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Article

Moderate Anthropogenic Noise Exposure Does Not Affect Navy Bottlenose Dolphin (Tursiops truncatus) Whistle Rates

by
Jessica J. Sportelli
1,*,
Kelly M. Heimann
2 and
Brittany L. Jones
3
1
National Marine Mammal Foundation, San Diego, CA 92106, USA
2
Rosenstiel School of Marine, Atmospheric, and Earth Science, University of Miami, Miami, FL 33149, USA
3
Naval Information Warfare Center Pacific, San Diego, CA 92152, USA
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2024, 12(3), 441; https://doi.org/10.3390/jmse12030441
Submission received: 10 January 2024 / Revised: 28 January 2024 / Accepted: 26 February 2024 / Published: 1 March 2024
(This article belongs to the Section Marine Biology)
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Abstract

:
Bottlenose dolphins (Tursiops truncatus) rely on frequency- and amplitude-modulated whistles to communicate, and noise exposure can inhibit the success of acoustic communication through masking or causing behavioral changes in the animal. At the US Navy Marine Mammal Program (MMP) in San Diego, CA, dolphins are housed in netted enclosures in the San Diego Bay and exposed to noise from vessels, unmanned underwater vehicles, and other remote sensing devices. The acoustic behavior of 20 dolphins was monitored and whistle rates during noise events were quantified. Whistle rates during the onset of the event (i.e., the first 5 min) did not significantly differ from the pre-onset (5 min immediately preceding). Whistle rates were also not significantly different for the entire duration of the event compared to a matched control period. The noise’s frequency range (i.e., control, mid-frequency (0–20 kHz) or high-frequency (21–80 kHz)), signal-to-noise ratio, and sound pressure level were not significantly related to the dolphins’ whistle rate. Considering this is a location of frequent and moderate noise output, these results lend support to established guidelines on anthropogenic noise exposure for cetaceans, suggesting that moderate noise exposure levels may not impact communication efforts in bottlenose dolphins.

1. Introduction

Cetaceans rely on audition and acoustic communication to survive in an aquatic environment. The bottlenose dolphin (Tursiops truncatus), for example, produces echolocation clicks for navigation and hunting, relying on received echoes to map their surroundings [1,2,3,4], and narrow-band tonal sounds (i.e., whistles) for communication with conspecifics, e.g., [5,6]; for review, see [7]. Typical broadband echolocation click output can have peak frequencies between 40 and 130 kHz, with average peak-to-peak source levels (SLs) up to 220 dB re 1 µPa at 1 m (m) [2,8,9]. Whistles typically occur between 0.8 and 24 kHz, but have been reported to reach over 40 kHz [10,11]. Whistle SLs, on average, range from 138 to 158 dB re 1 µPa at 1 m [7,12,13,14,15]. Bottlenose dolphins produce a ‘signature whistle,’ which is a stereotyped frequency–time contour unique to each individual that is commonly emitted during separations from conspecifics [6,16]. They also emit non-signature whistles, such as shared whistles between affiliated individuals, that are often produced when traveling together [16,17,18]. Whistles serve a variety of communicative functions including individual identification [6,19], maintaining group cohesion [16], information sharing [19,20], and expressing distress [21,22,23,24]. For fission–fusion societies, like those of delphinids, acoustic communication is imperative for group cohesion in occluded waters, mother–calf bonding, social memory, and maintaining relationships [16,20,25,26,27]. To be successful, the animal must be able to hear and be heard in a noisy ocean environment. Bottlenose dolphin hearing sensitivity covers a broad range of frequencies from around 0.15 to 160 kHz, with a higher sensitivity between 15 and 100 kHz [28,29,30].
The ocean is a naturally noisy place and cetaceans must communicate in the presence of natural noise sources including wind, ice movements, earthquakes, and snapping shrimp (Alpheidae) [2,31]. Concerns about exposure to anthropogenic noise from commercial shipping, military sonar, oil and gas exploration, and off-shore construction has motivated significant research into the effects of noise on marine mammals. Behavioral changes in the presence of anthropogenic noise have been observed in many species of marine mammal, although the received levels (RLs) at which the change was observed varied depending on the sensitivity of the species [28,29,32,33,34,35,36,37]. For example, northern bottlenose whales (Hyperoodon ampullatus) exhibited an avoidance response from RLs between 117 and 126 dB re 1 µPa [37], North Atlantic right whales (Eubalaena glacialis) stopped feeding when exposed to RLs between 133 and 148 dB re 1 µPa [38], and long-finned pilot whales (Globicephala melas) had a higher probability of behavioral response to tactical sonars greater than 165 dB re 1 µPa [39]. Acoustic deterrents such as pingers attached to fishing nets were shown to significantly reduce the bycatch of short-beaked common dolphins (Delphinus delphis) and beaked whales (Ziphiidae) in California (SLs of the pingers were between 120 and 146 dB re 1 µPa at 1 m, with frequency range between 10 and 12 kHz) which suggested that beaked whales actively avoided the sound [40,41]. Disturbances in behavior were also recorded from dolphins in professional care while participating in a controlled dose–response noise exposure study [42]. The dolphins abandoned their trained behavior of swimming across the enclosure to touch a paddle when exposed to a frequency-modulated 0.5 s upsweep (center frequency ~3.5 kHz) at RLs of 175 and 185 dB re 1 µPa [42]. While no consistent trend has been found across different marine mammal species during various contexts, Finneran and Jenkins [29] suggested that the probability of a behavioral response would increase with increasing RL. In addition, it has been hypothesized that precursory noises that cue the onset of an impending loud sound may also activate a context-based response dependent on the animal’s previous history with the sound, proximity to the sound source, and the animal’s current behavior (e.g., feeding, travelling, or resting) [42,43].
Masking occurs when one sound (e.g., noise) interferes with a listener’s ability to hear or interpret a signal (e.g., a whistle from a conspecific) [30,44,45,46,47]. Masking can reduce the communication space available to an animal, with the severity of the masking dependent on the masker amplitude, similarities between the noise and signal in frequency and time, the position of the listener relative to the sound sources, and the signal-detection capabilities of the animal (e.g., critical ratio) [46]. Anti-masking responses by cetaceans have included increasing the amplitude of their call (i.e., the Lombard effect) [48,49], changing the duration of their calls, and/or changing their call rate [50,51,52]. Humpback whales (Megaptera novaeangliae) decreased their calling output in the presence of low-frequency fisheries’ sonar [53], and bowhead whales (Balaena mysticetus) decreased their calling output in the presence of air guns [34]. Blainville’s beaked whales (Mesoplodon densirostris) stopped echolocating when exposed to Naval sonar exercises [54]. The acoustic strategies to overcome masking were dependent on the species being recorded. For example, in the presence of vessel noise, endangered Southern Resident killer whales (Orcinus orca) elongated and increased the amplitude of their calls [48] while the endangered beluga whale (Delphinapterus leucas) population in the St. Lawrence Estuary decreased their calling rate [50]. Noise type may also affect the chosen anti-masking behavior. For example, bottlenose dolphins increased their whistle rate at the onset of noisy vessel approaches [55] while delphinids in the Southern California Bight that were exposed to mid-frequency active (MFA) sonar were observed to either stop calling altogether or increase the intensity of their vocal output [56]. For marine mammals, noise impacts on acoustic communication can have detrimental effects on reproductive success, calf survival, reproductive opportunity, and coordinated travel or foraging. Altering vocal output can also result in an increased energetic cost for the caller, as measured in Holt et al. [57], where the metabolic rate of two bottlenose dolphins was significantly higher while vocalizing compared to their resting metabolic rate.
Bottlenose dolphins at the U.S. Navy Marine Mammal Program (MMP) live in groups in natural sea-water enclosures in San Diego Bay, CA. These dolphins are incidentally exposed to anthropogenic noise events including echo-sounders, unmanned underwater vehicles (UUV), sonar, and other remotely operated sensing equipment that communicate with research and military teams onshore. Changes in whistle rates have been proposed to be an indicator of distress in bottlenose dolphins and beluga whales [23,55,57,58], yet few studies have focused on changes in bottlenose dolphin vocal behavior due to noise exposure, specifically, e.g., [55,59,60]. Therefore, this study examined whether exposure to anthropogenic noise (hereafter referred to as noise) had any effect on the whistle rates of the Navy’s dolphins.

2. Materials and Methods

The focal group consisted of 20 dolphins from the MMP in San Diego, CA. The dolphins were housed in modular, interconnected clusters of a dozen or more 9 m × 9 m and 9 m × 18 m floating netted enclosures, separated by sex. The focal group is made up of both wild caught animals and animals born within the program, with and age range between 6 and 45 years old. While data collection for this study only spanned three years, some of these animals have been a part of the MMP for up to 30 years. The MMP is AAALAC-accredited and follows the national standards of the United States Public Health Service Policy on the Humane Care and Use of Laboratory Animals and the Animal Welfare Act. All protocols and procedures in this study were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) of the Naval Information Warfare Center Pacific and the Navy Bureau of Medicine and Surgery (BUMED).
The National Marine Mammal Foundation’s Welfare Acoustic Monitoring System (NMMF WAMS), hereafter referred to as WAMS, was developed to monitor the vocal behavior of the focal dolphin population [61]. The WAMS array was composed of four HTI-99-HF hydrophones (High-Tech Inc., Long Beach, MS, USA, 2–125 kHz frequency response with integrated preamplifier) arranged in a tetrahedral shape (Figure 1A). Spacing between any two hydrophones was 150 mm, with all four mounted to a platform and attached to a frame structure made of PVC pipe. The hydrophone cables were run to a custom-built Sail DAQ sound card attached via USB to a Dell laptop. The array sat on the sea floor between the female and male dolphin enclosures (Figure 1B). Each hydrophone had a sample rate of 125 kHz, with an average receiver sensitivity of −163.8 dB re 1 V/µPa between the four hydrophones.
Noise was monitored using a custom LabVIEW program (version 19.0f2) [61] which analyzed the ambient power spectral density (PSD) levels in the bay from the WAMS acoustic recordings in both the mid-frequency (MF) and high-frequency (HF) domains. Definitions used to establish MF and HF ranges were based on the species of study (e.g., toothed whales) and their specific hearing sensitivities [29,62,63]. For the purposes of this work, frequency ranges for MF (1–20 kHz) and HF (20–80 kHz) described in Jones et al. [61] were used. As a starting point for identifying the presence of an anthropogenic event, MF noise thresholds were set to 129 dB re 1 µPa2/Hz, while HF noise thresholds were set to 113 dB re 1 µPa2/Hz—similar to RL values recorded in other studies on behavioral changes in the presence of noise, e.g., [37,38,40,41]. The system would “trigger” when a received PSD level surpassed one of the thresholds. Data points within the program’s window that did not exceed the noise thresholds appeared blue, while a noise that did exceed the threshold appeared red (Figure 2).
WAMS ran almost continuously over the three-year study period, between August 2019 and September 2022, and output 5 min (min) .wav files onto an external hard drive, along with a corresponding .jpeg of the 5 min noise analysis window from LabVIEW (e.g., Figure 2). The noise analysis program also output a “trigger file” .txt document. The trigger files contained the name of the 5 min .wav file, the time stamp (MM:SS) within the 5 min file that the trigger was set off, and the maximum MF and HF received PSD levels recorded at that time. As the thresholds were intentionally set at a low level, each trigger was manually confirmed to have been triggered based on an anthropogenic noise emission and not a biological sound (e.g., dolphin echolocation clicks). Human analysts reviewed each noise event and identified the onset of the noise as the first appearance of the noise on the spectrogram with a high enough signal-to-noise ratio (SNR) to be visible to the analysts. An event was considered over when an hour of no noise followed the last noise emission. Thirty-eight noise events were analyzed for the current study. While more than 38 noise events took place over the three-year recording period, issues between corrupted files and incomplete saved data sets prevented the inclusion of every noise event detected. The 38 events analyzed all had a clear beginning and end to the event, and at least one 5 min file preceding the beginning of each noise event had been saved.
The sound pressure level (SPL) for both the onset of the noise and at the moment that the LabVIEW system was triggered were calculated using the third octave band limits around the noise’s center frequency in a custom-built program in LabVIEW (version 1.0.0.8, Hanning window, FFT Size: 8192, Window Size: 20). Clips of similar durations from the same time were taken of the ambient background noise and analyzed using the same program to obtain the average PSD, again around the third-octave band relative to the noise’s center frequency. The PSD of the ambient noise was subtracted from the SPL of the noise signal to obtain the SNR. The SNR was then compared to critical ratios (CR) previously reported for bottlenose dolphins by Branstetter et al. [64]. For this study, we assumed the composite of CRs for delphinid species reported in Branstetter et al. [64] was a comprehensive representation of different masking scenarios and represented this specific species’ hearing abilities well [45,65]. If an event’s onset was not loud enough to be audible to the dolphin (i.e., the SNR was less than the CR) it was removed from analysis. Three events were so removed, resulting in the 38 onset events analyzed (Table 1).
Using Raven Pro 1.6 with user-defined spectrogram parameters (Hanning window, 250 kHz sample rate, 4096 DFT size, 1050 Hop Size, 50% overlap, 2100 sample size, 3 dB filter bandwidth at 171 Hz, 42 kHz max frequency, 10 s time window), whistles were manually counted between three analysts using the selection marquee tool. An inter-observer reliability test using 10% of the data was performed before a new analyst could be added to the project. Inter-observer reliability (>80%) was reached for two additional analysts (i.e., 83% and 81%). Whistles that were made up of multiple disconnected loops were counted as one multi-looped whistle if the spacing between the end of one contour and the start of the other contour was less than 0.25 s [66]. If the break in the two whistle contours was greater than 0.25 s it was counted as two separate whistles. If two distinct and different whistle types appeared within 0.25 s of each other or overlapped, they were also considered two whistles. Whistles also had to be greater than 0.25 s in duration to be counted, which excluded brief tonal sounds described as “chirps” [7,19,67]. Whistle rates were then calculated as number of whistles per min (i.e., duration of the noise event in min) per dolphin (i.e., 20).
The data were analyzed in two ways (Figure 3): (1) the whistle rate (whistles per min per dolphin) during the “onset” of the noise compared to the preceding 5 min and (2) the whistle rate during the entire duration of the noise event compared to a matched control date and time period. To understand if the onset of a noise event had an impact on whistle rate, whistles were counted 5 min before the event started (the pre-onset) and 5 min after the start of the event [56]. To account for the noise event’s full duration, which was highly variable (Table 1), whistle rates for the complete noise event were then compared to a matched recording with no identifiable noise activity (i.e., a control). Control dates started at the same time of day (HH:MM:SS), on the same day of the week, and had the same number of dolphins present during that day’s recording. Control recordings were at least 30 min in duration if an exact duration to the event could not be matched. Out of 38 noise events, only 30 complete events were included in the analysis for the control and event dataset, as a control period with an equal number of dolphins (i.e., 20) could not be matched for eight dates (Table 1). Finally, all statistical analyses were conducted in IBM SPSS v.24.

3. Results

The maximum SPLs received by the hydrophone for the onset of an event ranged from 102.6 to 148.3 dB re 1 µPa, while the maximum received SPL for the complete event ranged from 108.6 to 161.7 dB re 1 µPa (Table 1). The SPL data was normally distributed. Mean (±SE) whistle rates during the onset of the noise event (0.07 ± 0.02) did not significantly differ from the pre-onset period (0.09 ± 0.02; t(74) = 0.613, p = 0.097; Figure 4). Whistle rates were also not significantly different between the full event (0.06 ± 0.01) and matched control periods (0.06 ± 0.01; t(58) = 0.406, p = 0.499; Figure 4). A univariate ANOVA did not find a significant difference in whistle rates between the controls, MF, or HF events for either the pre-onset and onset data (F(2,73) = 0.241, p = 0.787) or the control and full event data (F(2,57) = 0.09, p = 0.914).
A Pearson’s correlation was conducted to assess the relationship between increasing SNR and whistle rate. The SNR data was non-normally distributed. For the onset data, when the pre-onset (SNR values were set to zero) was included, no correlation was found between SNR and whistle rate (r(74) = −0.010, p = 0.933; Figure 5). When pre-onset was removed and only the SNR values of the noise condition were compared to whistle rate, a weak positive trend was found (r(36) = 0.316, p = 0.053). No significant correlation was found between the controls and full event when the control’s SNR (SNR was set to zero) were included (r(58) = −0.019; p = 0.886; Figure 5) nor was a significant correlation found when the SNRs of the controls were removed (r(28) = 0.164; p = 0.385). A Pearson’s correlation was also conducted to assess the SPL of the noise in relation to whistle rate. Similar to the SNR results, the onset data showed a weak positive trend (r(36) = 0.209; p = 0.053) while no significant correlation was found for the full event data (r(28) = 0.019, p = 0.922; Figure 6).

4. Discussion

Dolphins at the MMP did not change their whistle rate when exposed to moderate but variable levels of anthropogenic noise within San Diego Bay. This held true across multiple tests comparing the whistle behavior for the initial 5 min of an event to the preceding 5 min, and the full duration of the event to matched control periods. Interestingly, whistle rates did not change as the SNR (Figure 5) or SPL (Figure 6) of the noise increased. Changes in whistle rate have been an observed and suggested as an anti-masking response in other cetaceans [34,50,53] but were not found in Navy dolphins.
In Houser et al.’s [42] dose-exposure study, the dolphins showed desensitization to SPLs of 160 dB re 1 µPa and below for exposure to a simulated sonar signal (1-s duration, 3250–3450 Hz) during the performance of a trained behavior. The maximum received SPL in the present study was 161.7 dB re 1 µPa, just above the received level below which desensitization was noted, but it is important to note that the dolphins recorded here were not engaged in any trained behaviors or receiving reinforcement during the exposure events. Not only are the dolphins exposed to daily maritime activity in the San Diego Bay, they have been desensitized to other relatively loud noise sources such as the presence of MMP boat engines, recall pingers used for training, and pressure washing from pen cleaning. Additionally, these dolphins have been a part of the MMP for going on 30 years, and habituation to their bay environment would be expected. Dolphins themselves produce whistles at SLs of 138–158 dB re 1 µPa at 1 m [7,12,13,14,15]; therefore, it is understandable that exposure to noise at similar amplitudes did not result in a change in whistle production. Analyzing whistle rate at the initial 5 min of the event was an attempt to identify if the sudden onset of the noise elicited a change in whistle behavior that may have been difficult to detect in a full noise event where desensitization was possible, and a weak positive correlation was found when comparing the increasing SNR and SPL values to the onset whistle rates. This suggests that there was a slight adjustment in vocal output when the noise event began at a higher level. Whistle rates returned to their normal output as the event continued, as we did not find a significant correlation for the complete noise event. As Navy dolphins are housed in a natural sea-water environment within San Diego Bay, their exposure to anthropogenic noise is likely similar to wild counter-parts living in urban waters (e.g., the wild dolphin population that also inhabit the San Diego Bay). These results may apply to other coastal dolphin populations exposed to similar anthropogenic noise levels, but future work should explore this question.
Buckstaff [55] observed a higher whistle rate in a wild group of dolphins at the onset of noisy vessel approaches. Buckstaff’s onset whistle rate was 1.4, which was double the 0.07 found here [55]. Their “during” whistle rate was around 1 whistle per dolphin per min, compared to the 0.06 rate presented here for the full event duration [55]. Buckstaff suggested that the increased whistle rate during vessel onset reflected a motivation for the animals to stick together and promote the reunions of a spread-out pod. The dolphins at the MMP are housed together, where social groupings of dolphins are controlled and managed daily. While the Navy dolphins could not move away from the sound source, as wild animals can, they were safe from vessel approaches and had no potential for ship strikes. Therefore, it could be that the increase in whistles recorded in Buckstaff’s study were related to coordinating avoidance as opposed to a response to the vessel noise exposure itself, and would explain the lower whistle rates recorded for the Navy dolphins who did not need to coordinate. Additionally, changes in whistle rate were suggested to be a proxy for measuring distress levels in dolphins and beluga whales [23,55,57,58]. While the consistent whistle rates presented here suggested that Navy dolphins did not express distress through their vocalizations during these noise exposures, whistle rate is not the only indicator of stress. Therefore, to assess if the Navy dolphins are experiencing distress from these noise events, future studies should include the addition of physiological and behavioral markers such as cortisol levels, swimming patterns within the enclosure, the success of completing trained behaviors, or changes in respiration rate [42]. The noise events recorded for this study were opportunistic and not in the control of the analysts, therefore, it was not possible to observe both physical behavior and vocal behavior during the noise event. A more controlled methodology including playback of the noise events recorded here could make that possible. Future research should further identify the different noise types recorded, in order to test whether different variables such as source type, inter-signal interval and previous exposures to that noise affect acoustic response.

5. Conclusions

The whistle rates of a group of 20 dolphins at the MMP did not change in response to exposure to varying levels of anthropogenic noise in San Diego Bay, CA; the frequency band of the noise, the calculated SNR, and the received SPL of the noise did not seem to affect whistle rate. Acoustic communication for cetaceans is necessary for survival and reproduction; therefore, it is encouraging that whistle communication was not affected by noise exposure between 102.1 and 161.7 dB re 1 µPa. While significant changes in behavior and acoustic output add to our understanding of noise impacts on cetaceans, non-significant results are also useful to inform current policies and regulations. Noise pollution remains a concern for marine mammals. Continuing to update regulations with up-to-date research should continue on local, federal, and international levels.

Author Contributions

Conceptualization, J.J.S. and B.L.J.; Methodology, J.J.S. and B.L.J.; Formal analysis/Investigation, J.J.S. and K.M.H.; Writing—original draft preparation, J.J.S.; Writing—review and editing, K.M.H. and B.L.J.; Supervision, B.L.J.; Funding acquisition, B.L.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Office of Naval Research grant number N00014-21-1-2414.

Institutional Review Board Statement

The U.S. Navy Marine Mammal Program (MMP), Naval Information Warfare Center (NIWC) Pacific, houses and cares for a population of bottlenose dolphins and California sea lions in San Diego Bay (CA, USA). The Secretary of Navy Instruction 3900.41H directs that Navy marine mammals be provided the highest quality of care. NIWC Pacific is accredited by AAALAC International and adheres to the national standards of the U.S. Public Health Service policy on the Humane Care and Use of Laboratory Animals and the Animal Welfare Act. NIWC Pacific’s animal care and use program is routinely reviewed by the Institutional Animal Care and Use Committee (IACUC) and the U.S. Navy Bureau of Medicine and Surgery (BUMED). BUMED agreed with the approval of NIWC Pacific IACUC protocol #143-2021 and assigned NRD #1264 to this study.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are not publicly available. Data may be made available upon the approval of a formal sample request to the MMP. PAMGuard is freely available for download at https://www.pamguard.org/ (accessed on 25 February 2024). The custom WAMS program is currently publicly available through the PAMGuard Plugins download page at https://www.pamguard.org/downloads.php?cat_id=9 (accessed on 25 February 2024).

Acknowledgments

The authors are extremely grateful to the U.S. Navy Marine Mammal Program for allowing us to passively monitor the whistle behavior of the dolphins under their care. Thank you to Jason Mulsow of the National Marine Mammal Foundation (NMMF) for your sound calculations, guidance, advice, and edits to this manuscript. Thank you to Dorian Houser (NMMF) as well for your initial guidance and edits, and thank you to Katelin Lally for your contributions to data analysis in the early days of this study. A special thank you goes to the animal care supervisors Jaime Bratis, Megan Sereyko-Dunn, Amanda Naderer, and Sarah Hammar, and to the animal care staff, without whom the Sound and Health studies would not be possible. This is National Marine Mammal Foundation Contribution #383 to peer-reviewed scientific literature.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Representation of array placement and recording schematics. (A). The WAMS array, made up of 4 HTI-99_HF hydrophones mounted to a PVC frame. (B). Adapted from Jones et al., 2021 [61] Figure 1, an illustrated representation of the placement of the WAMS array between the focal dolphin enclosures, and the shack that houses the sound card and Dell laptop. Open water leading to the San Diego Bay is to the right of the right-most group of enclosures.
Figure 1. Representation of array placement and recording schematics. (A). The WAMS array, made up of 4 HTI-99_HF hydrophones mounted to a PVC frame. (B). Adapted from Jones et al., 2021 [61] Figure 1, an illustrated representation of the placement of the WAMS array between the focal dolphin enclosures, and the shack that houses the sound card and Dell laptop. Open water leading to the San Diego Bay is to the right of the right-most group of enclosures.
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Figure 2. Example of anthropogenic noise event and triggered LabVIEW system. Spectrogram of a HF anthropogenic noise event (above) with the corresponding noise amplitude values (below). User-defined spectrogram parameters were set in Raven 1.6 (Hann window function, 50% overlap, Hop Size 1050, DFT Size 4096, sample size 2100, 10-s time window (x-axis), 64 kHz frequency band (y-axis)). Noise analysis window settings (Hanning window function, 50% overlap, Fs 250000, analysis window size of 0.5 s) included 300 s on the x-axis and amplitude (dB re 1 µPa2/Hz) on the y-axis. Blue closed circles indicate MF levels below the set threshold of 129 dB re 1 µPa2/Hz, while red closed circles indicate MF levels above the threshold. Blue open circles indicate HF levels detected below the set threshold of 113 dB re 1 µPa2/Hz, and red open circles indicate HF levels detected above the threshold. The noise example here represents 10 s of the 300 s noise analysis window and is outlined by the black box.
Figure 2. Example of anthropogenic noise event and triggered LabVIEW system. Spectrogram of a HF anthropogenic noise event (above) with the corresponding noise amplitude values (below). User-defined spectrogram parameters were set in Raven 1.6 (Hann window function, 50% overlap, Hop Size 1050, DFT Size 4096, sample size 2100, 10-s time window (x-axis), 64 kHz frequency band (y-axis)). Noise analysis window settings (Hanning window function, 50% overlap, Fs 250000, analysis window size of 0.5 s) included 300 s on the x-axis and amplitude (dB re 1 µPa2/Hz) on the y-axis. Blue closed circles indicate MF levels below the set threshold of 129 dB re 1 µPa2/Hz, while red closed circles indicate MF levels above the threshold. Blue open circles indicate HF levels detected below the set threshold of 113 dB re 1 µPa2/Hz, and red open circles indicate HF levels detected above the threshold. The noise example here represents 10 s of the 300 s noise analysis window and is outlined by the black box.
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Figure 3. Schematic of data analysis. Spectrogram example of a HF noise event, time in min is represented on the x-axis, frequency is represented on the y-axis (0–40 kHz). The distinction between the pre-onset and onset data groups are outlined in blue, while the distinction between the full noise event is outlined in green. The noise event starts at the 0 min mark. The pre-onset and onset data spans the 5 min preceding the start of the noise event (−5) and the first 5 min of the event (5). The complete noise event begins at the beginning of the noise emission (0) and ends when an hour of no noise is followed by the last noise emission (n).
Figure 3. Schematic of data analysis. Spectrogram example of a HF noise event, time in min is represented on the x-axis, frequency is represented on the y-axis (0–40 kHz). The distinction between the pre-onset and onset data groups are outlined in blue, while the distinction between the full noise event is outlined in green. The noise event starts at the 0 min mark. The pre-onset and onset data spans the 5 min preceding the start of the noise event (−5) and the first 5 min of the event (5). The complete noise event begins at the beginning of the noise emission (0) and ends when an hour of no noise is followed by the last noise emission (n).
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Figure 4. Whistle rates per event. Whistle rate (whistles per min per dolphin) for the pre-onset and onset dataset (blue bars, left of the grey vertical line) and for the control and event dataset (green bars, right of the grey vertical line). Error bars represent 2 standard errors (SE).
Figure 4. Whistle rates per event. Whistle rate (whistles per min per dolphin) for the pre-onset and onset dataset (blue bars, left of the grey vertical line) and for the control and event dataset (green bars, right of the grey vertical line). Error bars represent 2 standard errors (SE).
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Figure 5. SNR to whistle rate correlation. Whistle rate (whistles per min per dolphin) (y-axis) compared to the signal-to-noise ratio (SNR) of the noise (x-axis). With pre-onset SNR values set to zero (dark blue dots), no significant linear relationship was found for the onset (light blue dots) data when compared to whistle rate. With the control SNR set to zero (dark green dots), no significant correlation was found for the for the full event data (light green dots) when compared to the whistle rate.
Figure 5. SNR to whistle rate correlation. Whistle rate (whistles per min per dolphin) (y-axis) compared to the signal-to-noise ratio (SNR) of the noise (x-axis). With pre-onset SNR values set to zero (dark blue dots), no significant linear relationship was found for the onset (light blue dots) data when compared to whistle rate. With the control SNR set to zero (dark green dots), no significant correlation was found for the for the full event data (light green dots) when compared to the whistle rate.
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Figure 6. SPL to whistle rate correlation. Whistle rate (whistles per min per dolphin) (y-axis) compared to the noise SPL (x-axis). No significant linear relationship was found for the onset (light blue dots) data when compared to whistle rate, and no significant correlation was found for the for the full event data (light green dots) when compared to the whistle rate.
Figure 6. SPL to whistle rate correlation. Whistle rate (whistles per min per dolphin) (y-axis) compared to the noise SPL (x-axis). No significant linear relationship was found for the onset (light blue dots) data when compared to whistle rate, and no significant correlation was found for the for the full event data (light green dots) when compared to the whistle rate.
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Table 1. Summary of data. Summary of number of noise event dates, noise event type broken down by mid-frequency (MF; 1–20 kHz) and high-frequency (HF; 20–80 kHz), and average noise event duration in minutes are presented here. Average duration is reported in minutes ± 1 standard deviation (SD). Also presented is the range of SPLs (dB re 1 µPa) received during the noise events.
Table 1. Summary of data. Summary of number of noise event dates, noise event type broken down by mid-frequency (MF; 1–20 kHz) and high-frequency (HF; 20–80 kHz), and average noise event duration in minutes are presented here. Average duration is reported in minutes ± 1 standard deviation (SD). Also presented is the range of SPLs (dB re 1 µPa) received during the noise events.
Data TypeN EventsMF EventsHF EventsAvg. Duration (min ± 1 SD)SPL Range (dB re 1 µPa)
Pre-Onset38--5-
Onset386324.90 ± 0.49102.6–148.3
Control30--54.67 ± 68.43-
Event3042673.31 ± 95.10108.6–161.7
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Sportelli, J.J.; Heimann, K.M.; Jones, B.L. Moderate Anthropogenic Noise Exposure Does Not Affect Navy Bottlenose Dolphin (Tursiops truncatus) Whistle Rates. J. Mar. Sci. Eng. 2024, 12, 441. https://doi.org/10.3390/jmse12030441

AMA Style

Sportelli JJ, Heimann KM, Jones BL. Moderate Anthropogenic Noise Exposure Does Not Affect Navy Bottlenose Dolphin (Tursiops truncatus) Whistle Rates. Journal of Marine Science and Engineering. 2024; 12(3):441. https://doi.org/10.3390/jmse12030441

Chicago/Turabian Style

Sportelli, Jessica J., Kelly M. Heimann, and Brittany L. Jones. 2024. "Moderate Anthropogenic Noise Exposure Does Not Affect Navy Bottlenose Dolphin (Tursiops truncatus) Whistle Rates" Journal of Marine Science and Engineering 12, no. 3: 441. https://doi.org/10.3390/jmse12030441

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