The use of passive towed arrays for surveying marine mammals

The first marine mammal acoustic survey using a towed passive hydrophone array was recently completed. Other cetacean surveys have used static, low-frequency arrays; the array used is a broadband, mobile system, receiving both baleen and toothed whale signals. For acoustic censusing and tracking (ACT) to become an accepted technique, it’s biases and assumptions must be described. ACT has a number of advantages, including a permanent record of all effort, compared to visual methods which are instantaneous samples, with no permanent record. This record permits measurement of perception and availability biases. A dynamic strip width can be calculated by continuously recording SPL. An area of 154 621 km2 was surveyed in the north central Gulf of Mexico during seven cruises, recording 1431 tapes. Recording effort occurred over 11 997 km of a total transect distance of 12 580 km, corresponding to a 95.4% level of effort; a concurrent visual survey yielded a 48.5% level of effort. A total of 157 contacts from 13 cetacean species were recorded, including 67 sperm whale (Physetermacrocephalus), 22 pantropical spotted dolphin (Stenellaattenuata), one baleen whale, 327 unidentified dolphin, and 30 unidentified cetacean contacts. Three-dimensional distributions were determined for sperm whale and pantropical spotted dolphin. Estimates of cetacean populations were computed based on this effort.