William E. Evans
Texas Inst. of Oceanogr., Texas A&M Univ., P.O. Box 1675, Galveston, TX 77553
Robert Benson
Texas A&M Univ., College Station, TX 77843-3367
When sonar detection of marine organisms is mentioned most every one visualizes active sonars. This is because development of passive sonar systems has been essentially limited to military use until recently. With the development of new designs, submarines could run deeper and more silent than in the past. The development of more sensitive listening systems was essential. With the development of improved passive sonars, a vast variety of biological noise mostly generated by cetaceans was documented. Most dolphins and whales are either direct or incidental noise makers. In many cases sound generating cetaceans can be identified as to family and in some cases to species (e.g., Physeter macrocephalus, Tursiops truncatus). In 1992 The Texas Institute of Oceanography, Center for Bioacoustics, Texas A&M University, in partnership with the National Marine Fisheries Service, Southeast Research Center, conducted a three-year acoustic/visual seasonal census of the cetaceans of the Gulf of Mexico. A linear towed array and sonobouy arrays were used. Thirteen of the 21 species of cetaceans known to occur in the Gulf of Mexico were detected acoustically using the towed array. Acoustic contacts were compared with the distribution and density of these species determined by the visual survey. The use of a mobile directional acoustic array had the advantage of detecting the presence of cetaceans 24 h a day. Because of this the number of acoustic contacts was significantly greater than those for the visual survey. In the case of the sperm whale it was possible to assess the distribution and also estimate the population density.