Source levels of harbor seal breeding vocalizations were estimated using a three-element planar hydrophone array near the Beardslee Islands in Glacier Bay National Park and Preserve, Alaska. The average source level for these calls was 144 dBRMS re 1 μPa at 1 m in the 40–500 Hz frequency band. Source level estimates ranged from 129 to 149 dBRMS re 1 μPa. Four call parameters, including minimum frequency, peak frequency, total duration, and pulse duration, were also measured. These measurements indicated that breeding vocalizations of harbor seals near the Beardslee Islands of Glacier Bay National Park are similar in duration (average total duration: 4.8 s, average pulse duration: 3.0 s) to previously reported values from other populations, but are 170–220 Hz lower in average minimum frequency (78 Hz).

Harbor seals (Phoca vitulina) are the most widely distributed pinniped in the northern hemisphere and occupy a variety of habitats including rocky reefs, islands, and glacier ice (Bigg, 1981). Although harbor seals may range widely during the post-breeding season (Lowry et al., 2001; Peterson et al., 2012; Womble and Gende, 2013), they typically exhibit a high degree of fidelity to pupping areas during the breeding season (Blundell et al., 2011).

Harbor seals, along with the majority of other phocid species, mate underwater (Van Parijs, 2003). During the breeding season, male harbor seals establish underwater territories and use breeding vocalizations, known as roars, to defend these areas against other males and possibly to attract females (Hanggi and Schusterman, 1994; Hayes et al., 2004b). Previous studies indicate that Pacific harbor seals in California (Phoca vitulina richardii) produce roars that are 2–10 s long and occupy the 300–1100 Hz band (Hanggi and Schusterman, 1994). Roars from Atlantic harbor seals (Phoca vitulina vitulina) have an average duration of approximately 5 s, and range in frequency from 250 to 1300 Hz (Van Parijs et al., 2000).

The one breeding vocalization source level reported for a single captive harbor seal is 145 dBRMS re 1 μPa at 1 m (Casey et al., 2016). Investigating the variability in source levels of harbor seal roars from multiple individuals in different habitats is important for estimating the intensity and detection range of these biologically important signals. Herein we present results of a study that measured the frequency characteristics and source levels of harbor seal roars in a wild population.

Four calibrated autonomous underwater hydrophones (AUHs) were deployed in Glacier Bay National Park and Preserve (GBNPP) near the Beardslee Islands adjacent to the Spider Island Reef complex, the largest terrestrial harbor seal pupping site in GBNPP. The number of harbor seals at Spider Reef typically peaks during the pupping period in June [215 seals ± 96 (SD), max = 453, years: 2004–2014] and the molting period in late July through August [308 seals ± 196 (SD), max = 723, years: 2004–2014] (Womble et al., 2010). AUHs were deployed in a diamond shaped planar array, at depths of 65–81 m, with a baseline of approximately 1 km between units. The array recorded continuously from May 27 to October 29, 2015 in the 15 Hz to 4 kHz range (hydrophone model ITC1032, 10 kHz sampling rate, low pass filter at 4 kHz, 16 bit resolution), which fully covers the frequency range of harbor seal breeding vocalizations (Hanggi and Schusterman, 1994; Van Parijs et al., 2000). Hydrophones were attached to aluminum moorings and acoustic recovery systems. Each AUH was outfitted with a highly precise clock (Q-Tech model number QT-2001, error of approximately 1 s per year) to allow for time-synchronization of the four channels and subsequent acoustic localization of vocalizing seals. Files from the AUHs were converted from .dat to .wav files using a custom written matlab script. The eastern-most hydrophone could not be time-aligned due to an erroneous clock and was therefore excluded from this analysis.

Acoustic data were subsampled over the duration of the harbor seal breeding season by randomly selecting one day for each week of peak vocal activity (June 1–July 31, L. Matthews and GBNPP, unpublished data). Within each sample day, four hours were selected that represented four non-overlapping light regimes based on angle of the sun–sunrise (June 1–July 20: 0200–0600 h, July 21–July 31: 0300–0700 h), day (June 1–July 20: 0600–2000 h, July 21–July 31: 0700–2000 h), sunset (June 1–July 31: 2000–0000 h), and night (June 1–July 20: 0000–0200 h, July 21–July 31: 0000–0300 h) (United States Naval Observatory, 2016). Overall, a total of 36 h of acoustic data were selected from nine recording days that spanned a two-month period. Spectrograms for each hour of multi-channel acoustic data were visually analyzed for the presence of harbor seal roar vocalizations. There were no other species present in the area that produce calls similar to harbor seal roars, allowing for accurate identification of these breeding vocalizations.

Previous studies describe four acoustic parameters that are most useful for comparative analyses of harbor seal roar vocalizations (Van Parijs et al., 2000): total duration (duration from the start to end of the roar), pulse duration (duration of the broadband component, which begins 1–2 s after the onset of the roar), minimum frequency, and peak frequency, where peak frequency is frequency component with the greatest amplitude (Fig. 1). These parameters were measured for roars that were detected in the acoustic data in which all four parameters were visible and the signal to noise ratio (SNR) exceeded 10 dB [spectrogram parameters: Hann window, discrete Fourier transform (DFT) size = 1024, analysis resolution = 9.7 Hz and 0.05 s, 50% overlap]. Call parameters were measured using Raven 1.5 (Bioacoustics Research Program, 2014). The frequency parameters were used to determine the appropriate frequency band for source level estimates.

Fig. 1.

Spectrogram of a harbor seal roar recorded in the Beardslee Entrance of Glacier Bay National Park and Preserve with measured call parameters. Call parameters are based on those reported in Van Parijs et al., 2000. [Spectrogram parameters: Hann window, discrete Fourier transform (DFT) size = 1024, analysis resolution = 9.7 Hz and 0.05 s, 50% overlap.]

Fig. 1.

Spectrogram of a harbor seal roar recorded in the Beardslee Entrance of Glacier Bay National Park and Preserve with measured call parameters. Call parameters are based on those reported in Van Parijs et al., 2000. [Spectrogram parameters: Hann window, discrete Fourier transform (DFT) size = 1024, analysis resolution = 9.7 Hz and 0.05 s, 50% overlap.]

Close modal

Harbor seal breeding vocalizations were localized using Raven 2.0 (Bioacoustics Research Program, 2016). Localization used near-field beamforming search for the set of time of arrival delays that gave maximum power from the beamformer output (Hawthorne and Salisbury, 2016). A simulated annealing algorithm was used to find the point in space that generated maximum power. Each localized roar was assigned a latitude and longitude position. A sound speed of 1472 m/s was used based on the results of Malme et al. (1982). A follow-up CTD cast done by the National Park Service in the study area in 2015 revealed no significant changes in the sound speed. The measured differences fell well within the accuracy range of the instrument. Bartlett's formula was used to estimate the variance of the energy output from the beamformer and resulted in error values for the northern and eastern bearings for each call. We used a maximum error of 100 m in either direction as a first quality control measure for the data and discarded locations with errors above this threshold.

The source level of a vocalization recorded at ranges greater than 1 m can be estimated by adding the received level of the call detected at the hydrophone (RL) to the estimated transmission loss between the call's origin and the hydrophone (TL) [Eq. (1)], where both RL and TL are in dB. Absorption is negligible at the frequencies of harbor seal roar vocalizations (Francois and Garrison, 1982) and was therefore not considered in source level calculations,

SL=RL+TL.
(1)

Source levels for harbor seal breeding vocalizations were calculated by (1) determining the distance to the roar's origin, (2) calculating the transmission loss of the roar, (3) calibrating the acoustic system and measuring the received level of the roar, and (4) determining the source level for each roar in dBRMS re 1 μPa at 1 m.

2D-distances between localized roars and hydrophones were calculated using the earth.dist function from the “fossil” package in r (Vavrek, 2011). Transmission loss has previously been described for the area, and was determined to be 15log(r), where r is the distance between the calling animal and the hydrophone (Fig. 29 in Malme et al., 1982). We used this equation and the previously calculated distances to estimate transmission loss for each localized roar.

Received levels were measured using Raven 1.5 (Bioacoustics Research Program, 2014) as follows. Individual selection tables were created for each acoustic file that contained the start and end times for each localization and a standardized bandwidth of 40 to 500 Hz to fully encompass the frequency range of harbor seal breeding vocalizations in GBNPP. Spectrograms were calibrated using the hydrophone sensitivity and pre-amplifier gain, and the “inband power” measurement in Raven was used to determine received levels. There is no evidence to support that harbor seal roars are directional, therefore directionality effects on source level estimates were assumed to be negligible.

Three source level estimates in dB re 1 μPa at 1 m were made for each roar (one for each hydrophone used in analysis) using Eq. (1). These three source level estimates in dB were converted to voltages, averaged, and then reconverted to dB to produce a single source level estimate for each localized breeding vocalization.

Background noise level in the 40–500 Hz band was measured for each of the localized roars for the two-second period preceding the call. A corresponding signal-to-noise ratio (SNR) was then calculated. Of these localized calls, we chose roars with an SNR ≥ 10 dB on all channels as a second measure to ensure that only high quality calls were used in source level estimates. As an additional quality control measure for the data, we removed all roars located outside of the array to ensure there was no effect of distance on the source level estimates.

The number of calling individuals was estimated for each hour of acoustic data in order to ensure that acoustic data were collected from multiple animals. Locations of calling animals were plotted and the number of acoustic hotspots—areas of high roar density—was counted as a proxy for the number of callers. Areas of high roar density were defined as geographic clusters of two or more calls that were separated by at least 100 m. Hayes et al. (2004a) estimated that male harbor seals defended underwater territories with an average area size of 0.04 km2 (n = 4). Given this territorial nature of harbor seal males, the authors believe the number of acoustic hotspots is an appropriate estimator for the minimum number of calling individuals (Hayes et al., 2004a).

A total of 6477 harbor seal breeding vocalizations (Fig. 1) were visually detected over the 36 h of acoustic data; call parameters were measured for 484 calls that met the SNR criteria. The average minimum frequency was 78 Hz ± 10 Hz (SD) and measurements ranged from 50 to 104 Hz. The average peak frequency was 119 ± 8 Hz (SD) and ranged from 98 to 147 Hz. The average total duration and pulse durations were 4.8 ± 1.1 s (SD) and 3.0 ± 1.0 s (SD) and ranged from 2.0 to 9.1 s and 0.9 to 7.1 s, respectively. Overall, harbor seal roars in GBNPP are similar in duration to other populations. However, the minimum frequency of roars in this location are approximately 170 Hz lower than Atlantic populations (Van Parijs et al., 2000) and approximately 220 Hz lower than previously reported values for Pacific populations (Hanggi and Schusterman, 1994).

Approximately 18% (n = 1155) of the visually detected roars were successfully localized. Reasons for unsuccessful localizations included the roars being too faint or undetectable on one or more channels, high localization error, and noise from the moorings or biological sounds such as killer whales or other harbor seals. Figure 2 illustrates the range of SNR values for all the localized roars; 554 of these roars had an SNR ≥ 10 dB for all three channels. There were 539 roars localized within the interior dimensions of the array (Fig. 3). These roars were used for the final source level estimates. The average source level for harbor seal breeding vocalizations was 144 dBRMS re 1 μPa at 1 m and the median source level was 144 dBRMS re 1 μPa at 1 m (40–500 Hz bandwidth, n = 539, 95% CI: 143.61, 143.95 dBRMS re 1 μPa). Source level measurements ranged from 129 to 149 dBRMS re 1 μPa (Fig. 4). On average, the range of source level values for one roar from the three hydrophones was 8 dBRMS re 1 μPa. The average errors associated with the localization in the eastern and northern bearings were 34.2 and 18.6 m, respectively. There were 23 h, out of 36 h that were analyzed, in which harbor seal breeding vocalizations were detected. The estimated number of callers in each file varied from 1 to 6, with an average of 2.3 ± 1.4 (SD). Given this result, we surmise that the source level estimates presented here represent a minimum of six different individuals.

Fig. 2.

Distribution of the signal-to-noise ratios for localized calls. Black indicates calls that were removed from analysis, while the grey bars represent calls with an SNR ≥ 10 dB.

Fig. 2.

Distribution of the signal-to-noise ratios for localized calls. Black indicates calls that were removed from analysis, while the grey bars represent calls with an SNR ≥ 10 dB.

Close modal
Fig. 3.

Map of hydrophone array in the Beardslee Entrance and acoustic localizations (black points) used in the source level analysis (n = 539). Light grey points indicate roars that were removed from analysis based on their position outside of the array. Stars represent locations of hydrophones (North AUH: 58.51648°N, 135.97198°W, South AUH: 58.50585°N, 135.97225°W, East AUH: 58.51106°N, 135.96348°W, West AUH: 58.51186°N, 135.98110°W). The easternmost hydrophone, represented by the light grey star, was not used to localize harbor seal breeding vocalizations.

Fig. 3.

Map of hydrophone array in the Beardslee Entrance and acoustic localizations (black points) used in the source level analysis (n = 539). Light grey points indicate roars that were removed from analysis based on their position outside of the array. Stars represent locations of hydrophones (North AUH: 58.51648°N, 135.97198°W, South AUH: 58.50585°N, 135.97225°W, East AUH: 58.51106°N, 135.96348°W, West AUH: 58.51186°N, 135.98110°W). The easternmost hydrophone, represented by the light grey star, was not used to localize harbor seal breeding vocalizations.

Close modal
Fig. 4.

Histogram representing the range of calculated source levels for all calls used in analysis (n = 539).

Fig. 4.

Histogram representing the range of calculated source levels for all calls used in analysis (n = 539).

Close modal

This is the first study to report source levels for roars from a wild population of harbor seals. The average source level is 1 dB lower than the previously reported value estimated from a solitary captive male, but the previously reported value does fall within the range of measurements seen in this study (Casey et al., 2016). The source levels reported here are similar to harp seal underwater vocalizations that are thought to function for breeding purposes (103 to 180 dB re 1 μPa at 1 m), but are lower than pinniped breeding vocalizations from Antarctic seals, including Weddell seals (148 to 193 dB re 1 μPa at 1 m) and leopard seals (153 to 177 dB re 1 μPa at 1 m) (Rogers, 2014; Rossong and Terhune, 2009; Thomas and Kuechle, 1982). It is possible that the differences between species is due to body size, as male harbor seals and male harp seals are similar in body length and weight, while Weddell and leopard seals are much larger.

Future studies should investigate the potential variation in acoustic parameters, including source levels, of harbor seal breeding vocalizations produced by males in glacier ice habitats in tidewater glacier fjords, which host some of the largest seasonal aggregations of harbor seals in Alaska (Calambokidis et al., 1987). The Spider Reef complex likely accounts for <15% of harbor seals in GBNPP; the majority of seals occur in glacier ice habitat Johns Hopkins Inlet in GBNPP (Mathews and Pendleton, 2006; Womble et al., 2010). Given the management concern regarding acoustic disturbance of marine mammals in Alaska, future studies will also investigate the effects of vessel noise on harbor seal acoustic behavior.

The authors would like to thank the crew of the M/V Lite Weight—Captain Paul Weltzin and Deckhand John Martin—for their assistance in deploying and recovering the hydrophone array. We would also like to thank Dean Hawthorne for his assistance in acoustic localization. This project was made possible by funds from the National Park Foundation's Alaska Coastal Marine Grant program and the Marine Mammal Commission. Additional thanks to the reviewers for comments and feedback that improved the final version of this manuscript.

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