Underwater Calls: 20 New Hawaiian Monk Seal Vocalizations Identified by Acoustic Monitoring

Overview

The Hawaiian monk seal is among the world's most endangered marine mammals, with roughly 1,600 individuals remaining. In this episode, scientists describe a new understanding of their vocal communication after deploying underwater recorders across key habitats and identifying 25 distinct call types, 20 of which are newly described.

Methods & Findings

Researchers used passive acoustic recorders called hydrophones, or sound traps, placed in shallow coastal waters (<10 m) to capture weeks to months of underwater sounds. After processing over 4,500 hours of data, they cataloged more than 23,000 vocalizations and 25 call types, including six elemental calls like croak, hum, growl, and whoop, plus 19 combinational calls. These complex sequences are unusual among seals and may link to specific behaviors.

Conservation Outlook

Low-frequency monk seal calls (<1 kHz) are vulnerable to masking by ship noise, which could affect mating and foraging success. Future work will include year‑long recordings and seal-mounted video-acoustic tracking to connect calls with behaviors, guiding protection measures such as boat speed limits and space around animals.

Overview

This episode highlights a landmark study of Hawaiian monk seal vocalizations, a crucial area given the species’ endangered status with about 1,600 individuals remaining. By deploying underwater recorders across important monk seal habitats in the Hawaiian archipelago, researchers detected 25 distinct call types, 20 of which were previously undescribed. The work expands our understanding of how these marine mammals communicate in the wild, complementing earlier on‑land observations and the 2021 underwater-call descriptions from seals in human care.

Key voices in the episode include Kirby Parnell, a PhD candidate at the University of Hawaii at Manoa, who led the call-type analysis, and Dana Jones, executive director of the Hawaiian Monk Seal Preservation Association, who frames the conservation context. The hosts describe the scale of the effort, including 4,500+ hours of acoustic data, more than 23,000 vocalizations heard, and 25 distinct calls identified, six elemental and 19 combinational.

Methods: How the Calls were Discovered

To scientifically describe the seals’ calls, researchers recorded underwater sounds using self-contained hydrophones, often securing them to concrete blocks on the seabed. In shallow waters, these devices capture signals around the animals’ typical range, and in some cases, the recordings lasted weeks or months. One unit was damaged when a curious monk seal chewed it, illustrating the challenges of working with wild animals. After retrieval, data were downloaded and manually processed, a time-consuming but essential step that yielded the 23,000+ vocalizations and 25 call types.

Kirby Parnell explains that the team logged each call’s start and end times, with dense periods showing dozens of vocalizations in minutes. The work also benefited from interns who assisted with data processing over several years, emphasizing the collaborative, extended nature of the study.

Findings: The Call Types and Their Implications

Six elemental call types, such as croak, hum, growl, and whoop, form the building blocks, while 19 combinational calls combine these units without silent gaps. Examples include combinations like croak-hum-growl-whoop and growl-rumble-whoop. This level of combinatorial structure is unusual among pinnipeds and may enable more precise behavioral signaling, potentially tying specific sequences to mating, foraging, or social interactions.

Low-frequency vocalizations, typically below 1 kilohertz, align with a frequency range shared by large ships. This overlap raises concerns about masking, which could reduce the distance over which seals can hear each other, affecting mating opportunities and coordinated behaviors. Researchers note that the monk seals often lead relatively solitary lives except during breeding, when females with pups and males patrolling beaches heighten vocal activity.

The Road Ahead: Behavioral Links and Conservation Actions

With a foundation in underwater acoustics, the researchers plan to deploy longer-term recorders, up to a year or more, to identify seasonal patterns that may link calls to breeding or foraging. They are also piloting seal-mounted video and audio trackers to provide behavioral context for vocalizations, a step toward associating call types with actions like prey detection or courtship. The whine is highlighted as a potential foraging cue, based on by‑product observations and social media clips; scientists hope SEAL spy cameras will help confirm its function.

Understanding how human activity affects monk seal communication could inform conservation measures, such as vessel-speed limits and quiet-water windows, to minimize disruption to critical behaviors. As Dana Jones notes, recognizing that these are not threats to be alarmed by but signals to protect, is essential for protecting an endangered species that shares Hawaii’s coastal waters with a large human population and extensive boating and ecotourism.

Quotes

"oh my gosh, we have data" - Kirby Parnell

"It's basically like a seal Fitbit with a video camera and acoustic recorder" - Host

"the whine, it sounds like" - Kirby Parnell

"We could definitely tell when the males were on the move looking for a girlfriend" - Dana Jones