cetaceans

The sea is actually a very noisy place. Sound is a convenient mediator of information in water, since it travels far, irrespective of daylight and visibility. This has been exploited by all sorts of animals living in the sea. However, the acoustical properties of water put special demands on sea animals trying to use sounds. This has lead to a rich variety of adaptations in both sound generators, receptors and use in the sea. Among the noisiest sea creatures is a Crustacean, the snapping shrimp Alpheus japonicus, producing a pistol shot like, broadband (<20-200 kHz) sound with their claws. They do so in connection with prey capture, resulting in the immobilizing of small fish. It is not the sound itself, but a powerful jet of water ejected through a hole in the claw, that blocks out the fish's linear system. The sound is produced when two smooth surfaces, pressed tightly in the open claw, are suddenly forced apart when the claw is closed. Less noisy are fish, mostly limiting themselves to silent, low frequency grunts used in social contexts. Sometimes the fish bladder is used as a resonance chamber, amplifying the sound. Hearing is mostly limited to the low frequency range, even if recent research shows that e.g. herrings Clupea harengus may hear up to over 10 kHz. Birds are not believed to display much underwater social behaviour, nor sounds. But loons (Gaviidae), penguins (Spheniscidae), cormorants (Phalacrocoracidae), diving ducks such as the eider duck Somateria mollissima, and auks (Alcidae) all make noise when swimming, mainly cavitation noise. All marine mammals, when they returned to a life in the sea, had to re-adapt to the acoustical properties of water. In order to be effective, both the terrestrial sound generation and reception systems had be remodelled. This has occurred to a various extent in present day marine mammals. Water dwelling Carnivores, such as the polar bear Thalarctos maritimus, may be good swimmers and divers, but have not developed special underwater adaptations in neither sound generation nor hearing. The otter Lutra lutra and the mink Mustela vison chase and catch fish under water, but then apparently rely on vision and touch (vibrissae). They too have normal terrestrial ears. Propeller-like noise from swimming minks, which is believed to be cavitation noise from the front paws, has been reported. Pinnipeds are much more adapted to life in water, although they have maintained an important link to terrestrial life in connection with reproduction. Therefore they have a sound repertoire for use both in air and in water. Their calls are low frequency, often composed of trains of pulses, but FM sounds also occur. These calls may be heard at great distances. 0ne species, the leopard seal Hydrurga leptonyx, is reported to produce ultrasonic sounds in connection with fish chasing. Often the throat region is seen to be moving and/or inflated in connection with the seals' underwater sounds, indicating that the larynx may be involved in their production, but most often no air is expelled into the water. Or cavity resonance may be involved, e.g. in the bell-like sound of walrus, which is believed to be produced the pharyngeal pouches. Seal hearing is acute and partly adapted to the water physics. The Cetaceans have cut every link to terrestrial life, with no need for compromises in their acoustical adaptations. Baleen whales use very low frequency and sometimes very intense sounds, in order to reach far with their communication signals. Their sound generation mechanism is largely unknown, but may involve resonance in the lungs and/or trachea. Their hearing shows clear morphological adaptations to underwater demands, but so far their characteristics are rather unknown. Odontocetes have gone the farthest in their acoustical adaptation, and have developed the most sophisticated sonar in the animal kingdom. In at least the Delphinids the sonar clicks are concentrated into a narrow beam pointing forward, approximately along the long axis of the rostrum. This is done by means of the melon, a fatty tissue structure in front of the blowhole. Most Delphinids also produce FM whistles in the 5-25 kHz range. The most well-known of these is the so called signature whistle, which is believed to be like an acoustical fingerprint. The sonar clicks as well as the whistles are produced by means of a new sound generation system in the nasal cavities, powered by air pressure created in the bony nares. The main part of the sonar click is purely a tissue phenomenon, whereas the whistles are produced in the air within the nares. The air used during sound production is collected in diverticula just below the blowhole, and is then pulled back to the bony nares to be used again. In its extreme, the pulsed sounds may be powerful enough to debilitate prey. This has been suggested for the sperm whale, where the large head, with the spermaceti organ, may be a huge sound amplifier. The hearing in most Delphinids is extremely acute, ranging from below 1 kHz to 150 kHz. The sounds are entering via the lower jaw, and is guided to the middle ear by mean of a special fatty tissue channel. The hearing is not affected by water depth, indicating that the air in the middle ear is not involved in the sound transmission.

The Cetacean Sound Library at the 'Centro Interdisciplinare di Bioacustica e Ricerche Ambientali' of the University of Pavia (Italy) was created to keep underwater acoustic recordings of cetaceans occurring in the Mediterranean Sea. Recordings were made during research cruises organized by the Centro itself and other Institutions to study the acoustic behaviour, distribution and biology of cetaceans in the Mediterranean Sea. About 137 hours of recordings belonging to sperm whales (which until recently was the target species), striped dolphins, risso's dolphins, bottlenose dolphins, common dolphins, pilot whales, and other sound sources (including man-made noises) are present. Software tools were developed to visualize and analyze stored recordings and to make easier their retrieval and interpretation. As tapes are browsed, a content-database is kept updated to allow access to the cut of interest and to related data such as cruise tracks, bathymetric charts and photographs. Whenever possible sperm whale recordings are linked to a digitized photo-ID (fluke) of the individual. To further develop the sound library, during the last two years a useful partnership with the Italian Navy was initiated, and the Centro is utilised to evaluate biological sounds recorded in ASW (Anti Submarine Warfare) operations.

In dolphins, directional, broad-band pulsed sounds are used for echolocation but due to their high directionality, they are also well suited for addressing intra-specific, social signals. Studies on pulsed communication sounds in free swimming dolphins have so far been restricted to the audible range, not dealing with the directional aspect. Preliminary studies at our facility indicate that dolphin use this option in their intra- specific communication. We have developed a new Pc-compatible datalogger concept for recording and store high frequency pulsed sounds produced by free swimming bottlenose dolphins Tursiops truncatus. The recording unit, called 'MOSART' (MObile Subsurface Acoustic Recording of Transients), is designed to record the highly directional high frequency sounds received by a dolphin, not those produced by it. The 'MOSART' will be attached to the dorsal in by means of a non- invasive saddle pack (Trac Pac). The 'MOSART' has a broad-band hydrophone, and specially developed sound processing electronics and software. The sounds are stored on a 1.8 inch hard disk. Several recording options will be adjustable by means of computer software settings, (e.g. time delay before recording onset, trigger amplitude threshold, high pass filter limit and sampling rate). To maximise the recording time, only a limited and pre-set number of fully sampled clicks will be collected. For the rest of the clicks only a time stamp will be stored. In this way at least 86 minutes of continues click trains, with an average repetition rate of 500 pps, may be recorded. The stored data will be transferred to a PC for analysis. It will be compared with traditional broad band recordings with fixed hydrophones and correlated with video recordings of the dolphin behaviour.

Acoustic censusing of marine mammals is an advancing technique. This presentation will discuss advantages and disadvantages of acoustic censusing compared to visual censusing. When considering using acoustics to describe marine mammal populations, several questions need to be addressed, including type of population estimate (relative or absolute), array design, localization requirements, frequency bandwidth, and species diversity of the sampled population. Major advantages of acoustic censusing include greater detection ranges, fewer environmental limitations, and a complete record of all contact cues. Given recordings of each contact, the signals can be further analyzed for source identification and localization. Acoustic censusing difficulties relate to determining source identity, group size, and detection distance. Fundamental choices such as array design can have major impacts. Finally, the overall advantages of acoustic censusing, particularly when done concurrently with a visual survey lead to useful data. In the recently completed GulfCet I project, acoustic effort occurred along 95% of the survey track, compared to 49% for the concurrent visual survey. Population estimates from the acoustic survey were 316 (265-377) sperm whales and 36,946 (33,512-40,566) dolphins, compared to 313 (192-508) sperm whale and 18,584, (10,268-35,431) dolphins for the visual survey. Subsequent analysis can now be done, for example, on the effects of noise on marine mammals based on signals recorded during the survey.

Cluster analysis is a straightforward approach to classification of sounds, but the distance matrices required for input are problematical. Using a limited number of frequency time coordinates to represent each sound allows efficient handling of large data sets, as well as classification of data not used to define the clusters. Cross-correlation analysis offers the power of complete comparison of sounds, but no way to classify sounds not included in the cross- correlation. We evaluate McCowan's elegant parametric method of comparing dolphin whistles (PCA of 20 equally spaced frequency measurements) for agreement with less efficient but more powerful cross-correlation methods, on an independent data set of bottlenose dolphin whistles. Kmeans clustering of principal components produced different clusters from 3 other methods. Distance matrices generated with McCowan's 20 variables and with cross correlation analysis of time-normalized frequency contours were similar, but distance matrices based on non-normalized contours differed greatly. Our results suggest that McCowan's method might be improved by including duration as a variable in the PCA.

Recordings of sperm whales Physeter macrocephalus were collected via a towed passive hydrophone array. The study area ranged from the 100-2000m isobaths from the Florida-Alabama to the Texas-Mexico borders in the northwestern Gulf of Mexico, USA. The study area was divided into 14 north/ south transects at 74km intervals. Seven cruises were conducted on a seasonal basis from 1992-1994. Sperm whale vocalizations were identified based upon their unique spectral characteristics such as frequency range, duration, and temporal pattern. There were a total of 67 on-effort acoustic sperm whale contacts (a - contact is defined as an encounter with a vocal whale or whale group) during 1,055 hrs (11,997 km) of acoustic recording; corresponding to a rate of 0.064 acoustic contacts/hr (0.005 contacts/km). Chi- square analyses were conducted based upon acoustic level of effort. The average bottom depth per contact was 1,244m (sd=414). A Chi-square analysis of depth categories indicated that more sperm whales were observed than expected at a depth range of 711-1,190m (Chi-square=10.24, p=0.017) along the continental slope. Days were divided into six equal time of day (TOD) categories, and a Chi-square analysis of TOD categories indicated no significant difference in the number of sperm whale contacts across TOD (Chi-square=4.47, p=0.45). A Chi-square analysis of Beaufort wind speeds indicated no significant difference in the number of sperm whale contacts across Beaufort wind speed categories (Chi-square=8.01, p=0.91). A Chi- square analysis of seasons indicated no significant difference in the number of sperm whale contacts across seasons (Chi-square=1.91, p=0.59). A Chi- square analysis of ocean surface dynamic height data indicated that more sperm whales were observed than expected at a low dynamic height range of -30 to –10cm (Chi-square=4.81; p=0.00016). There were sperm whale concentrations in the Mississippi River Canyon and Western Gulf which may be associated with the Loop Current and eddy formations, and decay.

There is a growing concern in the literature about the effects of low frequency sounds (LFS) on marine mammals. A primary way to assess these effects on marine mammals involves the study of disturbance reactions. Detailed research of disturbance reactions of submerged marine mammals requires 3- dimensional localization and tracking of the animals. Animals such as sperm whales Physeter macrocephalus are localized passively with the use of travel time differences (TTD) of their vocalizations received by multiple hydrophones at known positions. Classically, straight-line paths of sound propagation between source and receiver are used to calculate source position. A more accurate calculation of source position involves naturally occurring non- constant sound speeds. This gives rise to arced paths of sound propagation between source and receiver. An algorithm is used to recursively pinpoint source position in a medium with a non-constant sound speed. 5 hydrophone array configurations are tested, each with 30 randomly generated source positions. Average errors of the 150 source position calculations (x, y, z) are (±1.58m, ±1.70m, ±10.44m) for the straight line, and ±0.76m, ±0.87m, ±1.10m) for the algorithm. On average, the algorithm improves the source depth calculation by an order of magnitude.

Determination of the delphinid sonar signal generation site has eluded cetologists for decades. Recent interest in developing a bionic sonar system has reinvigorated the effort to characterize the apparatus and its operation. We studied activities within the pharyngeal and nasal cavities of two bottlenose dolphins Tursiops truncatus during echolocation. A high-speed dual-camera video system provided synchronized windows for recording two concomitant events: movements visible through an endoscope and oscilloscope traces of acoustic pressure at a hydrophone placed near the animal's head. Dolphins have two tissue complexes (Cranford et al. 1996), one located on either side, and just above, the membranous nasal septum. They apparently generate acoustic pulses by pushing air across sets of internal flips.' The acoustic pulse occurs coincident with one oscillatory cycle of the lips. Changes in acoustic pulse repetition rate and the lip's vibration cycles are simultaneous, indicating that their rates and periods are synchronous. We did not find other structures in the airways vibrating synchronously with each acoustic pulse generation event. The palatopharyngeal muscle complex compresses air for the system. These observations settle a long-standing controversy over the site of biosonar signal generation in odontocetes and open a vista of potential avenues for future investigations. (Work supported by the Office of Naval Research)

In examining the potential impact of human-made sound on sea mammals, it was considered that whale hearing sensitivity might diminish with increasing ambient pressure. To test the effect of depth, two white whales made 885 dives to a platform at 5, 100, 200 or 300 m in the Pacific Ocean. Each stationing on the platform up to 12 minutes at a time, whales whistled when they heard a 500 ms tone from a hydrophone. With increasing depth, air density increase in the middle ear, sinuses, and nasal cavity changed each whale's whistle response, but did not attenuate hearing as it does in the aerial ear (of humans and other land mammals tested in pressure chambers) due to middle ear impedance changes. The findings support theories that sound is conducted through whale head tissues to the ear without the usual ear drum/ossicular chain amplification of the aerial middle ear. These first ever hearing tests in the open ocean demonstrate that whales hear as well at depth as near the surface; therefore, zones of influence for human made sounds are just as great throughout the depths to which whales dive, or at least to 300 m.

In June 1995, a 12 days research cruise was organized in the Ligurian and North Tyrrhenian Sea to record cetacean sounds with the towed array of the University of Pavia. The cruise was supported by the Italian Navy within the ENCY 95 (European Nature Conservation Year) activities. The hydrophone was towed for 111 hours (out of 12 cruising days) at speeds up to 14 km/h; listening stations were held on a 24h schedule for at least 10 min every half an hour. One sperm whale was detected and located. It was heard at night and acoustically tracked for the following 8 hours. Within this period the whale was sighted at the surface 5 times, while 8 complete dives were continuously recorded on DAT tapes (about 6 hours of recording). The recordings are now archived at the Cetacean Sound Library held at the Centro. New methods of sound analysis were developed to make the analysis of such long recordings easier and to give compact pictures of whole dives. Our real-time analysis software was modified with new procedures able to 1) automatically detect and count clicks, 2) measure and save inter-click intervals, 3) save packed representations of the click sequences and display autocorrelograms to show the evolution of inter-click intervals over long periods of time. The analysis of the recordings shows that all the recorded dives were characterized by a typical and constant clicking pattern at their beginning. The duration of the acoustical emission, measured from the first click to the last click of each dive, was on average 27 minutes 30 seconds, while the silence related to the surfacing was on average 13 minutes and 11 seconds.