Rhyming in the cold: First evidence of soniferous fish in the Southern Ocean

Acoustic ecology of Southern Ocean fish is currently unknown due to lack of dedicated fish acoustic research from those remote/inaccessible areas. The objective of this study was to investigate the seasonal and diel acoustic occurrence pattern of fish relative to environmental conditions at the sub-Antarctic Prince Edward Islands (PEIs) in the Southern Ocean. To collect our passive acoustic data, we used an autonomous recorder deployed at ~167 m water depth on an oceanographic mooring from mid-2021 to early 2023. Ski-Monkey III towed camera footages from dives deployed around the PEIs were reviewed to identify the potential source of the underwater recorded fish sounds. Three types of sounds (pops, grunts and drum calls) were detected and validated using random forest model based on their characteristics. Pops and grunts were produced in a chorus and singlet form. Pops were the most frequently detected sounds and were detected in December 2021 through May 2022 whereas grunts were detected in January through March 2022. Drum calls were rare and were detected as single units on few occasions in December 2021 through March 2022. These seasonal fish occurrences correspond to the spawning season of fish in the Southern Ocean, suggesting the use of acoustic cues during spawning. From camera footages, painted notothen (Lepidonothen larseni) was the only fish species found around acoustic recorder location, and pops were attributed to this abundant species. Fish sound occurrence increased after sunrise and sunset. Sea surface temperature between 5.2 and 8 °C was the primary predictor of fish acoustic occurrence, underscoring the vulnerability of these fish to environmental change. This study provides the first evidence of the seasonal occurrence and diel behaviour of soniferous Southern Ocean fish and the potential to use bioacoustics to monitor fish spawning season, which is important for the conservation and management of these fish species.