Magnus Wahlberg

Sperm whale clicks: Directionality and source level revisited

In sperm whales (Physeter catodon L. 1758) the nose is vastly hypertrophied, accounting for about one-third of the length or weight of an adult male. Norris and Harvey [in Animal Orientation and Navigation, NASA SP-262 (1972), pp. 397-417] ascribed a sound-generating function to this organ complex. A sound generator weighing upward of 10 tons and with a cross-section of 1 m is expected to generate high-intensity, directional sounds. This prediction from the Norris and Harvey theory is not supported by published data for sperm whale clicks (source levels of 180 dB re 1 microPa and little, if any, directionality). Either the theory is not borne out or the data is not representative for the capabilities of the sound-generating mechanism. To increase the amount of relevant data, a five-hydrophone array, suspended from three platforms separated by 1 km and linked by radio, was deployed at the slope of the continental shelf off Andenes, Norway, in the summers of 1997 and 1998. With this system, source levels up to 223 dB re 1 microPa peRMS were recorded. Also, source level differences of 35 dB for the same click at different directions were seen, which are interpreted as evidence for high directionality. This implicates sonar as a possible function of the clicks. Thus, previously published properties of sperm whale clicks underestimate the capabilities of the sound generator and therefore cannot falsify the Norris and Harvey theory.

Investigating the Potential Use of Environmental DNA (eDNA) for Genetic Monitoring of Marine Mammals

The exploitation of non-invasive samples has been widely used in genetic monitoring of terrestrial species. In aquatic ecosystems, non-invasive samples such as feces, shed hair or skin, are less accessible. However, the use of environmental DNA (eDNA) has recently been shown to be an effective tool for genetic monitoring of species presence in freshwater ecosystems. Detecting species in the marine environment using eDNA potentially offers a greater challenge due to the greater dilution, amount of mixing and salinity compared with most freshwater ecosystems. To determine the potential use of eDNA for genetic monitoring we used specific primers that amplify short mitochondrial DNA sequences to detect the presence of a marine mammal, the harbor porpoise, Phocoena phocoena, in a controlled environment and in natural marine locations. The reliability of the genetic detections was investigated by comparing with detections of harbor porpoise echolocation clicks by static acoustic monitoring devices. While we were able to consistently genetically detect the target species under controlled conditions, the results from natural locations were less consistent and detection by eDNA was less successful than acoustic detections. However, at one site we detected long-finned pilot whale, Globicephala melas, a species rarely sighted in the Baltic. Therefore, with optimization aimed towards processing larger volumes of seawater this method has the potential to compliment current visual and acoustic methods of species detection of marine mammals.

Echolocation signals of wild harbour porpoises, Phocoena phocoena.

SUMMARY Field recordings of harbour porpoises (Phocoena phocoena) were made in the inner Danish waters with a vertical array of three or four hydrophones. The back-calculated source level ranged from 178 to 205 dB re 1μ Pa pp @ 1 m with a mean source level of 191 dB re 1 μPa pp @ 1 m. The maximum source level was more than 30 dB above what has been measured from captive animals, while the spectral and temporal properties were comparable. Calculations based on the sonar equation indicate that harbour porpoises, using these high click intensities, should be capable of detecting fish and nets and should be detectable by porpoise detectors over significantly larger distances than had previously been assumed. Harbour porpoises in this study preferred a relatively constant inter-click interval of about 60 ms, but intervals up to 200 ms and down to 30 ms were also recorded.

Estimating source position accuracy of a large-aperture hydrophone array for bioacoustics

A linear error propagation analysis was applied to a hydrophone array used to locate sperm whales [see Mohl et al., J. Acoust. Soc. Am. 107, 638–648 (2000)]. The accuracy of two-dimensional (2D) and three-dimensional (3D) array configurations was investigated. The precision in source location was estimated as a function of inaccuracies in measurements of sound velocity, time-of-arrival differences (TOADs), and receiver positions. The magnitude of additional errors caused by geometric simplification was also assessed. The receiver position uncertainty had the largest impact on the precision of source location. A supplementary vertical linear array consisting of three receivers gave information on the vertical bearing and distance to the sound sources. The TOAD data from an additional receiver as well as from surface reflections were used to form an overdetermined location system. This system rendered positions within two standard deviations of the estimated errors from the original 3D array.

Acoustic behaviour of echolocating porpoises during prey capture

SUMMARY Porpoise echolocation has been studied previously, mainly in target detection experiments using stationed animals and steel sphere targets, but little is known about the acoustic behaviour of free-swimming porpoises echolocating for prey. Here, we used small onboard sound and orientation recording tags to study the echolocation behaviour of free-swimming trained porpoises as they caught dead, freely drifting fish. We analysed porpoise echolocation behaviour leading up to and following prey capture events, including variability in echolocation in response to vision restriction, prey species, and individual porpoise tested. The porpoises produced echolocation clicks as they searched for the fish, followed by fast-repetition-rate clicks (echolocation buzzes) when acquiring prey. During buzzes, which usually began when porpoises were about 1–2 body lengths from prey, tag-recorded click levels decreased by about 10 dB, click rates increased to over 300 clicks per second, and variability in body orientation (roll) increased. Buzzes generally continued beyond the first contact with the fish, and often extended until or after the end of prey handling. This unexplained continuation of buzzes after prey capture raises questions about the function of buzzes, suggesting that in addition to providing detailed information on target location during the capture, they may serve additional purposes such as the relocation of potentially escaping prey. We conclude that porpoises display the same overall acoustic prey capture behaviour seen in larger toothed whales in the wild, albeit at a faster pace, clicking slowly during search and approach phases and buzzing during prey capture.

The acoustic behaviour of diving sperm whales observed with a hydrophone array

Abstract Clicks of male sperm whales ( Physeter catodon Linneaus) were recorded off Northern Norway using a hydrophone array. The 3-D diving behaviour in one complete (34 min) and four incomplete (6–36 min) dives was observed using passive acoustic localization. The vertical swim speed was 0.8–1.4 m/s, and the horizontal swim speed was 0.2–2.6 m/s. The maximum dive depth of an individual track ranged from 358 to 1494 m. The bottom depth was 500 to 1600 m and a scattering layer was present at 250 m. Clicks were usually emitted with inter-click intervals (ICI) of 0.5–2 s in blocks of 29–249 clicks, interspaced by pauses of 6–117 s. The length of blocks decreased with depth in a manner predictable from a pneumatic sound generator. Click sequences with shorter inter-click intervals, denoted creaks and believed to indicate feeding events, were located at depths from 278 to 1245 m. This study suggests that sperm whales exploit prey in a large part of the water column below the scattering layer and use an air-driven sound production system.

Biosonar adjustments to target range of echolocating bottlenose dolphins(Tursiops sp.) in the wild

SUMMARY Toothed whales use echolocation to locate and track prey. Most knowledge of toothed whale echolocation stems from studies on trained animals, and little is known about how toothed whales regulate and use their biosonar systems in the wild. Recent research suggests that an automatic gain control mechanism in delphinid biosonars adjusts the biosonar output to the one-way transmission loss to the target, possibly a consequence of pneumatic restrictions in how fast the sound generator can be actuated and still maintain high outputs. This study examines the relationships between target range (R), click intervals, and source levels of wild bottlenose dolphins (Tursiops sp.) by recording regular (non-buzz) echolocation clicks with a linear hydrophone array. Dolphins clicked faster with decreasing distance to the array, reflecting a decreasing delay between the outgoing echolocation click and the returning array echo. However, for interclick intervals longer than 30–40 ms, source levels were not limited by the repetition rate. Thus, pneumatic constraints in the sound-production apparatus cannot account for source level adjustments to range as a possible automatic gain control mechanism for target ranges longer than a few body lengths of the dolphin. Source level estimates drop with reducing range between the echolocating dolphins and the target as a function of 17 log(R). This may indicate either (1) an active form of time-varying gain in the biosonar independent of click intervals or (2) a bias in array recordings towards a 20 log(R) relationship for apparent source levels introduced by a threshold on received click levels included in the analysis.

Source parameters of echolocation clicks from wild bottlenose dolphins (Tursiops aduncus and Tursiops truncatus)

The Indian Ocean and Atlantic bottlenose dolphins (Tursiops aduncus and Tursiops truncatus) are among the best studied echolocating toothed whales. However, almost all echolocation studies on bottlenose dolphins have been made with captive animals, and the echolocation signals of free-ranging animals have not been quantified. Here, biosonar source parameters from wild T. aduncus and T. truncatus were measured with linear three- and four-hydrophone arrays in four geographic locations. The two species had similar source parameters, with source levels of 177-228 dB re 1 μPa peak to peak, click durations of 8-72 μs, centroid frequencies of 33-109 kHz and rms bandwidths between 23 and 54 kHz. T. aduncus clicks had a higher frequency emphasis than T. truncatus. The transmission directionality index was up to 3 dB higher for T. aduncus (29 dB) as compared to T. truncatus (26 dB). The high directionality of T. aduncus does not appear to be only a physical consequence of a higher frequency emphasis in clicks, but may also be caused by differences in the internal properties of the sound production system.

Effects of offshore wind farms on marine wildlife—a generalized impact assessment

Marine management plans over the world express high expectations to the development of offshore wind energy. This would obviously contribute to renewable energy production, but potential conflicts with other usages of the marine landscape, as well as conservation interests, are evident. The present study synthesizes the current state of understanding on the effects of offshore wind farms on marine wildlife, in order to identify general versus local conclusions in published studies. The results were translated into a generalized impact assessment for coastal waters in Sweden, which covers a range of salinity conditions from marine to nearly fresh waters. Hence, the conclusions are potentially applicable to marine planning situations in various aquatic ecosystems. The assessment considered impact with respect to temporal and spatial extent of the pressure, effect within each ecosystem component, and level of certainty. Research on the environmental effects of offshore wind farms has gone through a rapid maturation and learning process, with the bulk of knowledge being developed within the past ten years. The studies showed a high level of consensus with respect to the construction phase, indicating that potential impacts on marine life should be carefully considered in marine spatial planning. Potential impacts during the operational phase were more locally variable, and could be either negative or positive depending on biological conditions as well as prevailing management goals. There was paucity in studies on cumulative impacts and long-term effects on the food web, as well as on combined effects with other human activities, such as the fisheries. These aspects remain key open issues for a sustainable marine spatial planning.

Estimated transmission beam pattern of clicks recorded from free-ranging white-beaked dolphins (Lagenorhynchus albirostris).

Recordings were made from white-beaked dolphins in Icelandic waters using a four-hydrophone array in a star configuration. The acoustic signals were amplified and sampled to a hard disk at a rate of 800 kHz per channel. The 3 and 10 dB beamwidths were calculated to be 8 degrees and 10 degrees, respectively, indicating a narrower transmission beam for white-beaked dolphins than that reported for bottlenose dolphins (Tursiops truncatus). The beamwidth was more similar to that found for belugas (Delphinapterus lucas). The measured beam pattern included large side lobes, perhaps due to the inclusion of off-axis clicks, even after applying several criteria to select only on-axis clicks. The directivity index was calculated to be 18 dB when using all data for angles from 0 degrees-50 degrees. The calculated sound radiation from a circular piston with a radius of 6 cm driven by a white-beaked dolphin click had a beam pattern very similar to the measured beam pattern for the main transmission lobe of the white-beaked dolphin. The directivity index was 29 dB. This is the first attempt to estimate the directionality index of dolphins in the field.