MEPS 313:285-294 (2006)  -  doi:10.3354/meps313285

Behavioral reactions of free-ranging harbor porpoises Phocoena phocoena encountering standard nylon and BaSO4 mesh gillnets and warning sound

Sven Koschinski1,*, Boris M. Culik1, Edward A. Trippel2, L. Ginzkey3

1Leibniz Institute of Marine Sciences at Kiel University (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany
2Fisheries and Oceans Canada, Biological Station, St. Andrews, New Brunswick E5B 2L9, Canada
3Federal Armed Forces Underwater Acoustics and Marine Geophysics Research Institute (FWG), Klausdorfer Weg 2–24, 24148 Kiel, Germany

ABSTRACT: Field tests suggest that high-density nets can reduce harbor porpoise Phocoena phocoena by-catch in demersal gillnet fisheries. However, it is not clear whether acoustic reflectivity or twine stiffness are responsible for this. We conducted sonar tests in a tank in the frequency range of 110 to 190 kHz and found that the target strength of the high-density BaSO4 net was 7.2 dB higher at 150 kHz than that of the standard nylon net. In a fjord on Vancouver Island, Canada, we investigated porpoise surfacing and echolocation behavior as they encountered 2 surface gillnets (45 × 9 m, 165 mm mesh size) made of (1) standard 100% nylon and (2) a mix of BaSO4 and nylon. The distribution of click intervals shifted to longer intervals when the BaSO4 net was used (median = 51 ms vs. 45.2 ms for the standard net; Kolmogorov-Smirnov test, p < 0.001), indicating a greater target distance. We estimated that porpoises are able to detect BaSO4 nets 4.4 m in advance of standard nylon nets. However, an unexpected low percentage of echolocating porpoise groups within 50 m of the center of nets (standard 30.6%, BaSO4 19.3%) indicates that additional measures may be necessary to reduce by-catch. A subsequent experiment showed that transmission of 2.5 kHz tones as a warning sound increased biosonar use by a factor of 4 compared to controls (16.7% for controls vs. 71.4% for groups during ensonification; chi2-test, p < 0.001). The combination of reflective nets and warning sounds may be a promising mitigative tool.

KEY WORDS: Harbor porpoise · Phocoena phocoena · Barium sulfate · Reflective gillnet · By-catch mitigation

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