seals we initially encountered moved to an adjacent beach. With the help of Hawaii Marine Animal Response (HMAR) and Barbara Billand, we spoke to beachgoers about our mission and cleared a safe location on the beach for a second attempt. Working alongside scientists and veterinarians at the HMSRP, we successfully instrumented RK72 (“Kekoa”) with a CATS Cam and a satellite tag. He also obtained new flipper tags in the process (K22, K23). Since then, his satellite tag has pinged near Barber’s Point, Ko Olina, and the Honolulu Airport.

Figure 4. CATS Cam and satellite tag on the back of RK72 (top left). New flipper tags (top right). RK72 swimming with his new instruments (bottom). NMFS permit 22677.

On our fourth day of effort, we scouted for seals with Hawaii State Parks on the North Shore and got lucky again. THREE seals were on the beach, and we chose to instrument R330 (“Squinty”). He also received his first flipper tags (3CA, 3CB) and the vet team was able to gather biological samples as part of the HMSRP’s population health monitoring research. After releasing him, he moved down the beach and approached the two other seals. They vocalized at each other (yay for acoustic data!) and he then proceeded to lazily roll back into the water, four times, on top of his freshly attached CATS Cam. As the team was packing up, we saw multiple breaching humpback whales — an amazing finale to our tagging expedition. R330’s satellite tag has since pinged around Ka’ena Point.

Figure 5. Placing the CATS Cam on R330's back (top left). New flipper tags (top right). Video of R330 rolling back into the water after interacting with two other seals (bottom). NMFS permit 22677.

What’s Next?


Instrumenting seals is only half of the story. Because CATS Cams store data on the tag, we must now retrieve it meaning that we must find the seals on a sandy beach again. The satellite tag helps us determine the general location of the seal but only while satellites are over Hawaii (morning and evening hours). Therefore, we rely heavily upon seal sightings from the public and our partners who are currently helping us search for RK72 and R330. Once these seals are sighted on the beach, the HMSRP team will recapture the seal, remove the tag, and release the seal. The seals will be left with a bit of mesh and foam on their backs until they molt (something that seals do once per year). We will walk away with a goldmine of behavioral and acoustic data which will shed light on the cryptic underwater life of an endangered species.


In collaboration with Hawaii State Parks, we successfully retrieved R330's CATS Cam this week! I'm currently offloading the 89 GB of data recorded on the tag (see a sneak peak video below). We are still searching for RK72 - his satellite tag has pinged near the Honolulu Airport the last few days. Mahalo to the airport security team who have been looking for RK72 this week! If you happen to see RK72 (flipper tags K22, K23) hauled out or a seal with a CATS Cam swimming around, please call the NOAA Marine Wildlife Hotline at (888) 256-9840.

Personal Thoughts

This project is part of my PhD research at UH Manoa that has taken five years to accomplish. I could write an entire second blog about the obstacles we’ve faced in progressing this research. So much time and effort has been put into this project — everything from obtaining appropriate permits, grant writing, and tag development — all whilst the pandemic was happening. Throughout this experience, I have had the privilege to learn from and work alongside scientists and conservationists who have studied Hawaiian monk seals since the 80s. I’m truly grateful for my collaborators and mentors and for the opportunity to work with Hawaiian monk seals.


Research activities were conducted under NMFS permit #22677 and are funded by a NOAA Ocean Acoustics Program grant.

Figure 6. A happy graduate student waiting on the glue to dry (left). A happy team after a successful instrumentation (right). NMFS permit 22677.

Figure 7. R330's satellite locations the morning of Feb. 1st offshore Ka’ena Point (left). RK72's satellite locations near Barber's Point (right). NMFS permit 22677.

Figure 8. Video of R330 approaching and vocalizing at two other seals after instrumentation. NMFS permit 22677.

Figure 9. Sneak peak video of R330 capturing a dead eel. NMFS permit 22677.


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