Time and tide: Seasonal, diel and tidal rhythms in Wadden Sea Harbour porpoises (Phocoena phocoena)

Odontocetes have evolved a rich diversity of prey- and habitat-specific foraging strategies, which allows them to feed opportunistically on locally and temporally abundant prey. While habitat-specific foraging strategies have been documented for some odontocete species, this is less known for the harbour porpoise (Phocoena phocoena). We collected multiple years of acoustic data using echolocation click loggers to analyse porpoise occurrence and buzzing behaviour, indicating feeding, in the German Wadden Sea (North Sea). Seasonal, diel and tidal effects were studied using Generalised Estimating Equations (GEE-GAMs). Locally season, time of day and tidal time significantly influenced the probability of porpoise detections and detection of foraging sequences (buzzes). Hunting strategies, and therefore frequency of buzzes, were likely affected by prey distribution and large differences between POD locations indicated that porpoises used highly specific behaviour adapted to tide and time of day to efficiently feed on the available prey. Strong seasonal and spatial variation in diel and tidal effects underline the importance of long-term observations. Studies on porpoise behaviour are often based on short-term observations and might rather reflect a seasonal than a general pattern. The results of this study show clearly that significant changes in porpoise behaviour can be found in short and long-term observations. Here some features are based on short term determinants and others are stable over years and care should be taken about drawing general conclusions based on local patterns. Highly variable spatio-temporal patterns indicate a high flexibility of porpoises in a highly variable environment and address a challenge for complex conservation management plans.

Temporary hearing threshold shift in a harbor porpoise (Phocoena phocoena) after exposure to multiple airgun sounds

In seismic surveys, reflected sounds from airguns are used under water to detect gas and oil below the sea floor. The airguns produce broadband high-amplitude impulsive sounds, which may cause temporary or permanent threshold shifts (TTS or PTS) in cetaceans. The magnitude of the threshold shifts and the hearing frequencies at which they occur depend on factors such as the received cumulative sound exposure level (SELcum), the number of exposures, and the frequency content of the sounds. To quantify TTS caused by airgun exposure and the subsequent hearing recovery, the hearing of a harbor porpoise was tested by means of a psychophysical technique. TTS was observed after exposure to 10 and 20 consecutive shots fired from two airguns simultaneously (SELcum: 188 and 191 dB re 1 μPa²s) with mean shot intervals of around 17 s. Although most of the airgun sounds' energy was below 1 kHz, statistically significant initial TTS_1-4 (1-4 min after sound exposure stopped) of ∼4.4 dB occurred only at the hearing frequency 4 kHz, and not at lower hearing frequencies tested (0.5, 1, and 2 kHz). Recovery occurred within 12 min post-exposure. The study indicates that frequency-weighted SELcum is a good predictor for the low levels of TTS observed.

Effects of exposure to sonar playback sounds (3.5 - 4.1 kHz) on harbor porpoise (Phocoena phocoena) hearing

Safety criteria for naval sonar sounds are needed to protect harbor porpoise hearing. Two porpoises were exposed to sequences of AN/SQS-53C sonar playback sounds (3.5-4.1 kHz, without significant harmonics), at a mean received sound pressure level of 142 dB re 1 μPa, with a duty cycle of 96% (almost continuous). Behavioral hearing thresholds at 4 and 5.7 kHz were determined before and after exposure to the fatiguing sound, in order to quantify temporary threshold shifts (TTSs) and hearing recovery. Control sessions were also conducted. Significant mean initial TTS_1-4 of 5.2 dB at 4 kHz and 3.1 dB at 5.7 kHz occurred after 30 min exposures (mean received cumulative sound exposure level, SEL_cum: 175 dB re 1 μPa²s). Hearing thresholds returned to pre-exposure levels within 12 min. Significant mean initial TTS_1-4 of 5.5 dB at 4 kHz occurred after 60 min exposures (SEL_cum: 178 dB re 1 μPa²s). Hearing recovered within 60 min. The SEL_cum for AN/SQS-53C sonar sounds required to induce 6 dB of TTS 4 min after exposure (the definition of TTS onset) is expected to be between 175 and 180 dB re 1 μPa²s.

Hearing thresholds of a male and a female harbor porpoise (Phocoena phocoena)

To study intra-species variability in audiograms, the hearing sensitivity of a six-year-old female and a three-year-old male harbor porpoise was measured by using a standard psycho-acoustic technique under low ambient noise conditions. The porpoises' hearing thresholds for 13 narrow-band sweeps with center frequencies between 0.125 and 150 kHz were established. The resulting audiograms were U-shaped and similar. The main difference (25 dB) in mean thresholds between the two porpoises was at the high-frequency end of the hearing range (at 150 kHz). Maximum sensitivity (47 dB re 1 μPa for the female and 44 dB re 1 μPa for the male) occurred at 125 kHz. The range of most sensitive hearing (defined as within 10 dB of maximum sensitivity) was from 16 to ∼140 kHz. Sensitivity declined sharply above 125 kHz. All five porpoises for which a valid behavioral audiogram now exists were rehabilitated stranded animals, all were tested with similar psycho-acoustic techniques, and all had similar audiograms. The present study provides further evidence to confirm that the hearing range and sensitivity of the first three harbor porpoises, which have been used in secondary research and on which policy decisions have been based, are representative of those of young harbor porpoises in general.

Year-round spatiotemporal distribution of harbour porpoises within and around the Maryland wind energy area

Offshore windfarms provide renewable energy, but activities during the construction phase can affect marine mammals. To understand how the construction of an offshore windfarm in the Maryland Wind Energy Area (WEA) off Maryland, USA, might impact harbour porpoises (Phocoena phocoena), it is essential to determine their poorly understood year-round distribution. Although habitat-based models can help predict the occurrence of species in areas with limited or no sampling, they require validation to determine the accuracy of the predictions. Incorporating more than 18 months of harbour porpoise detection data from passive acoustic monitoring, generalized auto-regressive moving average and generalized additive models were used to investigate harbour porpoise occurrence within and around the Maryland WEA in relation to temporal and environmental variables. Acoustic detection metrics were compared to habitat-based density estimates derived from aerial and boat-based sightings to validate the model predictions. Harbour porpoises occurred significantly more frequently during January to May, and foraged significantly more often in the evenings to early mornings at sites within and outside the Maryland WEA. Harbour porpoise occurrence peaked at sea surface temperatures of 5°C and chlorophyll a concentrations of 4.5 to 7.4 mg m^-3. The acoustic detections were significantly correlated with the predicted densities, except at the most inshore site. This study provides insight into previously unknown fine-scale spatial and temporal patterns in distribution of harbour porpoises offshore of Maryland. The results can be used to help inform future monitoring and mitigate the impacts of windfarm construction and other human activities.

Pile driving playback sounds and temporary threshold shift in harbor porpoises (Phocoena phocoena): Effect of exposure duration

High intensity underwater sounds may cause temporary hearing threshold shifts (TTSs) in harbor porpoises, the magnitude of which may depend on the exposure duration. After exposure to playbacks of pile driving sounds, TTSs in two porpoises were quantified at 4 and 8 kHz with a psychophysical technique. At 8 kHz, the pile driving sounds caused the highest TTS. Pile driving sounds had the following: pulse duration 124 ms, rate 2760 strikes/h, inter-pulse interval 1.3 s, duty cycle ∼9.5%, average received single-strike unweighted broadband sound exposure level (SELss) 145 dB re 1 μPa(2)s, exposure duration range 15-360 min (cumulative SEL range: 173-187 dB re 1 μPa(2)s). Control sessions were also carried out. Mean TTS (1-4 min after sound exposure stopped in one porpoise, and 12-16 min in the other animal) increased from 0 dB after 15 min exposure to 5 dB after 360 min exposure. Recovery occurred within 60 min post-exposure. For the signal duration, sound pressure level (SPL), and duty cycle used, the TTS onset SELcum is estimated to be around 175 dB re 1 μPa(2)s. The small increase in TTS between 15 and 360 min exposures is due to the small amount of sound energy per unit of time to which the porpoises were exposed [average (over time) broadband SPL ∼144 dB re 1 μPa].

Hearing frequency thresholds of harbor porpoises (Phocoena phocoena) temporarily affected by played back offshore pile driving sounds

Harbor porpoises may suffer hearing loss when exposed to intense sounds. After exposure to playbacks of broadband pile driving sounds for 60 min, the temporary hearing threshold shift (TTS) of a porpoise was quantified at 0.5, 1, 2, 4, 8, 16, 32, 63, and 125 kHz with a psychoacoustic technique. Details of the pile driving sounds were as follows: pulse duration 124 ms, rate 2760 strikes/h, inter-pulse interval 1.3 s, average received single strike unweighted sound exposure level (SEL) 146 dB re 1 μPa(2) s (cumulative SEL: 180 dB re 1 μPa(2) s). Statistically significant TTS only occurred at 4 and 8 kHz; mean TTS (1-4 min. after sound exposure stopped) was 2.3 dB at 4 kHz, and 3.6 dB at 8 kHz; recovery occurred within 48 min. This study shows that exposure to multiple impulsive sounds with most of their energy in the low frequencies can cause reduced hearing at higher frequencies in harbor porpoises. The porpoise's hearing threshold for the frequency in the range of its echolocation signals was not affected by the pile driving playback sounds.

High frequency components of ship noise in shallow water with a discussion of implications for harbor porpoises (Phocoena phocoena)

Growing ship traffic worldwide has led to increased vessel noise with possible negative impacts on marine life. Most research has focused on low frequency components of ship noise, but for high-frequency specialists, such as the harbor porpoise (Phocoena phocoena), medium-to-high frequency noise components are likely more of a concern. To test for biologically relevant levels of medium-to-high frequency vessel noise, different types of Automatic Identification System located vessels were recorded using a broadband recording system in four heavily ship-trafficked marine habitats in Denmark. Vessel noise from a range of different ship types substantially elevated ambient noise levels across the entire recording band from 0.025 to 160 kHz at ranges between 60 and 1000 m. These ship noise levels are estimated to cause hearing range reduction of >20 dB (at 1 and 10 kHz) from ships passing at distances of 1190 m and >30 dB reduction (at 125 kHz) from ships at distances of 490 m or less. It is concluded that a diverse range of vessels produce substantial noise at high frequencies, where toothed whale hearing is most sensitive, and that vessel noise should be considered over a broad frequency range, when assessing noise effects on porpoises and other small toothed whales.

Frequency of greatest temporary hearing threshold shift in harbor porpoises (Phocoena phocoena) depends on the noise level

Harbor porpoises may suffer hearing loss when they are exposed to high level sounds. After exposure for 60 min to a 6.5 kHz continuous tone at average received sound pressure levels (SPLav.re.) ranging from 118 to 154 dB re 1μPa, the temporary hearing threshold shifts (TTSs) of a harbor porpoise were quantified at the center frequency (6.5 kHz), at 0.5, 1.0, and 1.3 octaves above the center frequency (9.2, 13.0, and 16.0 kHz), and at a frequency assumed to be ecologically important for harbor porpoises (125 kHz, the center frequency of their echolocation signals) by means of a psychoacoustic technique. The hearing frequency at which the maximum TTS occurred depended on the SPLav.re. The higher the SPLav.re., the higher the TTS induced at frequencies higher than the exposure frequency; below 148 dB re 1 μPa, the maximum TTS was at 6.5 kHz, whereas above 148 dB re 1 μPa, the maximum TTS was at 9.2 kHz. The hearing threshold of the harbor porpoise for the center frequency of its echolocation signals (125 kHz) was not affected at the highest SPLav.re. to which the animal was exposed.

Effect of level, duration, and inter-pulse interval of 1-2 kHz sonar signal exposures on harbor porpoise hearing

Safety criteria for underwater low-frequency active sonar sounds produced during naval exercises are needed to protect harbor porpoise hearing. As a first step toward defining criteria, a porpoise was exposed to sequences consisting of series of 1-s, 1-2 kHz sonar down-sweeps without harmonics (as fatiguing noise) at various combinations of average received sound pressure levels (SPLs; 144-179 dB re 1 μPa), exposure durations (1.9-240 min), and duty cycles (5%-100%). Hearing thresholds were determined for a narrow-band frequency-swept sine wave centered at 1.5 kHz before exposure to the fatiguing noise, and at 1-4, 4-8, 8-12, 48, 96, 144, and 1400 min after exposure, to quantify temporary threshold shifts (TTSs) and recovery of hearing. Results show that the inter-pulse interval of the fatiguing noise is an important parameter in determining the magnitude of noise-induced TTS. For the reported range of exposure combinations (duration and SPL), the energy of the exposure (i.e., cumulative sound exposure level; SELcum) can be used to predict the induced TTS, if the inter-pulse interval is known. Exposures with equal SELcum but with different inter-pulse intervals do not result in the same induced TTS.

Determining the detection thresholds for harbor porpoise clicks of autonomous data loggers, the Timing Porpoise Detectors

Timing Porpoise Detectors (T-PODs, Chelonia Ltd.) are autonomous passive acoustic devices for monitoring odontocetes. They register the time of occurrence and duration of high frequency pulsed sounds as possible odontocetes echolocation clicks. Because of evolution, five T-POD versions exist. Although the manufacturer replaced those by a digital successor, the C-POD, T-PODs are still used, and data from many field studies exist. Characterizing the acoustic properties of T-PODs enables the interpretation of data obtained with different devices. Here, the detection thresholds of different T-POD versions for harbor porpoise clicks were determined. While thresholds among devices were quite variable in the first T-POD generations, they became more standardized in newer versions. Furthermore, the influence of user-controlled settings on the threshold was investigated. From version 3 on, the detection threshold was found to be easily adjustable with version-dependent setting options "minimum intensity" and "sensitivity," enabling the presetting of standard thresholds. In version 4, the setting "click bandwidth" had a strong influence on the detection threshold, while "selectivity" in version 3 and "noise adaptation = ON" or "OFF" in version 4 hardly influenced thresholds obtained in the tank tests. Nevertheless, the latter setting may influence thresholds in a complex acoustic environment like the sea.

Hearing frequency thresholds of a harbor porpoise (Phocoena phocoena) temporarily affected by a continuous 1.5 kHz tone

Harbor porpoises may suffer hearing loss when exposed to intense sounds. After exposure to a 1.5 kHz continuous tone without harmonics at a mean received sound pressure level of 154 dB re 1 μPa for 60 min (cumulative sound exposure level: 190 dB re 1 μPa(2) s), the temporary hearing threshold shift (TTS) of a porpoise was quantified at 1.5, 2, 4, 6.5, 8, 16, 32, 63, and 125 kHz with a psychoacoustic technique. Significant TTS only occurred at 1.5 and 2 kHz. Mean TTS (1-4 min after sound exposure stopped) was ~14 dB at 1.5 kHz and ~11 dB at 2 kHz, and recovery occurred within 96 min. Control hearing tests before and after a 60 min low ambient noise exposure showed that normal variation in TTS was limited (standard deviation: ± 1.0 dB). Ecological effects of TTS depend not only on the magnitude of the TTS, its duration (depending on the exposure duration), and the recovery time after the exposure stopped, but also on the hearing frequency affected by the fatiguing noise. The hearing thresholds of harbor porpoises for the frequencies of their echolocation signals are not affected by intense low frequency sounds, therefore these sounds are unlikely to affect foraging efficiency.

Predictions from harbor porpoise habitat association models are confirmed by long-term passive acoustic monitoring

Survey based habitat association models provide good spatial coverage, but only a snapshot in time of a species' occurrence in a particular area. A habitat association model for harbor porpoises was created using data from five visual surveys of the Moray Firth, Scotland. Its predictions were tested over broader temporal scales using data from static passive acoustic loggers, deployed in two consecutive years. Predictions of relative abundance (individuals per kilometer of survey transect) were obtained for each 4 km × 4 km grid cell, and compared with the median number of hours per day that porpoises were acoustically detected in those cells. There was a significant, but weak, correlation between predicted relative abundance and acoustic estimates of occurrence, but this was stronger when predictions with high standard errors were omitted. When grid cells were grouped into those with low, medium, and high predicted relative abundance, there were similarly significant differences in acoustic detections, indicating that porpoises were acoustically detected more often in cells where the habitat model predicted higher numbers. The integration of acoustic and visual data added value to the interpretation of results from each, allowing validation of patterns in relative abundance recorded during snapshot visual surveys over longer time scales.

Hearing thresholds of two harbor seals (Phoca vitulina) for playbacks of multiple pile driving strike sounds

Pile driving, which creates high amplitude sounds with potentially negative impacts on the marine environment, is used to attach wind turbines to the sea bed. To quantify the distance at which pile driving sounds can be detected by harbor seals, unmasked hearing thresholds were obtained for series of five pile driving sounds recorded at 100 and 800 m from a pile driving location. The played back spectra resembled the spectra of sounds recorded under certain conditions 10-50 km from an offshore pile driving site. The lower the received level, the later within the series of sounds the harbor seals responded. The mean 50% detection threshold sound exposure levels for any sound in the series were: 40 (seal 01, 100 m), 39 (seal 01, 800 m), 43 (seal 02, 100 m), and 43 (seal 02, 800 m) dB re 1 μPa(2)s (add 9 dB for sound pressure level, dB re 1 μPa). The mean 50% detection thresholds based on detection of only the first sound of the series were ca. 5 dB higher. Detection at sea depends on the actual propagation conditions and on the degree of masking of the sounds by ambient noise, but the present study suggests that pile driving sounds are audible to harbor seals up to hundreds of kilometers from pile driving sites.

A practical weighting function for harbor porpoise underwater sound level measurements

Harbor porpoise (Phocoena phocoena) are subject to underwater noise disturbance from anthropogenic sources, especially shipping. The underwater audiograms of harbor porpoise were used to create a frequency weighting function, dBht(Phocoena phocoena), to permit estimation of the broadband perceived amplitudes of ambient and shipping noise. An equation was fit to the 0.02-20 kHz range of unmasked detection thresholds and normalizing to 0 dB at 20 kHz; dB = 46.4-35.6 log(kHz). The weighting function de-emphasizes the low frequency components of noise. Harbor porpoise hearing is less sensitive to low frequency shipping noise and, except at high amplitudes, estimating potential noise impacts using linear measurements will be misleading.

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for playbacks of multiple pile driving strike sounds

Pile driving is presently the most common method used to attach wind turbines to the sea bed. To assess the impact of pile driving sounds on harbor porpoises, it is important to know at what distance these sounds can be detected. Using a psychophysical technique, a male porpoise's hearing thresholds were obtained for series of five pile driving sounds (inter-pulse interval 1.2-1.3 s) recorded at 100 and 800 m from the pile driving site, and played back in a pool. The 50% detection threshold sound exposure levels (SELs) for the first sound of the series (no masking) were 72 (100 m) and 74 (800 m) dB re 1 μPa(2)s. Multiple sounds in succession (series) caused a ~5 dB decrease in hearing threshold; the mean 50% detection threshold SELs for any sound in the series were 68 (100 m) and 69 (800 m) dB re 1 μPa(2)s. Depending on the actual propagation conditions and background noise levels, the results suggest that pile driving sounds are audible to porpoises at least at tens of kilometers from pile driving sites.

The use of an air bubble curtain to reduce the received sound levels for harbor porpoises (Phocoena phocoena)

In December 2005 construction work was started to replace a harbor wall in Kerteminde harbor, Denmark. A total of 175 wooden piles were piled into the ground at the waters edge over a period of 3 months. During the same period three harbor porpoises were housed in a marine mammal facility on the opposite side of the harbor. All animals showed strong avoidance reactions after the start of the piling activities. As a measure to reduce the sound exposure for the animals an air bubble curtain was constructed and operated in a direct path between the piling site and the opening of the animals' semi-natural pool. The sound attenuation effect achieved with this system was determined by quantitative comparison of pile driving impulses simultaneously measured in front of and behind the active air bubble curtain. Mean levels of sound attenuation over a sequence of 95 consecutive pile strikes were 14 dB (standard deviation (s.d.) 3.4 dB) for peak to peak values and 13 dB (s.d. 2.5 dB) for SEL values. As soon as the air bubble curtain was installed and operated, no further avoidance reactions of the animals to the piling activities were apparent.

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for sweeps (1-2 kHz and 6-7 kHz bands) mimicking naval sonar signals

The distance at which active naval sonar signals can be heard by harbor porpoises depends, among other factors, on the hearing thresholds of the species for those signals. Therefore the hearing sensitivity of a harbor porpoise was determined for 1 s up-sweep and down-sweep signals, mimicking mid-frequency and low-frequency active sonar sweeps (MFAS, 6-7 kHz band; LFAS, 1-2 kHz band). The 1-2 kHz sweeps were also tested with harmonics, as sonars sometimes produce these as byproducts of the fundamental signal. The hearing thresholds for up-sweeps and down-sweeps within each sweep pair were similar. The 50% detection threshold sound pressure levels (broadband, averaged over the signal duration) of the 1-2 kHz and 6-7 kHz sweeps were 75 and 67 dB re 1 μPa(2), respectively. Harmonic deformation of the 1-2 kHz sweeps reduced the threshold to 59 dB re 1 μPa(2). This study shows that the presence of harmonics in sonar signals can increase the detectability of a signal by harbor porpoises, and that tonal audiograms may not accurately predict the audibility of sweeps. LFAS systems, when designed to produce signals without harmonics, can operate at higher source levels than MFAS systems, at similar audibility distances for porpoises.

Differences in the response of a striped dolphin (Stenella coeruleoalba) and a harbour porpoise (Phocoena phocoena) to an acoustic alarm

Small cetacean bycatch in gillnet fisheries may be reduced by deterring odontocetes from nets acoustically. However, different odontocete species may respond differently to acoustic signals from alarms. Therefore, in this study a striped dolphin and a harbour porpoise were subjected simultaneously to sounds produced by the XP-10 experimental acoustic alarm. The alarm produced 0.3s tonal signals randomly selected from a set of 16 with fundamental frequencies between 9 and 15kHz, with a constant pulse interval of 4.0s (duty cycle 8%) and a Source Level range of 133-163dB re 1muPa (rms). The effect of the alarm was judged by comparing the animals' respiration rate and position relative to the alarm during test periods with those during baseline periods. As in a previous study on two porpoises with the same alarm, the porpoise in the present study reacted strongly to the alarm by swimming away from it and increasing his respiration rate. The striped dolphin, however, showed no reaction to the active alarm. Based on harbour porpoise audiograms and the specific audiogram of the striped dolphin in the present study, and the low background noise levels during the experiment, both animals must have heard the alarm signals clearly. This study indicates that cetacean species are not equally sensitive to human-made noise disturbance. Therefore, source levels of acoustic alarms should be adapted to the species they are supposed to deter. In addition, alarms should be tested on each odontocete species for which they are intended to reduce bycatch.