Seal bomb explosion sound source characterization

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 lPa 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 lPa m 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 lPa m 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. VC 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1121/10.0006101 (Received 3 August 2020; revised 11 August 2021; accepted 13 August 2021; published online 14 September 2021) [Editor: D. Benjamin Reeder] Pages: 1821–1829


I. INTRODUCTION
Seal bombs, also 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. For example, seal bombs were used at least as early as 1980 in the eastern tropical Pacific (ETP) yellow fin tuna purse-seine fishery to control dolphin swimming direction during all stages of net setting (Cassano et al., 1990). More recently, 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).
A primary concern with the use of seal bombs is potential harm to marine mammals, especially animals in close proximity to the explosions. While non-hearing physical damage was estimated for close ranges (<4 m; Myrick et al., 1990a), hearing related impacts such as temporary threshold shift (TTS) and permanent threshold shift (PTS) or loss of hearing may occur at more distant ranges (e.g., Finneran, 2015). Furthermore, behavioral responses to explosions of the targeted animals, in addition to non-targeted marine mammals, may cause harm by altering biologically significant behaviors such as foraging (e.g., Southall et al., 2007).
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. We describe an experiment offshore of Southern California in which seal bombs were deployed and exploded at various ranges from an underwater sound recorder. 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.

A. Experiment overview
Experimental operations were conducted in late spring 2017, offshore of Southern California, when more than 600 seal bombs were individually exploded underwater over three days and recorded with an autonomous hydrophone. The free-floating autonomous hydrophone recorded these explosions at various ranges from less than 300 m to more than 8 km while deployed a few hundred meters beneath the sea surface above seafloor depths ranging from 635 to 870 m ( Fig. 1 and Table I). Global positioning system (GPS) receivers were attached to both the seal bomb deployment ship, Scripps Institution of Oceanography Research Vessel (R/V) Saikhon, and a sea surface float above the hydrophone a) Electronic mail: swiggins@ucsd.edu, ORCID: 0000-0002-9686-035X. b) to provide source-receiver ranges. These ranges along with measured sound pressures at the hydrophone receiver provided the measurements needed to estimate seal bomb source levels.

B. Seal bombs
Seal bombs are similar to pyrotechnic firework salutes that generate a loud report (i.e., bang) along with a bright flash when their explosive material, flash powder, is ignited. Flash powder is a low explosive that deflagrates (i.e., burns and builds up and then decreases pressure over the time of the explosion), although at a much faster rate than black powder (gunpowder), and should not be confused with detonation of high explosives, such as trinitrotoluene (TNT), where a shock wave (i.e., a wave front traveling faster than the speed of sound) is generated and maximum pressure is sudden but decays quickly and exponentially.
While there are different seal bomb manufacturers around the world using various amounts ($2-6 g) and different formulations of flash powder (Myrick et al., 1990b), this study was limited to one type of seal bomb, the Seal Cracker Device, manufactured by Stoneco Energetics Systems LLC (Prescott Valley, AZ). The Seal Cracker Devices, from here onward referred to as seal bombs, were $8.3 cm long Â 1.7 cm diameter cardboard tubes wrapped with yellow paper and a bright orange label with a $6.7 cm long Â 0.3 cm diameter green fuse protruding from one of the plastic-plugged ends [ Fig. 2(a)]. Inside the tube were two chambers: a lower one with silica sand used to provide weight so that the seal bomb will sink upon deployment and an upper one containing flash powder and the unlit end of the fuse for deflagration initiation [ Fig. 2(b)]. The seal bomb had a visco fuse that had a black powder core and was coated with nitrocellulose for water resistance so that it would continue to burn after deployment underwater. The fuse burn duration before explosion was about 8 s. Explosion depths were estimated to be 1-4 m (Myrick et al., 1990b).
The seal bombs used in this study had a charge mass of 2.33 g of flash powder and used a standard formulation of about 64.0% potassium perchlorate (KClO 4 ) as the oxidizer and a fuel of 25% aluminum powder and 10% sulfur (Stonebraker, 2018). This charge mass was similar to common M-80 salutes classified as consumer fireworks.
During the experiment, the free end of the seal bomb fuse was ignited using a standard home-improvement-style push-button torch hose kit connected to a 400 ml (14 fluid oz) bottle of propane. After ignition, the seal bomb was tossed by hand into the water 5-10 m starboard and off the rear quarter of the R/V Saikhon while under way at $3 m s À1  (11 km h À1 ; 6 kn). Seal bombs were deployed approximately every 30 s along a transit line, marked with the ship's GPS. Notes were logged for each seal bomb deployment including time, location, and type of explosion (good, dud, shallow) along with changes in deployment schedule due to deviations in ship track or pauses during marine mammal, fish, or bird presence to avoid their use in the proximity of marine animals.

C. Underwater recordings
To measure sound pressures of seal bomb explosions, recordings were made using an autonomous high-frequency acoustic recording package (HARP; Wiggins and Hildebrand, 2007). The HARP was configured to record at a 200 kHz sample rate with 16-bit samples onto laptop computer type hard disk drives. Since seal bombs generate high sound pressures and some source-receiver ranges were relatively short, the sensitivity of the hydrophone was reduced from standard HARP hydrophones by about 40 dB to prevent signal clipping. The hydrophone was constructed of two sensors: Benthos (North Falmouth, MA) AQ-1 for frequencies below 10 kHz and International Transducer Corporation (Santa Barbara, CA) 1042 for frequencies above. The sensors are specified as having approximately the same sensitivity of À201 dB re 1 V/lPa. The hydrophone signal conditioning electronics gain was set to be 10 dB with a full system peak clip level $200 dB re 1 lPa, and the full band frequency response (10 Hz-100 kHz) was calibrated in our lab at Scripps Institution of Oceanography so that absolute received sound pressures could be measured.
Typically, HARPs are deployed on the seafloor as bottom-mounted instruments or in a mooring configuration including an acoustic release system used for jettisoning ballast weight and instrument retrieval. For this study, the data logger housing and hydrophone were suspended beneath the sea surface in a multiple float and weight system such that the hydrophone was decoupled from vibrations and motions of the sea surface float (Fig. 3). The hydrophone was placed at 265 m depth, well below the thermocline to avoid problems with acoustic raypath refraction. Attached to a flagpole on the sea surface float about 1.5 m above the waterline in a plastic bag was a dog collar GPS [Garmin (Schaffhausen, Switzerland) Astro 32 with T5]. The dog collar transmitted positions every 2 min via radio frequencies to its receiver onboard the R/V Saikhon for logging. Float drift rate was less than 0.06 m s À1 (2 km h À1 ; 0.1 kn). The receiver for the dog collar GPS also was used to record the ship GPS positions.
After recovery of the recorder, the hard disk drives were removed, and disk image files of raw data disks were generated for archiving and processing. Processing raw data into working data included uncompressing and creating multiple 37.5-min audio (wav format) files with high precision time stamps. The audio files were used to make long spectrograms to provide a graphical index for the data, allowing quick and easy access to sound events of interest (see the acoustic analysis software package, Triton; Wiggins and Hildebrand, 2007).
Software was developed in MATLAB (Mathworks, Inc., Natick, MA), to filter, automatically detect, measure amplitudes, and save snippets of received seal bomb shots from the audio files. An eighth order Chebyshev type 2 lowpass filter (LPF) with a stop band edge at 10 kHz was used on the waveforms to reduce apparent high-frequency transient effects from the hydrophone. We did not anticipate any effect from the filter on sound pressure estimates as most of the energy for shallow depth explosions is below 1 kHz with FIG. 3. (Color online) Autonomous acoustic recorder mooring configuration. The large white float was at the sea surface and included a flag, flasher, and radar reflector to prevent being struck by nearby transiting vessels. Also, attached to the flag was a dog collar GPS receiver, which transmitted locations back to R/V Saikhon. Beneath the sea surface on the mooring line was a system of floats and weights to decouple the sea surface motion from the hydrophone at 265 m depth. Hydrophone depth was confirmed via Seabird temperature-pressure logger. the source spectrum falling off rapidly to at least 20 dB lower around 10 kHz (Weston, 1960). The detector was a simple energy detector with the 0-peak sound pressure threshold set to $16 Pa (i.e., peak sound pressure level threshold ¼ 144 dB re 1 lPa) to identify pulse first-arrival times. Snippet waveforms from 0.1 s before detection to 1.0 s after detection were saved as binary files. Additional software was developed to evaluate seal bomb shots, including metric calculations and plots.

D. Impulse metrics
Different metrics are used to describe different types of sound pressure signals, expressed in Pa. For example, continuous pressure wave signals from sources such as ships and sonar pings are typically reported as root mean square (rms) of the sound pressure, p(t), over a time window, T, where the time window is typically defined as the signal width 3 dB down from the peak sound pressure, the signal width 10 dB down from the peak sound pressure, or from 5% to 95% of the signal's total energy, described as À3 dB, À10 dB, and 90% rms, respectively. Impulsive sounds are usually not well-represented as rms because rms depends on the analysis window duration, which for transient signals is critical (e.g., Madsen, 2005). For example, the rms for a smoothly varying impulse, such as a Gaussian function or underwater explosion, is typically lower for 90% rms than for À3 dB rms because of a longer time window for the 90% rms metric. Impulsive or transient sounds, such as those from seismic air guns or underwater explosions, are often described as 0-peak (p pk ) or peak-to-peak (p pk-pk ) sound pressures; however, these metrics do not account for different pulse shapes and durations. The sound pressure exposure, with units Pa 2 s, accounts for the shape of the pulse and provides a useful comparable metric for transient signals by integrating the squared-pressure of the pulse waveform time series over a time window, Another useful and comparable metric for transient signals is the positive acoustic impulse, or pressure impulse, with units Pa s. The positive impulse is often used for studies on the effects of explosions on animals (Richardson et al., 1995) and is the integral of pressure over the duration of the pulse, Peak and rms sound pressures are often presented as levels in dB. To convert peak and rms pressures to levels, 20 times the base-10 logarithm of the pressures was used such that L ¼ 20 log 10 (P/P 0 ), where P 0 ¼ 1 lPa was the reference value of sound pressure, and L ¼ sound pressure level (SPL) or the peak sound pressure level (L pk ) when P was p rms or p pk , respectively. Similarly, for the sound pressure exposure, the sound exposure level (SEL) was calculated as SEL ¼ 10log 10 (E/E 0 ), where the reference value was E 0 ¼ 1 lPa 2 s. All metric terminology, units, and reference values were presented as per the International Organization for Standardization document for underwater acoustics (ISO 18405:2017(ISO 18405: , 2017.

E. Source level estimation
Source level is rarely measured in the field directly as it is referenced at 1 m range, which can be prohibitively close to the source, introducing complexities associated with the near field acoustic environment. Instead, SPLs were measured at ranges much greater than 1 m and range-dependent corrections for acoustic propagation loss, PL (reference value 1 m 2 ), were applied to estimate SLs at 1 m (reference value 1 lPa m) via the sonar equation, where values were in dB units (Urick, 1983). Similar to PL, but between two specified locations, neither of which was the source, was transmission loss (TL), expressed in dB and often a linear function of the base-10 logarithm of the range between the two locations such that TL ¼ X log 10 R 2 =R 1 ð Þ; where R 1,2 were the ranges in meters from the source to locations 1 and 2, and X was the regression coefficient, or slope, of a linear regression model of SPL versus log 10 (R). Estimating SLs from acoustic waves that propagate along straight paths is typically much less complicated than from raypaths with additional energy loss from refraction, reflection, and absorption. For example, the sound pressure loss in a homogeneous, unbounded, and non-absorptive medium from a source radiating outward equally in all directions is termed spherical spreading, and X ¼ 20 in Eq. (5) for short ranges and low frequencies (e.g., Urick, 1983). When the medium is bounded by top and bottom parallel planes, sound is reflected off of the planes and spreads cylindrically, propagating in a waveguide, resulting in a lower loss with X ¼ 10; however, additional losses at the bounding planes can occur due to surface roughness scattering, waveform destructive interference, and, in the case of the seafloor boundary, refraction into substructure. Water column refraction can increase or decrease losses via focusing or defocusing sound waves as they bend toward or away from a receiver in a non-homogeneous medium. These complexities in losses arising from environmental factors need to be considered when estimating how SPL varies with distance from a source, for instance, when evaluating source impact on marine mammals.

F. Sound speed profile
The speed of sound in the ocean typically varies with depth, which affects how sound travels from source to receiver, including causing sound raypath refraction (i.e., bending) and creating shadow zone regions where direct raypaths are attenuated. Sound speed is a function of salinity, temperature, and pressure, with the latter two parameters having the largest effect through the water column. Temperature and pressure were measured and recorded using a Seabird (Bellevue, WA) SBE-39 attached to the line between the data logger pressure housing and the hydrophone (Fig. 3). This configuration provided two casts per deployment day, one down when the recorder was deployed and one up when the recorder was recovered. The three days of recording provided six casts, which were averaged to provide an overall mean temperature profile for the experiment. This temperature profile was used to estimate the mean sound speed profile using the Chen and Millero (1977) equations with a constant 35 & salinity.
The sound speed profile was used to evaluate how raypaths travel between source and receiver in the area of the experiment. To estimate raypaths from source to receiver, we used BELLHOP, a ray tracing model software program run in MATLAB (Porter, 2011), along with the mean sound speed profile.

III. RESULTS
Over three experimental days, 648 seal bombs were deployed; 46 were logged as unexploded, and 542 were detected with the automatic detector (Table II). Unexploded seal bombs may not have been lit properly or had some other fault with the fuse or explosive. Seal bombs that were not detected either did not explode or likely had received sound pressures lower than the detector threshold due to sound propagation limitations such as long range or very shallow explosion depths (i.e., near the sea surface pressure release boundary).

A. Example single nearby shot
Seal bomb shots near the hydrophone receiver provided the highest received levels and best signal-to-noise ratio (SNR) for evaluating the arriving pulses. A shot from the closest-point-of-approach (CPA), where the explosion was nearly directly above the hydrophone on 2 June 2017, clearly shows four distinct pulses within the first 75 ms (Fig. 4). The direct arrival from the shot, $262 m from the hydrophone, was a fast rising and slower decaying pulse, which was then reflected off the sea surface, causing a phase reversal and resulting negative pulse [ Fig. 4(a)]. At about 37 ms after the first arrival, the first bubble pulse peaked, but with a slower rise time than the direct pulse, which was also reflected off the sea surface [ Fig. 4(b)]. The third positive and negative pulses at $56 ms were from the second bubble pulse and had the same initial steep character and phase as the first pulse and its sea surface reflection (SSR) [Fig. 4(c)]. The third bubble pulse and its SSR arrived $69 ms after the direct pulse and at lower amplitude than the first two bubble pulses. All SSRs were around 4 ms after preceding positive pulses, indicating an approximate shot depth of 3 m using a 1500 m s À1 sound speed. The time difference between the positive pulse and its SSR for the third bubble pulse was $0.25 ms less than for the first bubble pulse, indicating the third bubble pulse was shallower than the first. In general, with all recorded shots, the time between the direct first pulse arrival and the bubble pulses varied by a few milliseconds, showing slight variability in shot depth.
A more detailed evaluation of the first 2 ms from the CPA seal bomb shot (Fig. 5) showed the unfiltered (dotted) waveform had a leading transient with positive and negative pulses 30 ls apart, which we attributed to the hydrophone electronics. These transients only occurred on the direct and the second bubble pulses, likely due to their higher frequency content than the first and third bubble pulses, and were most prominent for close shots, decreasing with range. The LPF waveform showed a reduction in high frequencies and the leading transient but retained the pulse shape and area underneath the curve (i.e., pressure impulse, J p ), allowing various metrics to be calculated (Table III).

B. Peak sound pressure levels versus range
To examine our study area propagation environment, measured peak sound pressure levels from seal bomb shots were plotted against their ranges using a base-10 logarithm scale (Fig. 6). Three distinct regions grouped by ranges were apparent: low-loss (260-1200 m), high-loss (1500-2000 m), and variable-loss (2000-9000 m). A linear regression model for peak sound pressure levels versus log 10 (R) for the low-loss, short-range region provided a slope, or regression coefficient, X ¼ 18, which was slightly less lossy than spherical spreading. Closer inspection of this region showed three sub-regions (260-340, 400-800, and 800-1200 m), each with slightly different and decreasing slopes of approximately 20, 19, and 17, respectively, becoming less lossy with increased range due to refraction focusing and eventually creating causticlike effects $1200-1500 m. When the recorded pulse waveforms from the low-loss region were scaled by their range and regression coefficient (i.e., R X/20 ), they were all nearly identical for the first 500 ls of the pulse, showing low variability in shot pressure signatures at close ranges.
Refraction was the cause of high losses in the region between 1500 and 2000 m, with defocusing creating a TL slope of X % 130 in a region where direct raypaths were strongly attenuated. Greater than 2000 m range, the raypath arrivals were complicated by sound waves reflecting off the seafloor and sea surface, in some cases multiple times, and there was no clear range-dependency of TL, showing more than 10 dB of peak sound pressure level variability (Fig. 6).

C. Refraction
To better understand the slight decrease in TL as range increases from the CPA and then the large increase in loss around 1500-2000 m range shown in Fig. 6, twodimensional ray tracing in a depth-dependent sound speed model was performed and showed the effects of refraction. The sound speed profile for the model was estimated from an average of six depth-temperature casts during the experiment (Fig. 7).
The profile showed a large decrease in sound speed in the first $20 m of depth, resulting in a large sound speed gradient near the sea surface. The amount of raypath curvature (i.e., refraction) is directly related to the magnitude of the sound speed gradient, with raypaths bending more in higher-gradient environments and when traveling more perpendicular to the direction of the gradient. For example, a raypath initially traveling horizontally (perpendicular to the direction of the sound speed gradient) in the upper 10 m of this model curved downward away from the sea surface such that a receiver at the same depth as the source received levels less than spherical spreading (i.e., defocusing) and a receiver at a deeper depth received levels greater than spherical spreading (i.e., focusing) for sufficiently close ranges.
A graphical example of this refraction effect showed raypaths traced from a 3 m deep shot with angles relative to the sea surface from À3 to 45 in 2 increments for two FIG. 5. Sound pressure waveform from recorded seal bomb explosion at CPA. Seal bomb with 2.33 g of flash powder was exploded at 262 m range from the hydrophone receiver. Dotted line, the unfiltered pulse with electronic noise; solid line, the filtered pulse using an eighth order Chebyshev type 2 LPF with a stop band edge at 10 kHz to minimize hydrophone electronic noise induced leading transient with positive and negative pulses.  sound speed profiles: the one measured during this experiment and a homogeneous 1500 m s À1 profile exhibiting spherical spreading. In the refraction model [ Fig. 8(a)] at the receiver depth (blue horizontal line), the area focusing raypaths was shown at ranges greater than $1200 m up until the last ray (shot toward the sea surface) $1700 m range. Beyond 1700 m, a shadow zone resulted, an area void of raypaths with very high TLs. The homogeneous sound speed model produced straight rays and no acoustic shadowing or focusing [ Fig. 8(b)].

D. Estimated source metrics
To estimate seal bomb SLs from the sonar equation [Eq. (4)], we used the SPL measurements from the CPA shot at 262 m (Table III) and a spherical spreading PL (20 log 10 (262 m) ¼ 48 dB re 1 m 2 ). The resulting SL was 233 dB re 1 lPa m over a 0.120 ms time window (Table IV). Similarly, SEL and pressure impulse were measured over a 2 ms window at CPA (Table III) and spherical spreading PL was applied, resulting in a sound exposure source level of 197 dB re 1 lPa 2 m 2 s and pressure impulse estimated 1 m from the source (source pressure impulse) of 193 Pa m s. Using a longer time window to include the bubble pulses and the surface reflections (100 ms) in addition to the primary pulse increased the sound exposure source level by 6 dB to 203 dB re 1 lPa 2 m 2 s (Table IV).
We chose the CPA shot levels because it was the closest shot to the reference 1 m providing good SNR with the least amount of PL, its raypath was straight and direct without adverse refraction effects, and other shots near CPA were nearly identical to the CPA shot when scaled by range. Without closer range measurements and with water depths much greater than source/receiver propagation paths, spherical spreading was an appropriate propagation model for these low-frequency, omni-directional seal bomb sources (Urick, 1983). Further supporting spherical spreading in this region was the TL slope from CPA to $340 m, measured to be X ¼ 20 (Fig. 6).

IV. DISCUSSION AND CONCLUSIONS
To characterize seal bomb sound pressure signatures, we recorded calibrated underwater received levels of shots and found the environment (i.e., temperature profile) had a significant effect on sound propagation for sources near the sea surface due to raypath refraction or bending, highlighting the need for good PL models to properly estimate received levels from SLs. For example, in the acoustically refractive model with a source at 3 m depth [ Fig. 8(a)], a receiver near the sea surface and at $500 m range would not receive direct raypaths, only steep angle rays reflected off of the seafloor, and received levels would be less than predicted by spherical spreading. Conversely, the same receiver FIG. 7. (Color online) Sound speed and temperature profiles for study area. Sound speed profile was estimated based on the method of Chen and Millero (1977) using the mean (black line) temperature profile (inset) from two casts from each deployment (red, green, blue) and salinity of 35&.
FIG. 8. (Color online) Raypaths traced in two models with different sound speed profiles. Rays were shot from a depth of 3 m in 2 increments from À3 to 45 relative to the sea surface. The blue horizontal line represents the hydrophone receiver at 265 m depth. (a) Sound speed profile from Fig. 7 with a strong gradient near the sea surface created strong refraction with rays becoming closer together as the range increased until the maximum range was reached (red raypath was shot toward the surface at À3 but refracted downward). (b) Homogeneous sound speed throughout the model caused all raypaths to be straight and evenly spaced in angle without refraction-caused shadow or focusing zones. Note that depth and range were at different scales ($1:2). at $100 m range may receive sound at higher levels than predicted with spherical spreading because of raypath focusing, and waveforms would likely be complicated with constructive and destructive interference from SSRs due to low grazing angles both from source and to receiver at shallow depths. While our seal bomb estimated SL was a high SL in the ocean (e.g., Hildebrand, 2009), it was possible that nonlinear propagation with higher losses than spherical spreading occurred for seal bomb explosions at shorter ranges than measured in our experiment if seal bomb flash powder fast deflagration was similar to high explosive detonations generating a shockwave. For example, for a similar size charge of high explosives, the generated pulse would propagate nonlinearly to about 60 m with an additional $5 dB loss, or a factor of $1.8, before becoming linear propagation (Cole, 1948;Arons, 1954;Chapman, 1985), suggesting the SL could be as high as 238 dB re 1 lPa m. However, without additional measurements from shorter ranges than present here, we were required to use a spherical spreading PL model to estimate SL from the closest shot's received SPL.
While little is known about damage to marine mammals from underwater explosions, seal bomb source (i.e., at 1 m range) pressure impulse was estimated in this study to be at levels previously shown to cause tissue injury to mediumsize terrestrial mammals held underwater (Yelverton et al., 1973), and based on an open-water seal bomb explosion study, Myrick et al. (1990a) suggested seal bombs cause damage to dolphins and other marine mammals if exploded within 4 m range.
In addition to physical tissue damage from close explosions, impulsive sounds may cause TTS or PTS damage in marine mammal hearing (e.g., Finneran, 2015). The National Oceanic and Atmospheric Administration (NOAA) National Marine Fisheries Service (NMFS) estimated PTS threshold for unweighted peak sound pressure level for earless seals (phocid) was 218 dB re 1 lPa, that for eared seals (otariid) was 232 dB re 1 lPa, and those for cetaceans ranged from 202 to 230 dB re 1 lPa, with TTS estimated as 6 dB lower, or effectively at farther ranges than PTS (National Marine Fisheries Service, 2018). These thresholds are all at or lower than the levels estimated for seal bombs at short ranges. This suggests PTS and TTS may be occurring for animals near seal bombs used in fisheries that employ them to deter marine mammals from depredation and accidental bycatch.
Peak levels and SPL are often used to describe underwater signals; however, they are incomplete for characterizing impulsive signals such as explosions, because no information on pulse shape is provided. Pulse duration provides additional details on the amount of energy that was contained in the pulse and the rate at which it was released. Time-integrated metrics, such as pressure impulse and SEL, are more comparable for impulsive signals because they account for the total energy in the pulse, not just the sound pressure amplitude. Furthermore, the complete received waveform from a seal bomb, not just the first pulse, should be considered when evaluating impact because of additional impulsive sounds present from explosion bubble pulses and reflections off the sea surface. The total energy received was higher when the complete 100 ms waveform was used with sound exposure source level that was 6 dB higher than just the first 2 ms pulse. Furthermore, since seal bombs are often used repeatedly during fishing operations (Meyer-L€ obbecke et al., 2016), cumulative SEL over the full period of event activity should be used to estimate the total amount of sound energy emitted into the environment. Expanding one step further, to properly assess how SEL relates to auditory injury thresholds in marine mammals, filtering of the full period time series by animal auditory frequency weighting functions during cumulative SEL calculations should be conducted (Southall et al., 2019).