Detection ranges for acoustic based manatee avoidance technology

Abstract

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of watercraft collisions in Florida’s coastal waterways. Several boater warning systems, based upon manatee vocalizations, have been proposed to reduce the number of collisions. One aspect of the feasibility of an acoustically based system relies upon the distance at which a manatee vocalization is detectable. The magnitude of environmental noise and manatee vocalizations, as well as the acoustic spreading properties of the habitat will help to estimate the detection range of a vocalizing manatee. This study combines measured source levels of manatee vocalizations with the modeled acoustic properties of manatee habitats to develop a method for determining hydrophone spacing requirements for acoustically reliant manatee avoidance technologies. In quiet environments (background noise 70 dB) it was estimated that manatee vocalizations are detectable at approximately 250 m, with a 6 dB detection threshold. In louder environments (background noise 100 dB) the detection range drops to 2.5 m. Noise generated by boating traffic is also investigated. In a habitat with 90 dB of background noise, a passing boat with a maximum noise floor of 120 dB would be the limiting factor when it was within approximately 100 m of the hydrophone.