underwater

The underwater acoustic environment is inherently loud as a result of ambient sounds. In addition, there is an increasing amount of sound from anthropogenic sources, which produce noise within the hearing range of most fish (less than 1.0 kHz). For fish, the auditory system is one of the most important sensory systems because it provides information about ambient sounds, prey items, predators and potential mates. Since a fish's ability to accurately interpret its acoustic environment is essential for its survival, it is important to understand how noise affects sound perception ability of fish. This study examines the question: does noise exposure affect auditory sensitivity differently in hearing specialists (fish with coupling devices between the gasbladder or ancillary structures and inner ear, enhancing overall hearing with wider frequency range and lower threshold) and hearing generalists (fish without coupling devices, with narrow frequency range and higher threshold)? For this study, a hearing specialist species, the fathead minnow Pimephales promelas, and a hearing generalist species, the bluegill sunfish Lepomis macrochirus, were used to examine: (1) the immediate effects on auditory threshold of white noise exposure at various exposure durations (1-24 h at 0.3-4.0 kHz, 142 dB re: 1µPa); and (2) recovery time after noise exposure (1-14 days). Audiograms for noise-exposed fish were measured using the auditory brainstem response protocol, which records acoustically evoked brainwaves, and compared to control fish (no noise exposure). The results for specialists showed that their best hearing frequency range (0.8-2.0 kHz) was affected significantly more than other frequencies. In addition, recovery of hearing ability after noise exposure was found to be both frequency- and exposure-dependent. The results for hearing generalist fish will be presented and discussed (work supported by the Kentucky Academy of Science, National Organization for Hearing Research, NIMH-58198, Institute of Museum and Library Service-LL90187).

Sciaena umbra (Linneaus, 1758) is one of the four species of Sciaenidae that are present in the Mediterranean Sea. It lives in rocky reefs or in Posidonia grass meadows; it is a nocturnal predator, sexes are separated and eggs are buoyant. It has been known to produce sounds since 1947 at least, when Dijkgraaf described sounds of Sciaena umbra observed in the Zoological Station A. Dohrn of Naples. In the UNEP Asp-protocol of 1995 it was included in the list of species whose fishery needs regulating. In the Miramare Marine Reserve (north-eastern Adriatic Sea, Trieste, Italy), S. umbra is very abundant from May to September along the artificial rocky reefs under the pier of Miramare castle. Sounds were recorded digitally at 44.1 kHz sampling rate with a Reson TC 4032 hydrophone from the pier. Recording 10 minutes each hour for a whole day (24h) confirmed that sounds are present only from late afternoon until 1 a.m. Therefore all subsequent recordings of 10 minutes each hour were carried out from 19.00 to 24.00 only. Sounds are repetitive knocks composed of 1-11 pulses, more or less regularly spaced (131.6ms ± 33.8 SD), with the main frequency components within each pulse ranging between ca. 100 and 1,200 Hz. Based on time interval between sounds, there are at least three different acoustic patterns, named irregular, regular and chorus. Sounds are emitted from May till September with irregular patterns usually followed by regular ones during a daily session. Chorus patterns were recorded only in June and July and not consistently. If S. umbra is a serial spawner like most other Sciaenidae, chorus pattern might indicate spawning bouts.

Today, sounds produced by cetaceans are used for acoustic detection of individuals and groups in the wild. However, the detection probability ascertained by a concomitant visual survey has not been demonstrated extensively. We studied the finless porpoises Neophocaena phocaenoides in the Yangtze River from Wuhan to Poyang Lake in 1998 in China, using underwater sound monitoring with hydrophones (B&K 8103) placed along the sides of a research vessel, concurrent with visual observations. The peak to peak detection threshold was set at 133 dB re 1 µPa. With this threshold level, porpoises could be detected reliably within 300 m of the hydrophone. In a total of 774 km cruise, 588 finless porpoises were sighted by visual observation and 44,864 ultrasonic pulses were recorded by the acoustical observation system. The acoustic monitoring system could detect the presence of the finless porpoises 82% of the time. False alarms in the system occurred with a frequency of 0.9%. The performance of the acoustic detection system depends highly on the sound production rate and the directionality of the beam pattern. Echolocation click events of two finless porpoises were recorded with an acoustic data logger in an oxbow of the Yangtze River. They produced 3 to 4 click trains in a minute. A behavioural data logger attached to the identical animal provided the dive depth and the body angle simultaneously. Comparing the values of dive depth, the body angle and the level of water surface reflection, the 120 degrees off-axis sonar signals were found to have 160 dB peak-to-peak sound pressure level at one meter from the animal. The finless porpoises produced sonar signals frequently and the off-axis signal had sufficient level to be detected. High frequency acoustical observation is suggested as an effective method for field surveys of small cetaceans, which produce high frequency sonar signals.

Baleen whales live at extended timescales. Study of how these animals use their vocalisations for communication requires massive data sampling over long periods. The volume of data precludes traditional hands-on analysis techniques, at least during the first stages of data reduction. This paper describes a system for automating the sampling and analysis of baleen whale calls. Blue Balaenoptera musculus and fin B. physalus whale calls are very stereotypical. Blue whale 'A' and 'B' calls have fundamental frequencies of approximately 17 Hz, narrow bandwidth and well-defined harmonic structure, and typical duration of 15-25 sec. Fin whale 'pulses' have fundamental frequencies of approximately 17 Hz, but are broadband in nature and short (~ 1 sec) duration. The homogeneous call structure lends itself to automated detection. Stable acoustical differences in call structure lend themselves to automated species identification. We have benchmarked a series of bioacoustical call identification algorithms against a set of blue and fin whale calls while systematically manipulating the signal to noise ratio. The results demonstrated a typical tradeoff of speed versus accuracy. The best algorithm was inserted into the underwater sound recording system and its signal-detection theoretic performance was quantified. Results will be discussed with respect to technological and ecological aspects of baleen whale bioacoustics (Prolect CS-1082 of the Strategic Environmental Research and Development Program).

Amazonian manatees have historically been exploited in Brazil but there is little information on population status and trends. The difficulty of tracking individuals in turbid water habitats restrains the assessment of behavioural characteristics, which in turn could be helpful in determining some key parameters about their conservation status. Currently, acoustic signals are assumed to form the basis of manatee communication, for which prior empirical evidence has been reported. Therefore, if manatees can recognise each other by acoustical means, it should be possible to identify individual vocal patterns. Vocalisations were recorded of 14 captive Amazonian manatees, temporarily and individually housed. The vocalisations were digitised and seven variables were recorded. These were subjected to multivariate statistical treatment. Principal Components Analysis grouped the data indicating that some individuals could be distinguished on the basis of variables related to the fundamental frequency of vocalisations (axis 1 of the PCA). We have also observed a significant difference in axis 1 between sexes, with a tendency of females to have higher fundamental frequency values than males. No difference was observed between different age classes in axis 1 of the PCA. Axis 2 of the PCA was positively related with the signal duration, isolating a mother and calf pair with greater signal duration values than the others. An inverse relation between body size (total body length) and the range of the fundamental frequency was verified. This study reinforces the possibility of identifying individual manatees by their vocal patterns, hence making bioacoustics a useful tool for behavioural and social studies, in addition to providing needed information on conservation strategies for the species.

The advent of offshore exploration for oil and gas during the 1970s created a need for very accurate underwater position fixing for survey activities such as searching, mapping and photography, and for work tasks such as pipe laying, surface rig positioning, underwater structure positioning, towfish positioning, remotely operated vehicle (ROV) navigation, subsea construction, mining, drilling and a vast range of other applications. The precision required dictates the use of underwater acoustic navigation and tracking techniques, and the best known of these employ arrays of hydrophones, beacon (also called pingers), transponders (receiver-transmitters) and responders (transmitter- receivers). For most of the tasks listed above the positioning is carried out only after a series of procedures to fix the position of the array on the sea bed; these may include transponder deployment, array baseline calibration and possibly absolute calibration in terms of geodetic co-ordinates by integrated satellite, surface and underwater data telemetry. In this paper, we consider how techniques and systems developed for industrial applications may be adapted for tracking echo-locating cetaceans (dolphins, porpoises and whales), in particular the principles of position fixing in three-dimensional space in real time. This is a problem that presents a serious engineering challenge.

Computer classification of dolphin whistles requires a number of steps from detection of a whistle in background noise to classifying the whistle among a database of whistle types. Detection was achieved by using a broadband noise reduction technique and a filter for tracking relatively slowly changing FM tones. After whistle detection, its characteristic time-frequency-intensity contour was extracted using a tracing algorithm with an 'inertial' following rule to avoid crossover when multiple whistles were present. The data requirements for each extracted contour were reduced using an encoding technique that splits each whistle into segments based on frequency slopes and using a curve fitting routine to represent the contour within each segment. Classification is achieved using hidden Markov models to represent the possible segment sequences within each class, and three distance measures based on the segment curves. In this way class similarity percentages can be calculated for each class, and a candidate whistle can be assigned to the most probable class.

In gill-net fisheries around the world, very large numbers of small cetaceans are killed each year as bycatch (IWC report 1994). If commercial fishing is to continue in areas where the incidental catch of cetaceans is significant then technical improvements to the method of fishing are required to mitigate the impact. Modifying fishing gear by the addition of acoustic alarms which signal the position of the net by the transmission of low level sounds was pioneered by Lien and his colleagues in Newfoundland and this technique successfully reduced baleen whale interactions with set fishing nets and traps. These simple devices, which were constructed by the fishermen, were also tested in a bottom set gillnet fishery in the Bay of Fundy where harbour porpoise mortality is high. Encouraging results from this Canadian research resulted in an improved device being developed in the USA for use in a subsequent Gulf of Maine study. The low frequency signals developed as baleen whale deterrents were also tested on captive harbour porpoises at Harderwijk in Holland where it became very evident that the frequencies used were inappropriate. The technology used in these early devices is electro- acoustically inefficient and the operating (battery) costs rather high. In Europe studies of a variety of potential acoustic deterrent devices were carried out by Loughborough University which included tests with a harbour porpoise of a wide variety of signal frequencies and waveforms. These sounds were synthesised digitally and the behaviour of a 9ee swimming animal, contained within a large floating net enclosure, were observed. The new generation of micro-controller based beacon-mode alarms developed at Loughborough University is discussed here. These devices synthesise the sounds shown to be most aversive to a porpoise and implement new features intended to minimise habituation rates and maximise battery life. A preliminary field test with wild harbour porpoises in Scotland during September 1996 showed that these devices induce a dramatic avoidance behaviour, displacing the animals in a short test to a range greater than 640 m. The design and engineering of this new technology device is discussed in the context of preparing them for a large-scale commercial fishery trial in Denmark.

As part of the EU funded CETASEL project recordings were made of small cetaceans on the edge of the continental shelf between SW Eire, Biscay and North Spain. Various passive acoustic systems were attached to a pelagic trawl operating in relatively deep water (200m). These systems were then used in conjunction with a surface observer program and remotely operated television systems to study small cetacean behaviour around a moving fishing net. A multiple hydrophone system has been developed to track echolocating cetaceans. Comparison of the arrival times of echolocation 'clicks' on a number of spatially separated hydrophones were made allowing estimation of the positioning of the cetacean in relation to the hydrophone array. Additional lower frequency (3-20 kHz) recordings were made on a single hydrophone also attached to the trawl. A number of examples of lower frequency 'whistles' signals including multi-path (bottom and surface reflections) were observed. With knowledge of the hydrophone and water depth, comparison of the arrival time of the various multi-path components of a signal has also allowed the estimation of the range and depth of its source from the receiver. A simplified ray path model has been developed to simulate various source, receiver geometrys. Arrival times of the multi-path signals were calculated and compared with those seen during sea trials. A number of assumptions have been made in initial models including a constant sound velocity depth profile and the treatment of the surface and seabed as a simple reflecting surfaces. Initial results have shown a number of examples with a reasonable correlation between estimated position of a submerged cetacean and the associated surface observations. Examples of multiple (positioning) solutions were however found, these are in the main due to imprecision in the knowledge of the hydrophone and water depth geometry and inaccuracies in the initial timing measurements. The use of correlation techniques and stand alone depth measurement devices is therefore proposed for future measurements and analysis using this technique. It is felt that within certain constraints this technique can provide valuable additional information regarding cetacean behaviour in the wild. The addition of more complex time measurement techniques and better ray path modelling will hopefully provide a useful analysis tool in the study of cetaceans.

Oceans form two-thirds of the surface area of our planet. Our understanding of this environment even after several decades of research is best described as rudimentary. The watery abyss is home to the world's largest and most diverse biological ecosystems. Monitoring of these ecosystems and the study of the physical and biological evolution of the oceans is of paramount importance to our increased comprehension and understanding of the oceans. Efficient monitoring relies on the collection of data over long periods of time and in diverse sea conditions. Stationary or moving sonar systems have already proven invaluable for this task. However, most of these systems are fixed in their mode of operation, possessing little or no flexibility. Thus, when the environment changes data retrieval often becomes difficult or impossible. Changing the characteristics of the monitoring device to match the local sea conditions is a possible solution to this problem. In this paper the hardware and software implementation of a system that realises this function is presented together with some preliminary transmission tests in the Mediterranean Sea. The system, VERTLINK is designed to collect, store and transmit physical and bio-acoustical data to an end-user. The system, a half-duplex underwater communication link, com- prises two units-a sea-bed unit and a surface unit. The sea-bed unit, under the control of the surface unit, records oceanographic data and then transmits it to the sea-surface unit. The latter is linked to a computer station on-board a survey vessel via a physical cable or to shore station by means of a radio link. Using this system the end-user is able to change the way the sea-floor unit acquires and transmits data. The modulation rate/technique can be changed from 2-DPSK to 8-DPSK, allowing data retrieval to be changed according to sea noise conditions. The power level can be varied from 20% to 100%, to achieve more efficient use of power resources and the sea-floor unit can be told to release.