Measurement of the acoustic reflectivity of sirenia (Florida manatees) at 171 kHz.
The Florida manatee (Trichechus manatus latirostris) is an endangered sirenian. At present, its adult population (approximately 2200) seems stable, but tenuous. Manatee-boat collisions are a significant proportion (approximately 25%) of mortalities. Here, the potential use of active sonar for detecting manatees by quantifying sonic reflectivity is explored. In order to estimate reflectivity two methods were used. One method measured live reflections from captive animals using a carefully calibrated acoustic and co-registered optical system. The other method consisted of the analysis of animal tissue in order to obtain estimates of the sound speed and density and to predict reflectivity. The impedance measurement predicts that for a lateral view, the tissue reflectivity is close to 0.13, with a critical grazing angle of 28 degrees. Data measured from live animals indicate that substantial reflections can be recorded, however in many instances observed "empirical target strengths" were less than an experimentally dependent -48-dB threshold. Conclusions favor the hypothesis that the animals reflect substantial amounts of sound; however, the reflections can often be specular, and therefore impractical for observation by a manatee detection sonar operating at 171 kHz.
Jaffe
Jules S
JS
Marine Physical Lab, Scripps Institution of Oceanography, La Jolla, California 92093-0238, USA.
Simonet
Fernando
F
Roberts
Paul L D
PL
Bowles
Ann E
AE
D000162:Acoustics; D000273:Adipose Tissue; D000818:Animals; D001522:Behavior, Animal; D002138:Calibration; D017097:Electric Impedance; D012767:Ships; D012867:Skin; D020457:Trichechus manatus