Acoustic Tracking of Sperm Whales (Physeter macrocephalus) in the Central Mediterranean Sea Using the NEMO-OνDE Deep-Sea Observatory

Passive acoustic monitoring plays a critical role in the study of marine species, particularly in understanding the behavior of deep-diving endangered species like the Mediterranean sperm whale (Physeter macrocephalus). This paper presents an effective method for tracking sperm whales using synchronized acoustic data from four hydrophones. The tracking method estimates the location of sperm whales by measuring the time difference of arrival of detected clicks. The direction of arrival of the clicks and their reflections on the surface are then reconstructed to determine the position of the whale. The method was used to perform the first acoustic tracking study of sperm whale dives recorded in the Central Mediterranean Sea by the NEMO-OνDE cabled observatory, deployed at a depth of 2100 m in the Gulf of Catania. The data analyzed in this study were collected in August and October 2005 and include 49 five-minute recordings with the presence of sperm whale clicks. A Monte Carlo simulation revealed an estimated relative error of 2.7% in depth and 1.9% in the horizontal distance for the positioning of clicks. The algorithm successfully reconstructed 64 tracks of diving sperm whales and demonstrated its potential for monitoring within a 12 km radius. Moreover, a simultaneous tracking of a vessel and a sperm whale was performed, illustrating how the method can be used to study potential changes during dives in the presence of vessels. This method offers a reliable, non-invasive approach to studying sperm whale behavior, ecology, and interaction with anthropogenic activities.