Instrumenting free-swimming dolphins echolocating in open water.

Dolphins within the Navy Marine Mammal Program use echolocation to effectively locate underwater mines. They currently outperform manmade systems at similar tasks, particularly in cluttered environments and on buried targets. In hopes of improving manmade mine-hunting sonar systems, two instrumentation packages were developed to monitor free-swimming dolphin motion and echolocation during open-water target detection tasks. The biosonar measurement tool (BMT) is carried by a dolphin and monitors underwater position and attitude while simultaneously recording echolocation clicks and returning echoes through high-gain binaural receivers. The instrumented mine simulator (IMS) is a modified bottom target that monitors echolocation signals arriving at the target during ensonification. Dolphin subjects were trained to carry the BMT in open-bay bottom-object target searches in which the IMS could serve as a bottom object. The instrumentation provides detailed data that reveal hereto-unavailable information on the search strategies of free-swimming dolphins conducting open-water, bottom-object search tasks with echolocation.

Echolocation characteristics of free-swimming bottlenose dolphins during object detection and identification.

A biosonar measurement tool (BMT) was created to investigate dolphin echolocation search strategies by recording echolocation clicks, returning echoes, and three-dimensional angular motion, velocity, and depth of free-swimming dolphins performing open-water target detections. Trial start and stop times, locations determined from a differential global positioning system (DGPS), and BMT motion and acoustic data were used to produce spatial and acoustic representations of the searches. Two dolphins (LUT, FLP) searched for targets lying on the seafloor of a bay environment while carrying the BMT. LUT searched rapidly (< 10 s), produced few clicks, and varied click-peak frequency (20-120 kHz); FLP searched relatively slowly (tens of seconds) and produced many hundreds of clicks with stereotypical frequency-dependent energy distributions dominating from 30-60 kHz. Dolphins amplified target echo returns by either increasing the click source level or reducing distance to the target but without reducing source level. The distribution of echolocation click-peak frequencies suggested a bias in the dominant frequency components of clicks, possibly due to mechanical constraints of the click generator. Prior training and hearing loss accommodation potentially explain differences in the search strategies of the two dolphins.