Categorizing Active Marine Acoustic Sources Based on Their Potential to Affect Marine Animals

: Marine acoustic sources are widely used for geophysical imaging, oceanographic sensing, and communicating with and tracking objects or robotic vehicles in the water column. Under the U.S. Marine Mammal Protection Act and similar regulations in several other countries, the impact of controlled acoustic sources is assessed based on whether the sound levels received by marine mammals meet the criteria for harassment that causes certain behavioral responses. This study describes quantitative factors beyond received sound levels that could be used to assess how marine species are affected by many commonly deployed marine acoustic sources, including airguns, high-resolution geophysical sources (e.g., multibeam echosounders, sidescan sonars, subbottom proﬁlers, boomers, and sparkers), oceanographic instrumentation (e.g., acoustic doppler current proﬁlers, split-beam ﬁsheries sonars), and communication/tracking sources (e.g., acoustic releases and locators, navigational transponders). Using physical criteria about the sources, such as source level, transmission frequency, directionality, beamwidth, and pulse repetition rate, we divide marine acoustic sources into four tiers that could inform regulatory evaluation. Tier 1 refers to high-energy airgun surveys with a total volume larger than 1500 in 3 (24.5 L) or arrays with more than 12 airguns, while Tier 2 covers the remaining low/intermediate energy airgun surveys. Tier 4 includes most high-resolution geophysical, oceanographic, and communication/tracking sources, which are considered unlikely to result in incidental take of marine mammals and therefore termed de minimis . Tier 3 covers most non-airgun seismic sources, which either have characteristics that do not meet the de minimis category (e.g., some sparkers) or could not be fully evaluated here (e.g., bubble guns, some boomers). We also consider the simultaneous use of multiple acoustic sources, discuss marine mammal ﬁeld observations that are consistent with the de minimis designation for some acoustic sources, and suggest how to evaluate acoustic sources that are not explicitly considered here.

Table S1.Maximum densities of cetacean and sea turtle species reported in available model grids, along with calculations for the threshold radius Rt using equation (4) in main text and assuming probability p=0.01 (1% probability of taking a single animal) and random, uniform distribution of animals at the sea surface.The adjusted SL (SLit) is determined as outlined in the main text, using the appropriate Level B received SPL for cetaceans vs. turtles.90 th and 95 th percentile calculated for species with high reported densities.d Maximum density in each regional model grid for each species.Where models for each month were available for the Gulf of Mexico or U.S. Atlantic margin locations, August models were used.Values for the U.S. Pacific margin (Becker et al., 2020) are reported in animals per 1 km 2 and were scaled to densities per 100 km 2 .Grid cells having values other than "no data" are 11619 (U.S.Pacific margin), 7015 (Gulf of Mexico), and 12188 (U.S. Atlantic margin).e Threshold radius (Rt) calculated for the maximum density, 1% probability (p=0.01) of an animal being within the circle of radius Rt around a source, and a uniform distribution of the animals at the surface according to equation (4) in the text.Note that any received SPL value can be assigned at Rt. f Source level for incidental take (SLit) at threshold radius Rt corresponding to received SPL of 160 dB re 1 µPa, the current Level B take level for marine mammals used by the U.S. National Marine Fisheries Service.166 dB re 1 µPa is used in the SLit calculation for sea turtles.The calculation is done with spherical spreading in this table.Spherical spreading is a valid approximation for even shallow water depths when frequencies are hundreds to thousands of Hz.Actual units for SLit dB re 1 µPa @ 1 m.To adjust SLit for p=0.005 (0.5% probability), subtract 3 dB.g 90th percentile of animal densities per 100 km 2 for a given species.Only provided when the density exceeds 32 animals per 100 km 2 , which corresponds to Rt of 100 m and SLit of 200 dB re 1 µPa @ 1 m.h 95th percentile of animal densities per 100 km 2 for a given species.Only provided when the density exceeds 32 animals per 100 km 2 , which corresponds to Rt of 100 m and SLit of 200 dB re 1 µPa @ 1 m. a Exposure duration and ping exposure are defined in the main text and refer to the total time (including silence between pings) for transmission of pings that are received at >160 dB re 1 µPa threshold and the number of received pings above that threshold, respectively.b See main text for more explanation of single and dual swath MBES ping accumulation and the exposure of animals receiving pings on adjacent tracklines.c Sparker-1 refers to a 6 kJ Delta sparker, which is normally deployed at this power level only in deeper (> 500 m) waters.d Sparker-2 represents a more widely deployed configuration of the SIG ELC 820 sparker run at ~700 J, as is common at shallow (< 200 m) water depths.No animal is within the circle in either of these realizations.The p value for this combination of density and Rt is 0.099, meaning that there is nearly a 10% possibility that one animal would be within the red circle for any given realization.The large Rt value used here for this relatively high marine animal density is for illustrative purposes only.The calculations in the main text and in this supplement generally use p=0.01 (1% probability).For a user-defined distribution of animals, even one including clustering of animals in pods, the Monte Carlo approach could be used to determine Rt empirically for a given p value.2015), with SEL0 of 190 dB re 1 µPa.While this result describes safe distance as applied to Level A take (NMFS-OPR-59, 2018), it illustrates dependence on pulse length, which is a factor that the degree of exposure de minimis criterion (Factor 5) does not include.These results are not significant for incidental take in any absolute sense, but do demonstrate how pulse length can affect metrics associated with protection of marine animals.

Figure S1 .
Figure S1.Maximum density of any cetacean species in each grid cell for all modeled species (TableS1) for the U.S. Pacific margin(Becker et al., 2020)  and for the northern Gulf of Mexico and U.S. Atlantic margin(Roberts et al., 2016).When monthly density grids were available, the calculations were done for August.

Figure S2 .
Figure S2.Bathymetry used for calculations for Factors 2 and 4, taken from 1 arc second ETOPO grid (NOAA National Geophysical Data Center, 2009).

Figure S3 .
Figure S3.Rt calculated for 1% probability (p=0.01) of a single animal being within Rt of the source, using the maximum density of any cetacean species in each grid cell and assuming a uniform distribution of animals at the surface.An arbitrary radius of 25 m around a source is currently used by NMFS, but this yields a 1% probability (p=0.01) of an animal being within that distance of the source only immediately adjacent to the coast from Delaware to Florida on the U.S. Atlantic margin and offshore southern California on the U.S. Pacific margin.40 m may be a more appropriate arbitrary value, but 100 m is reasonable along much of the U.S. Atlantic and Gulf of Mexico margins, including on much of the continental shelf.The corresponding SLit calculated from these values is shown in Figure7aof the main text.

Figure S4 .
Figure S4.Two realizations of 350 random, uniformly distributed animals (blue dots) distributed in a 10 km x 10 km block at the ocean's surface.x and y coordinates are randomly chosen from a uniform distribution for 10 5 simulations.The red circle has ~100 m (Rt = 100 m) radius around the hypothetical source located at (5 km, 5 km).No animal is within the circle in either of these realizations.The p value for this combination of density and Rt is 0.099, meaning that there is nearly a 10% possibility that one animal would be within the red circle for any given realization.The large Rt value used here for this relatively high marine animal density is for illustrative purposes only.The calculations in the main text and in this supplement generally use p=0.01 (1% probability).For a user-defined distribution of animals, even one including clustering of animals in pods, the Monte Carlo approach could be used to determine Rt empirically for a given p value.

Figure S5 .
Figure S5.Comparison between empirically determined Rt values (points) using a Monte Carlo approach with the given p values and 10 5 realizations for each animal density, with the individual animal locations randomly and uniformly distributed on the surface of an 100 km 2 block.The solid analytical curves are

Figure
Figure S6.(Left) Maximum turtle densities scaled to animals per 100 km 2 for the U.S. Atlantic and northern Gulf of Mexico areas [Geo-marine, Inc., 2007a, b, c].Note that the coverage for these models is not as extensive as for cetaceans (FigureS1).(Right) Rt calculated using the turtle maximum density distribution for p=0.01 (1% probability of a single animal being within Rt of the source).

Figure S7 .
Figure S7.SLit for the maximum densities of turtles, using 166 dB re 1 µPa as the Level B received SPLB threshold applied at Rt. Spherical spreading assumed for water depths exceeding 10 m, and cylindrical spreading at shallower water depths.Note that SLit for turtles exceeds 200 dB re 1 µPa @ 1 m almost everywhere except the west coast of Florida and near the Louisiana coast.

Figure S9 .
Figure S9.The impact of duty cycle (ratio of pulse length to pulse repeat rate) on safe distance as a function of SL, calculated from Equation (5), adapted from Sivle et a. (2015), with SEL0 of 190 dB re 1 µPa.While this result describes safe distance as applied to Level A take (NMFS-OPR-59, 2018), it illustrates dependence on pulse length, which is a factor that the degree of exposure de minimis criterion (Factor 5) does not include.These results are not significant for incidental take in any absolute sense, but do demonstrate how pulse length can affect metrics associated with protection of marine animals.