underwater

Shipping has been responsible recently for many collisions with cetaceans in the Canary Islands. A series of experiments, including the playback of artificial sounds of different frequencies, was conducted to test a system designed to keep sperm whales apart from the ferries routes. The results showed that the whales did not react to low frequency playbacks which suggests sperm whales from an area which has heavy vessel traffic have a high tolerance for noise. After the collision of a ferry with two sperm whales, ears were extracted from the two individuals, in order to assess the health of their inner ears. CT scans showed that there were no fractures or other overt evidence of impact, or ship strike related injuries; however, ears from both animals had reduced auditory nerve volumes. 0ne animal also had patches of dense tissue in the inner ear. These findings are consistent with auditory nerve degeneration and fibrous growth in response to inner ear damage. In combination with the results from the playback experiment, these results suggest that low frequency sounds from shipping may be affecting hearing and increasing collision rates. These findings are however, preliminary, and histologic analyses are underway to determine whether the primary cause of the ear changes seen with CT are disease or noise induced.

The U.S. Navy's SURTASS LFA is an active sonar system under development since the late 1980's, for employment from a small number of auxiliary ships assigned to conduct undersea surveillance to detect, classify and track potential threat submarines. SURTASS LFA system development is important because traditional passive systems are facing increasing challenges in their detection capabilities due to improved submarine quieting technologies. SURTASS LFA hardware consists of an array of acoustic transmitting components suspended an average of 100 meters beneath the host ship, which travels at a maximum of 3-4 knots This vertical array of transducers transmits waveforms (100-500 Hz) which are much longer than other active sonar systems. Echoes reflected off objects such as submarines are then detected on passive towed horizontal line arrays (HLA), and processed and evaluated to identify and classify them. The U.S. Navy will soon be at the stage of transitioning the system from a test and evaluation status to the fleet for worldwide employment to enhance undersea warfare capabilities. At present, the U.S. Navy has only one system. If proposals are carried through to completion, a total of four systems will be procured, two each for the Atlantic and Pacific fleets. High sound levels can potentially be harmful, either by affecting hearing or by causing other physiological effects; however, the combination of high transmission losses in the marine environment and the implementation of proven mitigation measures (visual and acoustic monitoring, source ramp- up, sound pressure level monitoring, shut-down criteria), reduce the potential for harmful sound levels from the SIJRTASS LFA system reaching marine animals and divers to a negligible level. During the development of the SURTASS LFA system, the U.S. Navy prepares various environmental analyses prior to sea tests, which are coordinated with the appropriate agencies responsible for wildlife protection. In July, 1996, the U.S. Navy announced a proposal for operational employment of the system and the initiation of a worldwide environmental impact statement (EIS) to evaluate the potential environmental effects of such deployment. The EIS is one element of a synergistic plan composed of three primary thrusts; 1) a comprehensive scientific research program (SRP) under the aegis of some of the world's most prominent marine bioacousticians, using the SURTASS LFA system as a scientific tool to collect much-needed data on the potential effects of low frequency sound on marine mammals; 2) an intensive diver risk assessment involving in-water low frequency sound measurements with human subjects under the direction of the U.S. Navy Submarine Medical Research Laboratory; and 3) the EIS, which will be available to the public in draft form in 1998.

The role of underwater sound as a potential stressor in the marine environment is now widely recognised and the designers of sonars find themselves increasingly constrained by environmental legislation which requires them to consider the possible harmful effects of high power sound transmissions on marine life (e.g. fish and marine mammals) and on human beings. This paper describes a formal process of environmental impact assessment being developed in support of the procurement of future sonars in the UK. The basis of this process in environmental legislation is briefly reviewed but the main purpose of the paper will be to consider the complex scientific and technical issues surrounding environmental impact assessment. In particular, Environmental Assessment (EA) for sonar systems requires a process of cause and affect modelling to be undertaken. Sonars may produce both energy and substance pollution (e.g. explosives may release toxic compounds). The 'precautionary principle' which is enshrined in environmental legislation puts the onus on the polluter to prove that his particular form of pollution does not have a harmful effect on the environment. But how are environmental impact criteria defined? Toxic effects are relatively easy to test for and to quantify. Sound energy on the other hand is rather more difficult. Consideration of the hearing sensitivity of fish, for example, leads to the notion of safe exposure level and probability of avoidance. But how representative are the experiments on which these criteria are based (e.g. the impact of seismic airguns on fish catch rates)? How can we assess the reliability of the scientific evidence given the uncertainties elsewhere, e.g. poor or inadequate knowledge of sound propagation characteristics (including non-linear effects associated with impulsive sound sources), uncertainty in environmental conditions, natural variability and the cumulative effects of repeated exposure to sound energy transmissions? There have been few coincident measurements of sound intensity in the ocean at the ranges at which a particular species exhibits avoidance behaviour and many studies make simplifying assumptions regarding acoustic propagation, e.g. spherical spreading out to unrealistically long ranges, These and other topics will be reviewed with example calculations 'to illustrate particular aspects of the EA process which is being developed by the authors.

In recent years research into the whistle structure of different species of odontocete calls has attempted to identify distinctive individual, pod, or simply species-specific, features, that might assist in acoustic identification and discrimination. So far most of these studies have concentrated on the frequency modulated signal by examining these for characteristic features in the frequency-time domain using FFT analysis. Such analysis has concentrated on the centre frequency, frequency deviation, number of inflection points (frequency reversals), and more significantly the general shape or contour of the call. The Source Level of echolocation signals has also been studied for other reasons but this parameter appears to reflect some body size dependency. This paper discusses the feasibility of using the maximum Source Level of the narrow band social calls (whistles) as an additional cue when attempting to distinguish between species while studying free-ranging animals. The data used for spectral and statistical analysis was recorded from a Dutch fishing research vessel (FRV Tridens) during the 1996 and 1997 CETASEL project sea trials. A single hydrophone was attached to a pelagic trawl net fishing in relatively deep water (100-200 m depth) along the edge of the continental shelf in the Eastern North Atlantic sea areas SW of Eire, through Biscay and towards Finnisterre. Maximum sound pressure levels of social calls were extracted and converted into Source Levels (SL re 1µPa at 1m) using a calibrated 13 kHz pulsed cw pinger (Dukane) to provide a known SL reference. The necessary range information (distance between hydrophone and vocalising animal) was obtained by means of a multi-path echo-ranging method (Kaschner et al 1997). Parallel visual observation in good weather conditions provided the species identification and a time series analysis established the probability of the association between these sighted animals and the acoustic recordings of species whistles. On one occasion a very vocal group of bowriding Common Dolphins Delphinus delphis were recorded simultaneously in air, using a gun microphone directly above them at the bow, and underwater using the hydrophone attached to the trawl at a distance of 518m behind them. The maximum underwater Source Levels of individual signals which were recognisable in both recordings were calculated and compared with the other results in order to test the validity of the method described above. This paper discusses the methodology and limitations of this relatively simple technique but the results achieved to date suggest considerable potential for SL estimation in open sea conditions. Alternative, quieter, platforms than a pelagic trawler are recommended.

Kaschner, K. et al. (1997). Analysis and interpretation of cetaceans sounds obtained by a hydrophone attached to a pelagic trawl. In European Research on Cetaceans - 11. Proc. 11th Ann. Conf. ECS, Stralsund Germany (Eds. P.G.H. Evans) European Cetacean Society, Cambridge

The time resolution constant for Tursiops truncatus clicks is between 12 and 15 µs (Au 1993). However this extremely high theoretical value has never been considered as the actual time resolution of the dolphin auditory system. The "critical interval'' of 260 + 25 µs as a measure of the time resolution was derived from the series of echolocation and hearing experiments on dolphin discrimination between correlated stimuli (Dubrovskiy and Velmin, 1975). Having learned by experience how wrong one could be in construing results of dolphin's discrimination of correlated stimuli, we introduced uncorrelated noise stimuli (Zaslavskiy, Ryabov and Titov 1979, Zaslavskiy and Ryabov, 1991). The temporal masking, time intervals and pulse envelopes discrimination for the noise stimuli were studied. The actual time resolution of dolphin's auditory system of about 20-30 µs consistently manifested itself from these experiments. The dolphins have shown remarkable capability in analysing different characteristics of very short pulses in the time domain. The time resolution of the dolphin auditory system proved to be practically equal to the theoretical time resolution of echolocation clicks and at least ten times shorter than auditory integration time (Au 1993). Results gave no indication of the "critical interval". Experimental data were collected at the Karadag Department of Institute of Biology of Southern Seas, Crimea, USSR in 1977- 1989.

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.