Echolocation behavior of harbor porpoises Phocoena phocoena around chemically enhanced gill nets

Abstract

The echolocation behavior of harbor porpoises Phocoena phocoena around gillnets was monitored to test their response to chemically (BaSO 4 ) enhanced gill nets, designed to be more acoustically reflective than commercial nets. Field trials were conducted between 22 July and 31 August 2000 in the Bay of Fundy, Canada. Echolocation clicks were continuously monitored with Porpoise Echolocation Detectors (PODs). Commercial and experimental (chemically enhanced) gill nets were set for groundfish in water depths from 100 to 130 m. Echolocation occurrence (the proportion of 10 s intervals during which clicks were detected) and echolocation rate (the number of clicks h -1 ) were measured. The PODs varied in their detection ability, so comparisons between commercial and enhanced nets were made with individual PODs. Neither echolocation rate nor occurrence differed with net type for any POD. Significantly more echolocation was detected during the day than at night. Echolocation rate and echolocation occurrence varied with depth and location, possibly reflecting concomitant variation in the relative abundance of porpoises and/or their prey. We conclude that porpoises do not respond to the acoustic reflectivity of the modified nets. Rather, the effectiveness of these nets is apparently due to other factors, such as their physical properties, particularly their stiffness.