Echolocation Behavior of Dolphins and Porpoises
Echolocation is highly developed sensory ability of dolphins. Dolphins and porpoises in the wild is thought to evolve their echolocation system for the underwater navigation, recognition and foraging. Observation of the echolocation behavior of free ranging dolphins let us know the adaptive use of acoustic sensory systems in the water. Fortunately, the echolocation is the active sonar system, thus we can observe sensory efforts easily comparing with other underwater creature. Dolphins are thought to be suitable animals to study underwater 'sensory ethology'.

The ontogeny of echolocation in a Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis ), J. Acoust. Soc. Am. 122(2), 715-718.
Comparison of echolocation behaviour between coastal and riverine porpoises, Deep-Sea Research II 54(3-4), 290-297.
Biosonar behaviour of free-ranging porpoises, Proc. R. Soc. Lond. B 272, 797-801.
Off-axis sonar beam pattern of free-ranging finless porpoises measured by a stereo pulse event data logger, J. Acoust. Soc. Am. 117(5), 3325-3330.

Acoustical Survey of Marine Mammals
Marine mammal vocalizations are different from underwater noise. Some of them are tonal and long duration characteristics and others are ultrasonic high sound pressure level pulses. Using stationed, floating or moving platform deployed passive acoustical system, we have been monitoring baiji (China), finless porpoises (Japan&China), vaquita (Mexico), beluga (Alaska, USA) and dugong (Thailand). Number of animals could be counted acoustically that is useful for the population assessment. Sensing and communication behavior among conspecifics are another interesting topics.

Comparison of stationary acoustic monitoring and visual observation of finless porpoises (Neophocaena phocaenoides ), J. Acoust. Soc. Am., 125, 547-553. 
Acoustic sensors for rare porpoise, Nature 456, 431 (news report). 
Evidence of nighttime movement of finless porpoises through Kanmon Strait monitored using a stationary acoustic recording device, Fisheries Science 74, 970-976. 
First human-caused extinction of a cetacean species?, Biology Letters 3(5), 537-540.
Estimation of the detection probability for Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) with a passive acoustic method, J. Acoust. Soc. Am. 123(6), 4403-4411. 
Estimated detection distance of a baiji's(Chinese river dolphin,Lipotes vexillifer) whistles using a passive acoustic survey method, J. Acoust. Soc. Am.120(3), 1361-1365.
Feeding behavior of wild dugongs monitored by a passive acoustical method, J. Acoust. Soc. Am.120(3), 1356-1360.
Dugong (Dugong dugon) vocalization patterns recorded by automatic underwater sound monitoring systems. J. Acoust. Soc. Am. 119, 3726-3733.

Development of Dolphin Mimetic Sonar for Target Discrimination
Dolphins and porpoises depended on ultrasonic pulse sounds to catch prey in murky water or in darkness. With this biosonar, they can determine distance, size, material composition and even the shape and internal structure of underwater obstacles such as prey and conspecifics. Their sonar probably evolved to find and classify the target fish suitable to eat by themselves. We have much to learn from the dolphins and their sophisticated biosonar systems as we develop forthcoming fisheries echo sounders for identification and classification.ound production system of dolphins evolved for long time. 

Measurement of target strength spectrum of fish using sonar signals of dolphin, J. Acoust. Soc. Am. 124(6), 3440-3449. 

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