Hull-mounted hydrophones for passive acoustic detection and tracking of sperm whales (Physeter macrocephalus)

Acoustic monitoring has proven increasingly important for detection and tracking of marine mammals. Most systems rely on towed hydrophones that can be cumbersome to deploy and pose a risk in terms of fouling propellers and failing in rough seas. To alleviate these problems, an acoustic detection system designed for monitoring and tracking vocally active marine mammals, sperm whales in particular, has been developed. The system uses two hydrophones, designed for hull-mounting on each side of the keel of small as well as larger vessels. The system is relatively simple to use and does not require deployment of any equipment. Hydrophone outputs are amplified and filtered by a custom built conditioning circuitry. Bearing is estimated by manoeuvring the vessel to minimize the difference in level between the two hydrophones. Forward-aft ambiguity is resolved by turning the vessel. The system permits acoustic bearing estimations to be carried out while cruising at speeds of 5 knots. The theoretical maximum detection distances of sperm whales are several kilometres, depending on the relative orientation of the animal.