The underwater soundscape of the North Sea.

As awareness on the impact of anthropogenic underwater noise on marine life grows, underwater noise measurement programs are needed to determine the current status of marine areas and monitor long-term trends. The Joint Monitoring Programme for Ambient Noise in the North Sea (JOMOPANS) collaborative project was funded by the EU Interreg to collect a unique dataset of underwater noise levels at 19 sites across the North Sea, spanning many different countries and covering the period from 2019 to 2020. The ambient noise from this dataset has been characterised and compared - setting a benchmark for future measurements in the North Sea area. By identifying clusters with similar sound characteristics in three broadband frequency bands (25-160 Hz, 0.2-1.6 kHz, and 2-10 kHz), geographical areas that are similarly affected by sound have been identified. The measured underwater sound levels show a persistent and spatially uniform correlation with wind speed at high frequencies (above 1 kHz) and a correlation with the distance from ships at mid and high frequencies (between 40 Hz and 4 kHz). Correlation with ocean current velocity at low frequencies (up to 200 Hz), which are susceptible to nonacoustic contamination by flow noise, was also evaluated. These correlations were evaluated and simplified linear scaling laws for wind and current speeds were derived. The presented dataset provides a baseline for underwater noise measurements in the North Sea and shows that spatial variability of the dominant sound sources must be considered to predict the impact of noise reduction measures.

Basin-wide contributions to the underwater soundscape by multiple seismic surveys with implications for marine mammals in Baffin Bay, Greenland.

Seismic surveys increasingly operate in deeper Arctic waters with propagation conditions and marine mammal fauna different from the better-studied temperate, or shallow-water, regions. Using 31 calibrated sound recorders, we quantified noise contributions from four concurrent seismic surveys in Baffin Bay, Greenland, to estimate their potential impacts on marine mammals. The impact was cumulative as the noise level rose in response to the onset of each survey: on a minute-by-minute scale the sound-exposure-levels varied by up to 70 dB (20 dB on average), depending on range to the seismic vessel, local bathymetry effects and interference patterns, representing a significant change in the auditory scene for marine mammals. Airgun pulse energy did not decrease to ambient before arrival of the next pulse leaving very little low-frequency masking-free time. Overall, the measured values matched well with pre-season-modeling, emphasizing the importance of noise-modeling in impact assessments, if responses of focal marine mammals are known.

Echolocation in sympatric Peale's dolphins (Lagenorhynchus australis) and Commerson's dolphins (Cephalorhynchus commersonii) producing narrow-band high-frequency clicks.

An increasing number of smaller odontocetes have recently been shown to produce stereotyped narrow-band high-frequency (NBHF) echolocation clicks. Click source parameters of NBHF clicks are very similar, and it is unclear whether the sonars of individual NBHF species are adapted to specific habitats or the presence of other NBHF species. Here, we test whether sympatric NBHF species sharing the same habitat show similar adaptations in their echolocation clicks and whether their clicks display signs of character displacement. Wide-band sound recordings were obtained with a six-element hydrophone array from wild Peale's (Lagenorhynchus australis) and Commerson's (Cephalorhynchus commersonii) dolphins off the Falkland Islands. The centroid frequency was different between Commerson's (133+/-2 kHz) and Peale's (129+/-3 kHz) dolphins. The r.m.s. bandwidth was 12+/-3 kHz for both species. The source level was higher for Peale's dolphin (185+/-6 dB re 1 muPa p.-p.) than for Commerson's (177+/-5 dB re 1 muPa p.-p.). The mean directivity indexes were 25 dB for both species. The relatively low source levels in combination with the high directivity index may be an adaptation to reduce clutter when foraging in a coastal environment. We conclude that the small species-specific shifts in distribution of centroid frequencies around 130 kHz may reflect character displacement in otherwise-stereotyped NBHF clicks.

Feeding at a high pitch: source parameters of narrow band, high-frequency clicks from echolocating off-shore hourglass dolphins and coastal Hector's dolphins.

Toothed whales depend on echolocation for orientation and prey localization, and source parameters of echolocation clicks from free-ranging animals therefore convey valuable information about the acoustic physiology and behavioral ecology of the recorded species. Recordings of wild hourglass (Lagenorhynchus cruciger) and Hector's dolphins (Cephalorhynchus hectori) were made in the Drake Passage (between Tierra del Fuego and the Antarctic Peninsular) and Banks Peninsular (Akaroa Harbour, New Zealand) with a four element hydrophone array. Analysis of source parameters shows that both species produce narrow band high-frequency (NBHF) echolocation clicks. Coastal Hector's dolphins produce clicks with a mean peak frequency of 129 kHz, 3 dB bandwidth of 20 kHz, 57 micros, 10 dB duration, and mean apparent source level (ASL) of 177 dB re 1 microPa(p.-p.). The oceanic hourglass dolphins produce clicks with mean peak frequency of 126 kHz, 3 dB bandwidth of 8 kHz, 116 micros, 10 dB duration, and a mean estimated ASL of 197 dB re 1 microPa(p.-p.). Thus, hourglass dolphins apparently produce clicks of higher source level, which should allow them to detect prey at more than twice the distance compared to Hector's dolphins. The observed source parameter differences within these two NBHF species may be an adaptation to a coastal cluttered environment versus a deep water, pelagic habitat.

Stochastic resonance and signal detection in an energy detector--implications for biological receptor systems.

Stochastic resonance is demonstrated in a simple energy detector model, as a non-monotonic relationship between signal-to-noise ratio and detection of a sinusoid signal in bandpass-limited Gaussian noise. The behaviour of the model detecting signals of various intensities and signal-to-noise ratios was investigated. Significant improvement in detection was obtained by adding noise for mean signal intensities below the detection criterion of the detector. The range of usable noise levels, however, may be too small to be biologically meaningful. It is demonstrated that improving detection in the analysed model by adding noise to an otherwise undetectable signal is only at best as efficient as what can be obtained by adjusting the criterion to the signal-to-noise ratio. Improving detection by means of stochastic resonance is thus a sub-optimal strategy. It is speculated whether a demonstration of stochastic resonance in a biological system indicates any adaptive significance. More than anything, it indicates the presence of a mismatch between receptor sensitivity and the signal-to-noise ratio of the experiment, not the cause of this mismatch.

Arctiid moths and bat echolocation: broad-band clicks interfere with neural responses to auditory stimuli in the nuclei of the lateral lemniscus of the big brown bat.

Clicks emitted by arctiid moths interfere with the ranging ability of echolocating bats. To identify possible neural correlates of this interference, we recorded responses of single units in the nuclei of the lateral lemniscus to combinations of a broad-band click and a test signal (pure tones or frequency-modulated sweeps). In 77% of 87 units tested, clicks interfered with neural responses to the test stimuli. The interference fell into two categories: latency ambiguity and suppression. Units showing latency ambiguity responded to both the click and the test signal. However, when the click occurred within a window of approximately 3 ms before the onset of the test signal, the latency of the response to the test signal was affected. Units that were suppressed did not respond to clicks. Nevertheless, when a click was presented immediately before or simultaneously with a test signal, the response to the test signal was eliminated. Both types of units were found throughout the lateral lemniscus except for the columnar division of the ventral nucleus, where all units tested exhibited latency ambiguity. There is a close match between the single unit data and previous studies of range difference discrimination in the presence of clicks.