Underwater sounds produced by Galápagos sea lions, Zalophus wollebaeki

Underwater vocal repertoires in otariids remain poorly studied. However, anecdotal recordings of underwater vocalizations exist for several species, including California, and Steller sea lions, Northern, Galápagos, New Zealand, and Cape fur seals. Galápagos sea lions, which are endemic to the Galápagos Islands (Ecuador), form large, mixed-sex colonies year-round. Until now, vocalizations in this species have only been documented in air, with a focus on contact calls between females and pups. In June 2023, a high-frequency acoustic recorder (ST300HF) was deployed for continuous monitoring over a three-day period in Wreck Bay, San Cristóbal Island, Ecuador. The device was positioned at a depth of 4 m and recorded at a sampling rate of 96 kHz. The dataset included unknown underwater sounds suspected to be produced by Galápagos sea lions observed in the area. These recordings featured sequences of multiple belch-like sounds with frequency content up to 3 kHz and durations ranging from 0.4 to 2 s. On May 14, 2025, a GoPro video recorded during a 19-m dive off the west coast of Floreana Island captured both visual and acoustic evidence of a Galápagos sea lion producing similar underwater vocalizations. These sounds appeared to be associated with active prey searching behavior.