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Field tests suggest that high-density nets can reduce harbor porpoise Phocoena pho- coena 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 distrib- ution 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 dis- tance. 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 warn- ing sound increased biosonar use by a factor of 4 compared to controls (16.7% for controls vs. 71.4% for groups during ensonification; chi 2 -test, p < 0.001). The combination of reflective nets and warning sounds may be a promising mitigative tool.

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