mammals

The "Centro Interdisciplinare di Bioacustica e Ricerche Ambientali'' was founded in 1988 by the University of Pavia. Since 1989 it is endowed with a Laboratory of Marine Bioacoustics granted by the "lspettorato Centrale per la Difesa del Mare'' of the Italian Ministry of the Environment. The Cetacean Sound Library created at the Centro holds recordings made in research cruises organised to study the acoustic behaviour, distribution and biology of cetaceans in the Mediterranean Sea. More than 130 hours of recordings belong to Sperm whales (which till now has been the target species), Striped dolphins, Risso's dolphins, Bottlenose dolphins, Common dolphins, Pilot whales, and other sound sources including man-made noises. A catalogue based on a widely used database format was created in order to allow an easy and reliable access to the collected recordings and to related data such as digitised photographs of individual Sperm whales, video clips, behavioural observations, cruise tracks, bathymetric data, and others. In the last two years a useful partnership with the Italian Navy has been arranged, and the Centro is asked to evaluate biological sounds recorded in ASW (Anti Submarine Warfare) operations and surveys.

Studying the vocalisations of dolphins provides an insight into their underwater behaviour whereas visual methods are normally restricted to surfacing behaviour. Apart from whistles, which seem to be produced in a social context, the sonar emissions of dolphins consist of pulsed signals used for different echolocation purposes. By analysing the pulse periodicity, frequency and spectral components of 'clicks' and relating these to video recorded surfacing behaviour it should be possible to extract characteristic patterns which indicate different kinds of dolphin behaviour. Almost all the pulsed emissions recorded during a series of 24 hour long intensive studies suggest behaviour related to foraging. These recordings provide unambiguous conditions where the environmental constraints can be observed and understood. Goodson and Datta (1992) found that recognisable patterns occurred in the repetition rate of the echolocation signals during these sequences. They partitioned these pulse sequences into 4 distinct phases. These sub-classifications have been further examined using an extended data set from the same source. Graphical and statistical analysis of six complete sequences, recorded immediately prior to a visually observed fish capture, allows a better definition of the presence of these foraging partitions. An additional fifth foraging phase was identified. The sequences could be partitioned into separate identifiable segments and characterised as foraging phases. Possibilities exist for automating this analysis for on-line use in the field as well as for the application of this analysis approach to behaviour types other than foraging.

Ferry companies are increasingly utilising high-speed wave-piercing catamarans to provide fast alternatives to conventional services. The number of such ferries operating in the UK has doubled in the last 5 years, but the environmental impacts of this trend, including possible cetacean disturbance arising from noise pollution, have received little attention. In March 1997 a new high-speed ferry service began operating from Poole in Dorset, England, passing through the Durlston Marine Research Area, the site of a long-term bottlenose dolphin monitoring project. Recordings of the ferry were obtained, from portable and seabed mounted fixed hydrophones, in order to assess the potential for disturbance of the study animals. The most significant sound outputs are two sharp peaks around 500Hz. Apart from these, machinery noise also produces a continuous spectrum across the range 100Hz to above 5kHz. The other major noise source is from displaced water, contributing to noise levels in the higher part of the spectrum, particularly above 10kHz. For bottlenose dolphins, the ferry would appear unlikely to cause disturbance on acoustic grounds. In keeping with this, comparison of bottlenose dolphin sightings data before and since the commencement of the ferry service found no discernible change in the timing or frequency of dolphin activity in the study area. However, this was very much a preliminary short-term study and further data are required before firm conclusions can be made.

Most species of marine mammals seem highly reliant on and sensitive to underwater sounds. Sounds important to marine mammals may include calls from conspecifics, odontocete echolocation sounds, predator and prey sounds, and environmental sounds (e.g. surf or ice noise). Some man-made noises are known or suspected to have negative effects on marine mammals, including noise-induced masking, disturbance, hearing impairment, and possibly stress. However, marine mammals are adapted to a variable and often naturally noisy environment. Also, even when levels of man-made noise are well above natural ambient levels, negative effects on marine mammals are not always obvious. Data available up to early 1995 were summarised in the book "Marine Mammals and Noise'' (Richardson et al. 1995, Academic Press). Since then, advances have occurred in some but not all areas of particular concern:

(1) When can marine mammals hear man-made noise? Additional data are becoming available for some small- and moderate-sized odontocetes, pinnipeds, and manatees. There is still an urgent need for direct audiometric data from baleen and sperm whales.

(2) Does man-made noise mask important natural sounds? Data are available on masking in a few species of captive odontocetes and pinnipeds. However, we need data on masking processes and significance when free-ranging marine mammals are exposed to typical man-made sounds, including variable, non-tonal, and directional sounds.

(3) When does man-made noise disturb mammals, and when is disturbance strong enough to constitute harassment? Disturbance effects are graduated, not ''all or none''. Sometimes no disturbance is apparent even at short ranges with high received levels (RQ. At other times there is strong disturbance even at long ranges with low RLs. Strong and/or prolonged disturbance may have negative biological elects even if there is no physical damage. However, infrequent brief disturbances may have no biological significance, and if so should not be considered "harassment''. Additional controlled studies, both field and captive, are needed.

(4) What are the thresholds for noise-induced auditory impairment and non- auditory effects, and what types of man-made sounds could elicit them under field conditions? The first data on Temporary Threshold Shift (TTS) in marine mammals have been released. recently. TTS work with additional species and exposure conditions is needed. However, TTS results have limitations in establishing damage risk criteria (DRC), and relationships between TTS and harassment are uncertain.

(5) Noise-induced stress in marine mammals is almost entirely unstudied.

Mitigation measures sometimes used to reduce noise effects include seasonal and geographic restrictions, ramping up, and real-time monitoring plus localised mitigation. We need more data on the effectiveness of ramping up, visual and/or acoustic monitoring, and localised measures such as minimum approach distances, minimum altitudes, and shutdown radii. Progress is being made toward understanding noise effects on marine mammals, in focusing on the most serious issues, and in devising mitigation approaches. However, the issues are complex and the needed studies are often difficult. Some major emitters of underwater sound remain reluctant . to become involved in the process. It will take time, money and cooperation to conduct the needed studies, to determine which situations need mitigation, and to devise, test and implement effective yet practical mitigation measure.

This paper describes the first use of the Transmission Line Modelling (TLM) method for the time-domain numerical modelling of sound propagation in odontocete acoustic tissue. The validity of the technique is assessed by performing simulations on the highly specialised lipid materials distributed within the melon of the species Phocoena phocoena. The geometrical data for the simulations was obtained from Computer X-ray Tomography (CT) scans published by T W Cranford. A time-discrete waveform based on the output from the animal was synthesised for injection into the TLM simulation. The software described accepts a 24-bit bitmap for the geometrical data, and the injected samples in a spreadsheet file. Output is available as pressures in another spreadsheet and a series of bitmaps showing the propagating waves at different points in time. A number of bitmaps may be generated from the program and combined using a readily available commercial program into moving wave visualisations. The model clearly shows the acoustic energy contained by the waveguide effect of the graded-index composition of the melon. The investigation supports the assertion that the melon is a wave guiding structure rather than a conventional lens. The tissue structures prior to the melon therefore appear responsible for generating the oscillatory waveform.

The harbour porpoise, one of the smallest marine cetaceans of the Northern hemisphere, is a frequent victim of the continental shelf bottom-set gill-net fisheries with many thousands of animals being killed annually. The reasons for this high mortality are not clear as these animals possess a sophisticated active sonar system with which they detect, track and intercept small fish targets. The performance and limitations of this animal's sonar are therefore being studied in order to develop techniques that will minimise cetacean/ gillnet interactions. Sonar source levels and spectra were carefully determined in an enclosed environment from signals transmitted by two juvenile animals. This paper examines these signals and, in the context of the detailed structure of the vestibular air sacs, presents a new hypothesis suggesting that a passive biological mechanism exists in these animals which can explain the formation of their unusually narrow band echolocation signals. The knowledge gained to date in this and associated studies suggests that the harbour porpoise operates a relatively low power, short range sonar capable of detecting (ingestible size) individual fish out to a maximum range of about 30 m. The strongest acoustic emissions of this animal are expected to be around 120-140 kHz with Source Levels, in open water conditions, around 160-170 do re 1µpa at 1m. These sonar signals will be confined by the projecting aperture to a very directional beam pattern (approaching 8° in azimuth and 14° in elevation) projected directly ahead of the animal. The spectrum of the harbour porpoise's sonar pulse exhibits a single power peak and the -3 dB bandwidth is typically around 13 kHz. The harbour porpoise possesses a much shorter range, lower power, sonar than its larger delphinid cousins. In addition its more limited bandwidth may be expected to generate target echoes with less spectral coloration which in turn may impair its ability to correctly classify unfamiliar hazardous targets such as fishing nets from a safe distance.

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.