Target Strength of a Nylon Monofilament and an Acoustically Enhanced Gillnet: Predictions of Biosonar Detection Ranges

Abstract: Thousands of marine mammals die each year in fisheries-related entanglements. A substantial number of these animals entangle themselves in gillnets. Two populations in immediate danger are the coastal stock of the mid-Atlantic bottlenose dolphin, Tursiops truncatus, and the Gulf of Maine harbour porpoise, Phocoena phocoena. We investigated the efficacy of using an alternative net material made with barium sulphate hypothesized to be acoustically more reflective than traditional nets. By using simulated dolphin echolocation clicks, the target strength of the experimental net was compared with the target strength of a similar gauge nylon net. Results demonstrated that at angles greater than normal incidence, but less than 40°, the new barium sulphate net was acoustically more reflective than the nylon net; however, there was no significant difference in the target strength of the two nets at 0°. At angles greater than 40°, both nets were difficult to discern from background noise. Target strengths of the nets were used to calculate detection ranges for T. truncatus and P. phocoena. Both species should be able to detect the experimental nets at a distance greater than the nylon nets. For T. truncatus, this distance may be enough to reduce entanglement; however, because of P. phocoena‘s lower source level echolocation signals, they may not detect either net with echolocation in time to avoid contact.

Key Words: entanglement, gillnet, take reduction, Tursiops truncatus, Phocoena phocoena, target strength, acoustic reflectivity

Document Type: Research article

DOI: 10.1578/AM.30.2.2004.220

Page Numbers: 220-226

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