Target detection by an echolocating harbor porpoise (Phocoena phocoena)

Two echolocation experiments are described. They were conducted on the same harbor porpoise housed in a sea pen, one year apart at Neeltje Jans, The N etherlands. The aims were to determine the target detection ability of an e cholocating harbor porpoise, with the ultimate goal to predict the distance at which harbor porpoises can detect fishing nets. In experiment 1, the ma ximum distance at which the 3-year-old porpoise could detect a 7.62-cm diam eter water-filled stainless-steel sphere by echolocation was determined psy chophysically. The 50%-correct detection threshold was reached when the sph ere was at a distance of 26 m from the porpoise's rostrum. In experiment 2, conducted a year later, the maximum detection distance for a 5.08-cm water -filled stainless-steel sphere was 15.9 m. The target strengths of both tar gets were measured using simulated harbor porpoise echolocation signals and the results, coupled with transmission-loss calculations, indicated that t he echo levels received by the porpoise with the targets at the threshold r anges in the two experiments were only 1.3 dB apart. Together with informat ion on the target strengths of various fishing nets, the results of the pre sent study can be used to predict the distance at which the nets can be det ected by harbor porpoises. (C) 2999 Acoustical Society of America. [S0001-4 966(99)04803-1].