Sperm whale presence observed using passive acoustic monitoring from gliders of opportunity

Habitat use of the endangered Mediterranean sperm whale subpopulation remains poorly understood, especially in winter. The sustained presence of oceanographic autonomous underwater vehicles in the area presents an opportunity to improve observation effort, enabling collection of valuable sperm whale distribution data, which may be crucial to their conservation. Passive acoustic monitoring loggers were deployed on vertically-profiling oceanographic gliders surveying the north-western Mediterranean Sea during winter 2012-2013 and June 2014. Sperm whale echolocation usual click trains, characteristic of foraging activity, were detected and classified in the recordings, providing information about presence of sperm whale along the glider tracks. Widespread presence of sperm whales in the north-western Mediterranean Sea was confirmed. Winter observations suggest different foraging strategies between the Ligurian Sea, where mobile and scattered individuals forage at all times of day, and the Gulf of Lion, where larger aggregations target intense oceanographic features in the open ocean such as fronts and mixing events, with reduced acoustic presence at dawn. This study demonstrates the ability to successfully observe sperm whale behaviour from passive acoustic monitoring gliders. We identify possible mission design improvements that would lead to benefit from passive acoustic monitoring glider surveys to significantly improve sperm whale population monitoring and habitat use.