Separating underwater ambient noise from flow noise recorded on stereo acoustic tags attached to marine mammals

A.M.v.B.B. and P.B. were funded by The Netherlands Ministry of Defence. Fieldwork efforts and support for P.M. and F.S. was provided by the US Office of Naval Research [award numbers N00014-08-1-0984 and N00014-10-1-0355]. P.W. received a PhD studentship with matched funding from The Netherlands Ministry of Defence (administered by The Netherlands Organisation for Applied Scientific Research, TNO) and UK Natural Environment Research Council [NE/J500276/1].Sound-recording acoustic tags attached to marine animals are commonly used in behavioural studies. Measuring ambient noise is of interest to efforts to understand responses of marine mammals to anthropogenic underwater sound, or to assess their communication space. Noise of water flowing around the tag reflects the speed of the animal, but hinders ambient noise measurement. Here, we describe a correlation-based method for stereo acoustic tags to separate the relative contributions of flow and ambient noise. The uncorrelated part of the noise measured in digital acoustic recording tag (DTAG) recordings related well to swim speed of a humpback whale (Megaptera novaeangliae), thus providing a robust measure of flow noise over a wide frequency bandwidth. By removing measurements affected by flow noise, consistent ambient noise estimates were made for two killer whales (Orcinus orca) with DTAGs attached simultaneously. The method is applicable to any multi-channel acoustic tag, enabling application to a wide range of marine species.Publisher PDFPeer reviewe

Sperm whales reduce foraging effort during exposure to 1-2 kH z sonar and killer whale sounds

We would like to thank 3S partners and funders especially for enabling this research (NL Ministry of Defence, NOR Ministry of Defence, US Office of Naval Research, and World Wildlife Fund, Norway). PLT was supported by the Scottish Funding Council (grant HR09011) through the Marine Alliance for Science and Technology for Scotland.The time and energetic costs of behavioral responses to incidental and experimental sonar exposures, as well as control stimuli, were quantified using hidden state analysis of time series of acoustic and movement data recorded by tags (DTAG) attached to 12 sperm whales (Physeter macrocephalus) using suction cups. Behavioral state transition modeling showed that tagged whales switched to a non-foraging, non-resting state during both experimental transmissions of low-frequency active sonar from an approaching vessel (LFAS; 1-2 kH z, source level 214 dB re 1 μPa m, four tag records) and playbacks of potential predator (killer whale, Orcinus orca) sounds broadcast at naturally occurring sound levels as a positive control from a drifting boat (five tag records). Time spent in foraging states and the probability of prey capture attempts were reduced during these two types of exposures with little change in overall locomotion activity, suggesting an effect on energy intake with no immediate compensation. Whales switched to the active non-foraging state over received sound pressure levels of 131-165 dB re 1 μPa during LFAS exposure. In contrast, no changes in foraging behavior were detected in response to experimental negative controls (no-sonar ship approach or noise control playback) or to experimental medium-frequency active sonar exposures (MFAS; 6-7 kH z, source level 199 re 1 μPa m, received sound pressure level [SPL] = 73-158 dB re 1 μPa). Similarly, there was no reduction in foraging effort for three whales exposed to incidental, unidentified 4.7-5.1 kH z sonar signals received at lower levels (SPL = 89-133 dB re 1 μPa). These results demonstrate that similar to predation risk, exposure to sonar can affect functional behaviors, and indicate that increased perception of risk with higher source level or lower frequency may modulate how sperm whales respond to anthropogenic sound.Publisher PDFPeer reviewe

Underwater equal-latency contours of a harbor porpoise (Phocoena phocoena) for tonal signals between 0.5 and 125 kHz

Loudness perception can be studied based on the assumption that sounds of equal loudness elicit equal reaction time (RT; or “response latency”). We measured the underwater RTs of a harbor porpoise to narrowband frequency-modulated sounds and constructed six equal-latency contours. The contours paralleled the audiogram at low sensation levels (high RTs). At high-sensation levels, contours flattened between 0.5 and 31.5 kHz but dropped substantially (RTs shortened) beyond those frequencies. This study suggests that equal-latency-based frequency weighting can emulate noise perception in porpoises for low and middle frequencies but that the RT-loudness correlation is relatively weak for very high frequencies

Near-threshold equal-loudness contours for harbor seals (Phoca vitulina) derived from reaction times during underwater audiometry:a preliminary study

Equal-loudness functions describe relationships between the frequencies of sounds and their perceived loudness. This pilot study investigated the possibility of deriving equal-loudness contours based on the assumption that sounds of equal perceived loudness elicit equal reaction times (RTs). During a psychoacoustic underwater hearing study, the responses of two young female harbor seals to tonal signals between 0.125 and 100 kHz were filmed. Frame-by-frame analysis was used to quantify RT (the time between the onset of the sound stimulus and the onset of movement of the seal away from the listening station). Near-threshold equal-latency contours, as surrogates for equal-loudness contours, were estimated from RT-level functions fitted to mean RT data. The closer the received sound pressure level was to the 50 detection hearing threshold, the more slowly the animals reacted to the signal (RT range: 188-982 ms). Equal-latency contours were calculated relative to the RTs shown by each seal at sound levels of 0, 10, and 20 dB above the detection threshold at 1 kHz. Fifty percent detection thresholds are obtained with well-trained subjects actively listening for faint familiar sounds. When calculating audibility ranges of sounds for harbor seals in nature, it may be appropriate to consider levels 20 dB above this threshold. © 2011 Acoustical Society of America

Effect of broadband-noise masking on the behavioral response of a harbor porpoise (Phocoena phocoena) to 1-s duration 6-7 kHz sonar up-sweeps

Naval sonar systems produce signals which may affect the behavior of harbor porpoises, though their effect may be reduced by ambient noise. To show how natural ambient noise influences the effect of sonar sweeps on porpoises, a porpoise in a pool was exposed to 1-s duration up-sweeps, similar in frequency range (6-7 kHz) to those of existing naval sonar systems. The sweep signals had randomly generated sweep intervals of 3-7 s (duty cycle: 19). Behavioral parameters during exposure to signals were compared to those during baseline periods. The sessions were conducted under five background noise conditions: the local normal ambient noise and four conditions mimicking the spectra for wind-generated noise at Sea States 2-8. In all conditions, the sweeps caused the porpoise to swim further away from the transducer, surface more often, swim faster, and breathe more forcefully than during the baseline periods. However, the higher the background noise level, the smaller the effects of the sweeps on the surfacing behavior of the porpoise. Therefore, the effects of naval sonar systems on harbor porpoises are determined not only by the received level of the signals and the hearing sensitivity of the animals but also by the background noise. © 2011 Acoustical Society of America

Foraging behavior and disruption in blue, fin, and humpback whales in relation to sonar exposure:the challenges of generalizing responsiveness in species with high individual variability

Behavioral response studies (BRSs) are an important approach for quantifying responses of marine mammals to naval sonar exposure. Controlled exposure experiments (CEEs) are BRSs based on a formal experimental design. Impact assessment often requires prediction of the likelihood that an individual of any species present in an area will respond to a given dose of sonar. Empirical data exist for only a few species, and species are often grouped to enable estimation of responsiveness for unstudied species. In this study, data for three taxonomically close species were combined to quantitatively determine whether they could be grouped in terms of responsiveness. We focused on foraging responses of blue (Balaenoptera musculus), fin (Balaenoptera physalus), and humpback (Megaptera novaeangliae) whales in relation to sonar exposure. These species are lunge filter feeders, and the number of lunges within each phase of a CEE was used to test for changes in foraging in response to exposure. Humpback whales, which were exposed to the highest sound levels, were found to be more responsive during and after sonar exposure when compared with blue and fin whales. The lunge rates of blue and fin whales throughout the sonar exposures remained similar to baseline and no sonar control levels. The greatest challenge is the ability to generalize responsiveness in species for which responses at the individual level are probably rare and subtle. Moreover, the interpretation of these species’ similarities and individual differences in responsiveness is problematic given the contextual differences between each CEE. As in previous studies, our results reinforce the need for BRSs to incorporate environmental data collection that is relevant to the behavioral state of study animals

Separating underwater ambient noise from flow noise recorded on stereo acoustic tags attached to marine mammals

Sound-recording acoustic tags attached to marine animals are commonly used in behavioural studies. Measuring ambient noise is of interest to efforts to understand responses of marine mammals to anthropogenic underwater sound, or to assess their communication space. Noise of water flowing around the tag reflects the speed of the animal, but hinders ambient noise measurement. Here, we describe a correlation-based method for stereo acoustic tags to separate the relative contributions of flow and ambient noise. The uncorrelated part of the noise measured in digital acoustic recording tag (DTAG) recordings related well to swim speed of a humpback whale (Megaptera novaeangliae), thus providing a robust measure of flow noise over a wide frequency bandwidth. By removing measurements affected by flow noise, consistent ambient noise estimates were made for two killer whales (Orcinus orca) with DTAGs attached simultaneously. The method is applicable to any multi-channel acoustic tag, enabling application to a wide range of marine species.</p

Data from: Sperm whales reduce foraging effort during exposure to 1-2 kHz sonar and killer whale sounds

The time and energetic costs of behavioral responses to incidental and experimental sonar exposures, as well as control stimuli, were quantified using hidden state analysis of time series of acoustic and movement data recorded by tags (DTAG) attached to 12 sperm whales (Physeter macrocephalus) using suction cups. Behavioral state transition modeling showed that tagged whales switched to a non-foraging, non-resting state during both experimental transmissions of low-frequency active sonar from an approaching vessel (LFAS; 1–2 kHz, source level 214 dB re 1 μPa m, four tag records) and playbacks of potential predator (killer whale, Orcinus orca) sounds broadcast at naturally occurring sound levels as a positive control from a drifting boat (five tag records). Time spent in foraging states and the probability of prey capture attempts were reduced during these two types of exposures with little change in overall locomotion activity, suggesting an effect on energy intake with no immediate compensation. Whales switched to the active non-foraging state over received sound pressure levels of 131–165 dB re 1 μPa during LFAS exposure. In contrast, no changes in foraging behavior were detected in response to experimental negative controls (no-sonar ship approach or noise control playback) or to experimental medium-frequency active sonar exposures (MFAS; 6–7 kHz, source level 199 re 1 μPa m, received sound pressure level [SPL] = 73–158 dB re 1 μPa). Similarly, there was no reduction in foraging effort for three whales exposed to incidental, unidentified 4.7–5.1 kHz sonar signals received at lower levels (SPL = 89–133 dB re 1 μPa). These results demonstrate that similar to predation risk, exposure to sonar can affect functional behaviors, and indicate that increased perception of risk with higher source level or lower frequency may modulate how sperm whales respond to anthropogenic sound

Assessing the effectiveness of ramp-up during sonar operations using exposure models

Ramp-up procedures are used to mitigate the impact of sound on marine mammals. Sound exposure models combined with observations of marine mammals responding to sound can be used to assess the effectiveness of ramp-up procedures. We found that ramp-up procedures before full-level sonar operations can reduce the risk of hearing threshold shifts with marine mammals, but their effectiveness depends strongly on the responsiveness of the animals. In this paper, we investigated the effect of sonar parameters (source level, pulse-repetition time, ship speed) on sound exposure by using a simple analytical model and highlight the mechanisms that limit the effectiveness of ramp-up procedures.</p