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2017
Thode, A, Bonnel J, Thieury M, Fagan A, Verlinden C, Wright D, Berchok C, Crance J.  2017.  Using nonlinear time warping to estimate North Pacific right whale calling depths in the Bering Sea. Journal of the Acoustical Society of America. 141:3059-3069.   10.1121/1.4982200   AbstractWebsite

Calling depth distributions are estimated for two types of calls produced by critically endangered eastern North Pacific right whales (NPRWs) in the Bering Sea, using passive acoustic data collected with bottom-mounted hydrophone recorders. Nonlinear time resampling of 12 NPRW "upcalls" and 20 "gunshots" recorded in a critical NPRW habitat isolated individual normal mode arrivals from each call. The relative modal arrival times permitted range estimates between 1 and 40 km, while the relative modal amplitudes permitted call depth estimates, provided that environmental inversions were obtained from high signal-to-noise ratio calls. Gunshot sounds were generally only produced at a few meters depth, while upcall depths clustered between 10 and 25 m, consistent with previously published bioacoustic tagging results from North Atlantic right whales. A Wilcoxon rank sum test rejected the null hypothesis that the mean calling depths of the two call types were the same (p = 2.9 x 10(-5)); the null hypothesis was still rejected if the sample set was restricted to one call per acoustic encounter (p = 0.02). Propagation modeling demonstrates that deeper depths enhance acoustic propagation and that source depth estimates impact both NPRW upcall source level and detection range estimates. (C) 2017 Acoustical Society of America.

Bonnel, J, Caporale S, Thode A.  2017.  Waveguide mode amplitude estimation using warping and phase compensation. Journal of the Acoustical Society of America. 141:2243-2255.   10.1121/1.4979057   AbstractWebsite

In shallow water, low-frequency propagation can be described by modal theory. Acoustical oceanographic measurements under this situation have traditionally relied on spatially filtering signals with arrays of synchronized hydrophones. Recent work has demonstrated how a method called warping allows isolation of individual mode arrivals on a single hydrophone, a discovery that subsequently opened the door for practical single-receiver source localization and geoacoustic inversion applications. Warping is a non-linear resampling of the signal based on a simplistic waveguide model. Because warping is robust to environmental mismatch, it provides accurate estimates of the mode phase even when the environment is poorly known. However, the approach has issues with mode amplitude estimation, particularly for the first arriving mode. As warping is not invariant to time shifting, it relies on accurate estimates of the signal's time origin, which in turn heavily impacts the first mode's amplitude estimate. Here, a revised warping operator is proposed that incorporates as much prior environmental information as possible, and is actually equivalent to compensating the relative phase of each mode. Warping and phase compensation are applied to both simulated and experimental data. The proposed methods notably improve the amplitude estimates of the first arriving mode. (C) 2017 Acoustical Society of America.

2016
Thode, AM, Blackwell SB, Seger KD, Conrad AS, Kim KH, Macrander AM.  2016.  Source level and calling depth distributions of migrating bowhead whale calls in the shallow Beaufort Sea. Journal of the Acoustical Society of America. 140:4288-4297.   10.1121/1.4968853   AbstractWebsite

Automated and manual acoustic localizations of migrating bowhead whales were used to estimate source level and calling depth distributions of their frequency-modulated-modulated calls over seven years between 2008 and 2014. Whale positions were initially triangulated using directional autonomous seafloor acoustic recorders, deployed between 25 and 55m water depth near Kaktovik, Alaska, during the fall westward migration. Calling depths were estimated by minimizing the "discrepancy" between source level estimates from at least three recorders detecting the same call. Applying a detailed waveguide propagation model to the data yielded broadband source levels of 161 +/- 9 dB re 1 mu Pa-2 s at 1m (SEL) for calls received between 20 and 170Hz. Applying a simpler 15 log(10)(R) power-law propagation model yielded SEL source levels of 158 +/- 10dB. The most probable calling depths lay between 22 and 30m: optimal depths for long-range acoustic signal transmission in this particular environment. (C) 2016 Acoustical Society of America.

Thode, A, Wild L, Straley J, Barnes D, Bayless A, O'Connell V, Oleson E, Sarkar J, Falvey D, Behnken L, Martin S.  2016.  Using line acceleration to measure false killer whale (Pseudorca crassidens) click and whistle source levels during pelagic longline depredation. Journal of the Acoustical Society of America. 140:3941-3951.   10.1121/1.4966625   AbstractWebsite

False killer whales (Pseudorca crassidens) depredate pelagic longlines in offshore Hawaiian waters. On January 28, 2015 a depredation event was recorded 14m from an integrated GoPro camera, hydrophone, and accelerometer, revealing that false killer whales depredate bait and generate clicks and whistles under good visibility conditions. The act of plucking bait off a hook generated a distinctive 15Hz line vibration. Two similar line vibrations detected at earlier times permitted the animal's range and thus signal source levels to be estimated over a 25-min window. Peak power spectral density source levels for whistles (4-8 kHz) were estimated to be between 115 and 130 dB re 1 mu Pa-2/Hz @ 1m. Echolocation click source levels over 17-32 kHz bandwidth reached 205 dB re 1 mu Pa @ 1m pk-pk, or 190 dB re 1 mu Pa @ 1m (root-mean-square). Predicted detection ranges of the most intense whistles are 10 to 25km at respective sea states of 4 and 1, with click detection ranges being 5 times smaller than whistles. These detection range analyses provide insight into how passive acoustic monitoring might be used to both quantify and avoid depredation encounters. (C) 2016 Acoustical Society of America.

Seger, KD, Thode AM, Urban J, Martinez-Loustalot P, Jimenez-Lopez ME, Lopez-Arzate D.  2016.  Humpback whale-generated ambient noise levels provide insight into singers' spatial densities. Journal of the Acoustical Society of America. 140:1581-1597.   10.1121/1.4962217   AbstractWebsite

Baleen whale vocal activity can be the dominant underwater ambient noise source for certain locations and seasons. Previous wind-driven ambient-noise formulations have been adjusted to model ambient noise levels generated by random distributions of singing humpback whales in ocean waveguides and have been combined to a single model. This theoretical model predicts that changes in ambient noise levels with respect to fractional changes in singer population (defined as the noise "sensitivity") are relatively unaffected by the source level distributions and song spectra of individual humpback whales (Megaptera novaeangliae). However, the noise "sensitivity" does depend on frequency and on how the singers' spatial density changes with population size. The theoretical model was tested by comparing visual line transect surveys with bottom-mounted passive acoustic data collected during the 2013 and 2014 humpback whale breeding seasons off Los Cabos, Mexico. A generalized linear model (GLM) estimated the noise "sensitivity" across multiple frequency bands. Comparing the GLM estimates with the theoretical predictions suggests that humpback whales tend to maintain relatively constant spacing between one another while singing, but that individual singers either slightly increase their source levels or song duration, or cluster more tightly as the singing population increases. (C) 2016 Acoustical Society of America.

Thode, AM, Kim KH, Norman RG, Blackwell SB, Greene CR.  2016.  Acoustic vector sensor beamforming reduces masking from underwater industrial noise during passive monitoring. Journal of the Acoustical Society of America. 139:EL105-EL111.   10.1121/1.4946011   AbstractWebsite

Masking from industrial noise can hamper the ability to detect marine mammal sounds near industrial operations, whenever conventional (pressure sensor) hydrophones are used for passive acoustic monitoring. Using data collected from an autonomous recorder with directional capabilities (Directional Autonomous Seafloor Acoustic Recorder), deployed 4.1 km from an arctic drilling site in 2012, the authors demonstrate how conventional beamforming on an acoustic vector sensor can be used to suppress noise arriving from a narrow sector of geographic azimuths. Improvements in signal-to-noise ratio of up to 15 dB are demonstrated on bowhead whale calls, which were otherwise undetectable using conventional hydrophones. (C) 2016 Acoustical Society of America

2015
Seger, KD, Thode AM, Swartz SL, Urban J.  2015.  The ambient acoustic environment in Laguna San Ignacio, Baja California Sur, Mexico. Journal of the Acoustical Society of America. 138:3397-3410.   10.1121/1.4935397   AbstractWebsite

Each winter gray whales (Eschrichtius robustus) breed and calve in Laguna San Ignacio, Mexico, where a robust, yet regulated, whale-watching industry exists. Baseline acoustic environments in LSI's three zones were monitored between 2008 and 2013, in anticipation of a new road being paved that will potentially increase tourist activity to this relatively isolated location. These zones differ in levels of both gray whale usage and tourist activity. Ambient sound level distributions were computed in terms of percentiles of power spectral densities. While these distributions are consistent across years within each zone, inter-zone differences are substantial. The acoustic environment in the upper zone is dominated by snapping shrimp that display a crepuscular cycle. Snapping shrimp also affect the middle zone, but tourist boat transits contribute to noise distributions during daylight hours. The lower zone has three source contributors to its acoustic environment: snapping shrimp, boats, and croaker fish. As suggested from earlier studies, a 300 Hz noise minimum exists in both the middle and lower zones of the lagoon, but not in the upper zone. (C) 2015 Acoustical Society of America.

Crance, JL, Berchok CL, Bonnel J, Thode AM.  2015.  Northeasternmost record of a North Pacific fin whale (Balaenoptera physalus) in the Alaskan Chukchi Sea. Polar Biology. 38:1767-1773.: Springer Berlin Heidelberg   10.1007/s00300-015-1719-7   AbstractWebsite
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Blackwell, SB, Nations CS, McDonald TL, Thode AM, Mathias D, Kim KH, Greene CR, Macrander AM.  2015.  Effects of airgun sounds on bowhead whale calling rates: Evidence for two behavioral thresholds. Plos One. 10   10.1371/journal.pone.0125720   AbstractWebsite

In proximity to seismic operations, bowhead whales (Balaenamysticetus) decrease their calling rates. Here, we investigate the transition from normal calling behavior to decreased calling and identify two threshold levels of received sound from airgun pulses at which calling behavior changes. Data were collected in August-October 2007-2010, during the westward autumn migration in the Alaskan Beaufort Sea. Up to 40 directional acoustic recorders (DASARs) were deployed at five sites offshore of the Alaskan North Slope. Using triangulation, whale calls localized within 2 km of each DASAR were identified and tallied every 10 minutes each season, so that the detected call rate could be interpreted as the actual call production rate. Moreover, airgun pulses were identified on each DASAR, analyzed, and a cumulative sound exposure level was computed for each 10-min period each season (CSEL10-min). A Poisson regression model was used to examine the relationship between the received CSEL10-min from airguns and the number of detected bowhead calls. Calling rates increased as soon as airgun pulses were detectable, compared to calling rates in the absence of airgun pulses. After the initial increase, calling rates leveled off at a received CSEL10-min of similar to 94 dB re 1 mu Pa-2-s (the lower threshold). In contrast, once CSEL10-min exceeded similar to 127 dB re 1 mu Pa-2-s (the upper threshold), whale calling rates began decreasing, and when CSEL10-min values were above similar to 160 dB re 1 mu Pa-2-s, the whales were virtually silent.

Thode, A, Mathias D, Straley J, O'Connell V, Behnken L, Falvey D, Wild L, Calambokidis J, Schorr G, Andrews R, Liddle J, Lestenkof P.  2015.  Cues, creaks, and decoys: using passive acoustic monitoring as a tool for studying sperm whale depredation. Ices Journal of Marine Science. 72:1621-1636.   10.1093/icesjms/fsv024   AbstractWebsite

Since 2003, a collaborative effort (SEASWAP) between fishers, scientists, and managers has researched how Alaskan sperm whales locate demersal longline fishing activity and then depredate sablefish from gear. Sperm whales constantly produce relatively low-frequency biosonar signals whenever foraging; therefore, over the past decade, passive acoustic monitoring (PAM) has become a basic tool, used for both measuring depredation activity and accelerating field tests of potential depredation countermeasures. This paper reviews and summarizes past published PAM research on SEASWAP, and then provides a detailed example of how PAM methods are currently being used to test countermeasures. The review covers two major research thrusts: (i) identifying acoustic outputs of fishing vessels that provide long-distance "cues" that attract whales to fishing activity; and (ii) validating whether distinctive "creak" sounds can be used to quantify and measure depredation rates, using both bioacoustic tags and statistical comparisons between visual and acoustic depredation estimates during federal sablefish surveys. The latter part of the paper then provides an example of how PAM is being used to study a particular potential countermeasure: an "acoustic decoy" which transmits fishing vessel acoustic cues to attract animals away from true fishing activity. The results of an initial 2011 field trial are presented to show how PAM was used to design the decoy signals and monitor the efficacy of the deployment. The ability of PAM to detect both whale presence and depredation behaviour has reduced the need to deploy researchers or other specialists on fishing cruises. Instead, volunteer fishers can deploy "user-friendly" acoustic recorders on their gear, greatly facilitating the testing of various deterrents, and providing the industry and regulators a convenient and unobtrusive tool for monitoring both the scale and long-term spread of this behaviour across the Alaskan fishery.

Straley, J, O'Connell V, Liddle J, Thode A, Wild L, Behnken L, Falvey D, Lunsford C.  2015.  Southeast Alaska Sperm Whale Avoidance Project (SEASWAP): a successful collaboration among scientists and industry to study depredation in Alaskan waters. Ices Journal of Marine Science. 72:1598-1609.   10.1093/icesjms/fsv090   AbstractWebsite

In Alaskan waters, depredation on sablefish longline gear by sperm whales increases harvesting cost, negatively biases stock assessments, and presents a risk of entanglement for whales. The Southeast Alaska Sperm Whale Avoidance Project (SEASWAP), a collaborative effort involving industry, scientists, and managers, since 2003 has undertaken research to evaluate depredation with a goal of recommending measures to reduce interactions. Prior to 2003, little was known about sperm whale distribution and behaviour in the Gulf of Alaska (GOA). Although fishers were reporting increasing interactions, the level of depredation varied with no apparent predictor of occurrence across vessels. Between 2003 and 2007, fishers were provided with fishery logbooks and recorded information on whale behaviour, whale presence and absence, during the set, soak, and haul for 319 sets in the GOA. Data were evaluated for a vessel, area, and seasonal (month) effect in the presence and absence of sperm whales. Using catch per unit effort (cpue) as a metric, in kg/100 hooks, results indicated that depredation depended on both the vessel and the area. More whales associated with vessels from April to August. Sperm whales were also likely to be present when cpue was high, revealing that whales and fishers both knew the most productive fishing areas, but confounding the use of cpue as a metric for depredation. Using a Bayesian mark-recapture analysis and the sightings histories of photo-identified whales, an estimated N = 135 (95% CI 124, 153) sperm whales were associating with vessels in 2014. A spatial model was fitted to 319 longline sets and quantified a 3% loss in cpue, comparable to other global studies on sperm whale depredation. Through all phases of SEASWAP, our understanding of depredation has gained significantly. This successful collaboration should be considered as a model to create partnerships and build collaborations between researchers and fisher people encountering marine mammal interactions with fishing gear.

O'Connell, V, Straley J, Liddle J, Wild L, Behnken L, Falvey D, Thode A.  2015.  Testing a passive deterrent on longlines to reduce sperm whale depredation in the Gulf of Alaska. Ices Journal of Marine Science. 72:1667-1672.   10.1093/icesjms/fsv014   AbstractWebsite

In Alaska, sperm whale (Physeter macrocephalus) depredation on longline sets has increased since implementation of the Individual Fishing Quota programme in 1995. A collaborative effort (SEASWAP) between longliners, scientists, and managers has undertaken research to evaluate this depredation with a primary objective to develop and test a passive deterrent that would reduce depredation without reducing catch rate of sablefish (Anoplopoma fimbria). Commercial longliners, fishing for their own sablefish quotas during the regular season, deployed beaded gear (25 mm lucite beads attached to gangions) with control gear and set recorders to collect acoustic data. Beaded and control gear were randomly assigned by skate quad (672 hooks) with 5 quads in each longline set. Acoustic recorders were used to document sperm whale creak-pause events, representative of depredation of the longline gear. Although there were more sablefish per skate quad on the beaded gear and there was a decrease in depredation events on the beaded gear compared with the control, neither effect was significant (p = 0.205 and 0.364, respectively). The SEASWAP project is testing other deterrent strategies including gear modifications and the establishment of a sighting network to improve avoidance.

2014
Bonnel, J, Thode AM, Blackwell SB, Kim K, Macrander AM.  2014.  Range estimation of bowhead whale (Balaena mysticetus) calls in the Arctic using a single hydrophone. Journal of the Acoustical Society of America. 136:145-155.   10.1121/1.4883358   AbstractWebsite

Bowhead whales generate low-frequency calls in shallow-water Arctic environments, whose dispersive propagation characteristics are well modeled by normal mode theory. As each mode propagates with a different group speed, a call's range can be inferred by the relative time-frequency dispersion of the modal arrivals. Traditionally, at close ranges modal arrivals are separated using synchronized hydrophone arrays. Here a nonlinear signal processing method called "warping" is used to filter the modes on just a single hydrophone. The filtering works even at relatively short source ranges, where distinct modal arrivals are not separable in a conventional spectrogram. However, this warping technique is limited to signals with monotonically increasing or decreasing frequency modulations, a relatively common situation for bowhead calls. Once modal arrivals have been separated, the source range can be estimated using conventional modal dispersion techniques, with the original source signal structure being recovered as a by-product. Twelve bowhead whale vocalizations recorded near Kaktovik (Alaska) in 2010, with signal-to-noise ratios between 6 and 23 dB, are analyzed, and the resulting single-receiver range estimates are consistent with those obtained independently via triangulation from widely-distributed vector sensor arrays. Geoacoustic inversions for each call are necessary in order to obtain the correct ranges. (C) 2014 Acoustical Society of America.

Abadi, SH, Thode AM, Blackwell SB, Dowling DR.  2014.  Ranging bowhead whale calls in a shallow-water dispersive waveguide. Journal of the Acoustical Society of America. 136:130-144.   10.1121/1.4881924   AbstractWebsite

This paper presents the performance of three methods for estimating the range of broadband (50-500 Hz) bowhead whale calls in a nominally 55-m-deep waveguide: Conventional mode filtering (CMF), synthetic time reversal (STR), and triangulation. The first two methods use a linear vertical array to exploit dispersive propagation effects in the underwater sound channel. The triangulation technique used here, while requiring no knowledge about the propagation environment, relies on a distributed array of directional autonomous seafloor acoustics recorders (DASARs) arranged in triangular grid with 7 km spacing. This study uses simulations and acoustic data collected in 2010 from coastal waters near Kaktovik, Alaska. At that time, a 12-element vertical array, spanning the bottom 63% of the water column, was deployed alongside a distributed array of seven DASARs. The estimated call location-to-array ranges determined from CMF and STR are compared with DASAR triangulation results for 19 whale calls. The vertical-array ranging results are generally within +/- 10% of the DASAR results with the STR results providing slightly better agreement. The results also indicate that the vertical array can range calls over larger ranges and with greater precision than the particular distributed array discussed here, whenever the call locations are beyond the distributed array boundaries. (C) 2014 Acoustical Society of America.

Thode, AM, Wild L, Mathias D, Straley J, Lunsford C.  2014.  A comparison of acoustic and visual metrics of sperm whale longline depredation. Journal of the Acoustical Society of America. 135:3086-3100.   10.1121/1.4869853   AbstractWebsite

Annual federal stock assessment surveys for Alaskan sablefish also attempt to measure sperm whale depredation by quantifying visual evidence of depredation, including lip remains and damaged fish. A complementary passive acoustic method for quantifying depredation was investigated during the 2011 and 2012 survey hauls. A combination of machine-aided and human analysis counted the number of distinct "creak" sounds detected on autonomous recorders deployed during the survey, emphasizing sounds that are followed by silence ("creak-pauses"), a possible indication of prey capture. These raw counts were then adjusted for variations in background noise levels between deployments. Both a randomized Pearson correlation analysis and a generalized linear model found that noise-adjusted counts of "creak-pauses" were highly correlated with survey counts of lip remains during both years (2012: r(10) = 0.89, p = 1e-3; 2011: r(39) = 0.72, p = 4e-3) and somewhat correlated with observed sablefish damage in 2011 [r(39) = 0.37, p = 0.03], but uncorrelated with other species depredation. The acoustic depredation count was anywhere from 10% to 80% higher than the visual counts, depending on the survey year and assumptions employed. The results suggest that passive acoustics can provide upper bounds on depredation rates; however, the observed correlation breaks down whenever three or more whales are present. (C) 2014 Acoustical Society of America.

JM, S, GS S, AM T, J C, CR L, EM C, VM O?C, RD A.  2014.  Depredating sperm whales in the Gulf of Alaska: local habitat use and long distance movements across putative population boundaries. Endangered Species Research. 24:125-135.   10.3354/esr00595   AbstractWebsite

ABSTRACT: Satellite tags were attached to 10 sperm whales Physeter macrocephalus (1 whale was tagged in 2 different years) to determine the movements of sperm whales involved in removal of sablefish from longline fishing gear in the Gulf of Alaska (GOA). Tags transmitted from 3 to 34 d (median = 22) in 2007 and 7 to 158 d (median = 45) in 2009. Seven whales stayed in the GOA; all were associating with fishing vessels along the slope. Two whales headed south in June shortly after being tagged; one reached the inner third of the Sea of Cortez; the other’s last location was offshore Mexico at 14°N. A third whale stayed in the GOA until October and then headed south, reaching central Baja, Mexico, 158 d after tagging. The whales that travelled to lower latitudes followed no pattern in timing of departure, and at least 2 had different destinations. All whales passed through the California Current without stopping and did not travel to Hawaii; both are areas with known concentrations of sperm whales. Whales travelled faster when south of 56°N than when foraging in the GOA (median rate of median horizontal movement = 5.4 [range:4.1 to 5.5] and 1.3 [range:0.6 to 2.5] km h-1, respectively). Tagged sperm whales primarily travelled over the slope, but one spent considerable time over the ocean basin. Information on the timing and movement patterns of sperm whales may provide a means for fishermen to avoid fishing at whale hot spots, potentially reducing interactions between whales and fishermen.

2013
Walker, SC, Yardim C, Thode A, Arias-Castro E.  2013.  Using Fisher information to quantify uncertainty in environmental parameters estimated from correlated ambient noise. Journal of the Acoustical Society of America. 133:EL228-EL234.   10.1121/1.4792836   AbstractWebsite

Efforts to characterize environmental parameters from ambient noise must contend with uncertainty introduced by stochastic fluctuations of the noise itself. This Letter calculates the Fisher information and CramerRao bound of an unbiased correlated ambient noise parameter estimate. As an illustration, lower bounds on the error covariance of medium speed and attenuation parameters are obtained for a two-dimensional isotropic ambient noise scenario. The results demonstrate that an optimal sensor separation exists for obtaining the minimum error and the predictions are validated using simulated parameter inversions. The influences of record length, bandwidth, signal-to-noise, and spatial resolution are discussed. (C) 2013 Acoustical Society of America

Mathias, D, Thode AM, Straley J, Andrews RD.  2013.  Acoustic tracking of sperm whales in the Gulf of Alaska using a two-element vertical array and tags. Journal of the Acoustical Society of America. 134:2446-2461.   10.1121/1.4816565   AbstractWebsite

Between 15 and 17 August 2010, a simple two-element vertical array was deployed off the continental slope of Southeast Alaska in 1200 m water depth. The array was attached to a vertical buoy line used to mark each end of a longline fishing set, at 300 m depth, close to the sound-speed minimum of the deep-water profile. The buoy line also served as a depredation decoy, attracting seven sperm whales to the area. One animal was tagged with both a LIMPET dive depth-transmitting satellite and bioacoustic "B-probe" tag. Both tag datasets were used as an independent check of various passive acoustic schemes for tracking the whale in depth and range, which exploited the elevation angles and relative arrival times of multiple ray paths recorded on the array. Analytical tracking formulas were viable up to 2 km range, but only numerical propagation models yielded accurate locations up to at least 35 km range at Beaufort sea state 3. Neither localization approach required knowledge of the local bottom bathymetry. The tracking system was successfully used to estimate the source level of an individual sperm whale's "clicks" and "creaks" and predict the maximum detection range of the signals as a function of sea state. (C) 2013 Acoustical Society of America.

Blackwell, SB, Nations CS, McDonald TL, Greene CR, Thode AM, Guerra M, Macrander AM.  2013.  Effects of airgun sounds on bowhead whale calling rates in the Alaskan Beaufort Sea. Marine Mammal Science. 29:E342-E365.   10.1111/mms.12001   AbstractWebsite

This study assesses effects of airgun sounds on bowhead calling behavior during the autumn migration. In August-October 2007, 35 directional acoustic recorders (DASARs) were deployed at five sites in the Alaskan Beaufort Sea. Location estimates were obtained for >137,500 individual calls; a subsample of locations with high detection probability was used in the analyses. Call localization rates (CLRs) were compared before, during, and after periods of airgun use between sites near seismic activities (median distance 41-45km) and sites relatively distant from seismic activities (median distance >104km). At the onset of airgun use, CLRs dropped significantly at sites near the airguns, where median received levels from airgun pulses (SPL) were 116-129 dB re 1 Pa (10-450 Hz). CLRs remained unchanged at sites distant from the airguns, where median received levels were 99-108 dB re 1 Pa. This drop could result from a cessation of calling, deflection of whales around seismic activities, or both combined, but call locations alone were insufficient to differentiate between these possibilities. Reverberation from airgun pulses could have masked a small number of calls near the airguns, but even if masking did take place, the analysis results remain unchanged.

2012
Thode, AM, Kim KH, Blackwell SB, Greene CR, Nations CS, McDonald TL, Macrander AM.  2012.  Automated detection and localization of bowhead whale sounds in the presence of seismic airgun surveys. Journal of the Acoustical Society of America. 131:3726-3747.   10.1121/1.3699247   AbstractWebsite

An automated procedure has been developed for detecting and localizing frequency-modulated bowhead whale sounds in the presence of seismic airgun surveys. The procedure was applied to four years of data, collected from over 30 directional autonomous recording packages deployed over a 280 km span of continental shelf in the Alaskan Beaufort Sea. The procedure has six sequential stages that begin by extracting 25-element feature vectors from spectrograms of potential call candidates. Two cascaded neural networks then classify some feature vectors as bowhead calls, and the procedure then matches calls between recorders to triangulate locations. To train the networks, manual analysts flagged 219 471 bowhead call examples from 2008 and 2009. Manual analyses were also used to identify 1.17 million transient signals that were not whale calls. The network output thresholds were adjusted to reject 20% of whale calls in the training data. Validation runs using 2007 and 2010 data found that the procedure missed 30%-40% of manually detected calls. Furthermore, 20%-40% of the sounds flagged as calls are not present in the manual analyses; however, these extra detections incorporate legitimate whale calls overlooked by human analysts. Both manual and automated methods produce similar spatial and temporal call distributions. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.3699247]

Mathias, D, Thode AM, Straley J, Calambokidis J, Schorr GS, Folkert K.  2012.  Acoustic and diving behavior of sperm whales (Physeter macrocephalus) during natural and depredation foraging in the Gulf of Alaska. Journal of the Acoustical Society of America. 132:518-532.   10.1121/1.4726005   AbstractWebsite

Sperm whales have depredated black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has recently spread in intensity and geographic coverage. Over a three-year period 11 bioacoustic tags were attached to adult sperm whales off Southeast Alaska during both natural and depredation foraging conditions. Measurements of the animals' dive profiles and their acoustic behavior under both behavioral modes were examined for statistically significant differences. Two rough categories of depredation are identified: "deep" and "shallow.""Deep depredating" whales consistently surface within 500 m of a hauling fishing vessel, have maximum dive depths greater than 200 m, and display significantly different acoustic behavior than naturally foraging whales, with shorter inter-click intervals, occasional bouts of high "creak" rates, and fewer dives without creaks. "Shallow depredating" whales conduct dives that are much shorter, shallower, and more acoustically active than both the natural and deep depredating behaviors, with median creak rates three times that of natural levels. These results suggest that depredation efforts might be measured remotely with passive acoustic monitoring at close ranges. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.4726005]

Ponce, D, Thode AM, Guerra M, Urban RJ, Swartz S.  2012.  Relationship between visual counts and call detection rates of gray whales (Eschrichtius robustus) in Laguna San Ignacio, Mexico. Journal of the Acoustical Society of America. 131:2700-2713.   10.1121/1.3689851   AbstractWebsite

Daily acoustic calling rates of Eastern North Pacific (ENP) gray whales were measured on 6 days during 1 mo of their 2008 breeding season in the sheltered coastal lagoon of Laguna San Ignacio in Baja California, Mexico. Visual counts of whales determined that the numbers of single animals in the lower lagoon more than tripled over the observation period. All call types showed production peaks in the early morning and evening with minimum rates generally detected in the early afternoon. For four of the five observation days, the daily number of "S1"-type calls increased roughly as the square of the number of the animals in the lower lagoon during both daytime and nighttime. This relationship persisted when raw call counts were adjusted for variations in background noise level, using a simple propagation law derived from empirical measurements. The one observation day that did not fit the square-law relationship occurred during a week when the group size in the lagoon increased rapidly. These results suggest that passive acoustic monitoring does not measure gray whale group size directly but monitors the number of connections in the social network, which rises as roughly M2/2 for a group size M. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.3689851]

2011
Guerra, M, Thode AM, Blackwell SB, Macrander AM.  2011.  Quantifying seismic survey reverberation off the Alaskan North Slope. Journal of the Acoustical Society of America. 130:3046-3058.   10.1121/1.3628326   AbstractWebsite

Shallow-water airgun survey activities off the North Slope of Alaska generate impulsive sounds that are the focus of much regulatory attention. Reverberation from repetitive airgun shots, however, can also increase background noise levels, which can decrease the detection range of nearby passive acoustic monitoring (PAM) systems. Typical acoustic metrics for impulsive signals provide no quantitative information about reverberation or its relative effect on the ambient acoustic environment. Here, two conservative metrics are defined for quantifying reverberation: a minimum level metric measures reverberation levels that exist between airgun pulse arrivals, while a reverberation metric estimates the relative magnitude of reverberation vs expected ambient levels in the hypothetical absence of airgun activity, using satellite-measured wind data. The metrics are applied to acoustic data measured by autonomous recorders in the Alaskan Beaufort Sea in 2008 and demonstrate how seismic surveys can increase the background noise over natural ambient levels by 30 45 dB within 1 km of the activity, by 10-25 dB within 15 km of the activity, and by a few dB at 128 km range. These results suggest that shallow-water reverberation would reduce the performance of nearby PAM systems when monitoring for marine mammals within a few kilometers of shallow-water seismic surveys. (C) 2011 Acoustical Society of America. [DOI: 10.1121/1.3628326]

2010
Thode, A, Kim KH, Greene CR, Roth E.  2010.  Long range transmission loss of broadband seismic pulses in the Arctic under ice-free conditions. Journal of the Acoustical Society of America. 128:E181-E187.   10.1121/1.3479686   AbstractWebsite

In 2008 the Louis S. St-Laurent (LSSL) surveyed deep Arctic waters using a three-airgun seismic source. Signals from the seismic survey were detected between 400 km and 1300 km range on a directional autonomous acoustic recorder deployed in water 53 m deep off the Alaskan North Slope. Observations of received signal levels between 10-450 Hz versus LSSL range roughly fit a cylindrical transmission loss model plus 0.01 dB/km attenuation in deep ice-free waters, and fit previous empirical models in ice-covered waters. The transition between ice-free and ice-covered propagation conditions shifted 200 km closer to the recorder during the survey. (C) 2010 Acoustical Society of America

Thode, A, Skinner J, Scott P, Roswell J, Straley J, Folkert K.  2010.  Tracking sperm whales with a towed acoustic vector sensor. Journal of the Acoustical Society of America. 128:2681-2694.   10.1121/1.3495945   AbstractWebsite

Passive acoustic towed linear arrays are increasingly used to detect marine mammal sounds during mobile anthropogenic activities. However, these arrays cannot resolve between signals arriving from the port or starboard without vessel course changes or multiple cable deployments, and their performance is degraded by vessel self-noise and non-acoustic mechanical vibration. In principle acoustic vector sensors can resolve these directional ambiguities, as well as flag the presence of non-acoustic contamination, provided that the vibration-sensitive sensors can be successfully integrated into compact tow modules. Here a vector sensor module attached to the end of a 800 m towed array is used to detect and localize 1813 sperm whale "clicks" off the coast of Sitka, AK. Three methods were used to identify frequency regimes relatively free of non-acoustic noise contamination, and then the active intensity (propagating energy) of the signal was computed between 4-10 kHz along three orthogonal directions, providing unambiguous bearing estimates of two sperm whales over time. These bearing estimates are consistent with those obtained via conventional methods, but the standard deviations of the vector sensor bearing estimates are twice those of the conventionally-derived bearings. The resolved ambiguities of the bearings deduced from vessel course changes match the vector sensor predictions. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3495945]