Acoustic characterisation of bycatch mitigation pingers on shark control nets in Queensland, Australia

Abstract

The Queensland Shark Control Program (QSCP) uses pingers to prevent marine mammal entanglement in shark control nets along public beaches, in Queensland, Australia. Acoustic emissions of Fumunda F3 (designed for humpback whales) and F10 pingers (designed for dolphins) were measured and characterised. The acoustic signals consisted of tones (3 and 10 kHz, respectively) and harmonic overtones emitted for about 400 ms every 5 to 6 s. Directivity was more pronounced for the overtones. Broadband source levels were up to 135 dB re 1 µPa at 1 m for all pingers at all angles. Ambient noise was recorded in the vicinity of shark nets for 3 wk each quarter of 1 yr. Fish choruses, migrating humpback whales, dolphins, snapping shrimp, boats, sandpumps, wind and wave noise were identified. Beyond 1.5 km, pingers no longer contributed significantly to the ambient noise budget. Sound propagation was modelled to relate received pinger tones to measured ambient levels and to estimate the potential detection of pingers by local marine mammals (humpback whales, dugongs, dolphins). Mean transmitted levels were predicted to be audible over up to a few 100 m in range (depending on species). With currently 3 to 4 pingers per shark net of 200 m length, existing pinger type and arrangement were modelled to be adequate, even for marine mammals swimming straight at a net at top speed. Additional behavioural studies or long-term monitoring are needed to determine pinger efficacy.