Abstract
Small explosive charges, called seal bombs, used by commercial fisheries to deter marine mammals from depredation and accidental bycatch during fishing operations, produce high level sounds that may negatively impact nearby animals. Seal bombs were exploded underwater and recorded at various ranges with a calibrated hydrophone to characterize the pulse waveforms and to provide appropriate propagation loss models for source level (SL) estimates. Waveform refraction became important at about 1500 m slant range with approximately spherical spreading losses observed at shorter ranges. The SL for seal bombs was estimated to be 233 dB re 1 μPa m; however, for impulses such as explosions, better metrics integrate over the pulse duration, accounting for the total energy in the pulse, including source pressure impulse, estimated as 193 Pa m s, and sound exposure source level, estimated as 197 dB re 1 μPa2 m2 s over a 2 ms window. Accounting for the whole 100 ms waveform, including the bubble pulses and sea surface reflections, sound exposure source level was 203 dB re 1 μPa2 m2 s. Furthermore, integrating the energy over an entire event period of multiple explosions (i.e., cumulative sound exposure level) should be considered when evaluating impact.
Highlights
Seal bombs, known as explosive pest control devices and seal deterrent devices, among other names, are hand-thrown pyrotechnic devices capable of exploding underwater and are used as a means to deter marine mammals during commercial fishing operations
The SL for seal bombs was estimated to be 233 dB re 1 lPa m; for impulses such as explosions, better metrics integrate over the pulse duration, accounting for the total energy in the pulse, including source pressure impulse, estimated as 193 Pa m s, and sound exposure source level, estimated as 197 dB re 1 lPa2 m2 s over a 2 ms window
We describe an experiment offshore of Southern California in which seal bombs were deployed and exploded at various ranges from an underwater sound recorder
Summary
Known as explosive pest control devices and seal deterrent devices, among other names, are hand-thrown pyrotechnic devices capable of exploding underwater and are used as a means to deter marine mammals during commercial fishing operations. Underwater recordings of thousands of explosions per month were spatially and temporally correlated with commercial landings data of California market squid, suggesting that seal bombs were used extensively during squid fishing operations (Meyer-L€obbecke et al, 2016). Seal bomb source characterization is needed to provide metrics for managing marine noise pollution and mitigating effects on marine mammals due to high sound pressures from these explosions. The received sound pressure waveforms were analyzed, and various metrics were estimated to provide a characterization of the seal bomb source, including source level (SL), an important metric for marine noise management
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