Acoustic Harassment Devices (AHD) for salmon trap nets in the Baltic Sea

• Acoustic harassment devices (AHDs) were deployed at salmon-trap nets in the Baltic Sea to reduce gear and catch damage by grey seals (Halichoerus grypus). The AHDs emitted pulses of 250-500-ms duration, worked at a frequency of 15 kHz, and a source level of 179 dB re 1 μPa rms at 1 m. AHDs were deployed during three consecutive fishing seasons. Catches were significantly higher in traps with AHDs (25.5 kg d-1) than in controls (12.0 kg d-1), and catch damage was less (3.5 vs. 6.7 kg d-1). These results persisted over and between fishing seasons, but late in the season damage to the catches was common also in traps with AHDs. This study shows that the AHD may be a complementary mitigation tool in the seal-fishery conflict in certain types of fisheries, even though it is technically demanding, and for environmental reasons should be used with great care.

Nyckelord

Conflict; damage; fishery; grey seals; mitigation; salmon

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