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It is well known that dolphins possess a sonar capability which allows them to project acoustic energy and analyze returning echoes in order to detect and recognize objects under water. The use of acoustic energy is probably the most effective way to probe an underwater environment for purposes of navigation, obstacle avoidance, prey and predator detection, and object localization and detection. Acoustic and other mechanical vibrational energy propagates in water more efficiently than any form of energy including electromagnetic, thermal and optical energy. Since the natural habitats of many dolphin species include shallow bays, inlets, coastal waters, swamps, marshlands, and rivers that are often so murky or turbid that vision is severely limited, these animals must rely heavily on their active and passive sonar capabilities for survival. Some of the sonar capabilities of dolphins have been described in review articles by Nachtigall (1980) and Au (1988).

The acoustic backscatter of a diving Atlantic bottlenose dolphin (Tursiops truncatus) was measured using Simrad EK‐60 echosounders operating at 70, 120, and 200 kHz. The animal was trained to dive to a small platform suspended with a nylon rope from a small boat to various depths, with the maximum depth being 100 m. When the dolphin reached the platform, it was required to station on a bite plate attached to the platform until an acoustic signal was sent to release the animal. A video camera on the platform with the signal sent up on a separate line was used to monitor the animal on the bite plate. A time‐depth recorder was also attached to the dolphin to determine the average rate of decent and ascent. Echosounding records were collected while the dolphin descended to and ascended from the platform. A total of 15 echosounding records with the dolphin swimming within the beam of the transducers were collected. Preliminary analysis of the data indicated that target strength varied from about −26 dB to −40 dB with a slight frequency dependency. The echoes from the dolphin were structured with the maximum echoes originating from the vicinity of the lungs.

The clicks from an Atlantic bottlenose dolphin were obtained in a controlled experiment at the Marine Mammal Research Program (Kaneohe Bay, HI). A previously described phantom echo experiment using solid elastic spherical targets provides the foundation for an investigation of the specific scattering components used in a dolphin’s target identification and classification methods. A model of the scattering interaction of incident dolphin echolocation clicks and elastic targets is outlined. The frequency spectra from the echolocation clicks are combined with the resonance‐scattering representations of the scattered pressure fields to yield filter‐type integrals that can be viewed as filtering the form function of the scatterer through the spectral window of the incident echolocation clicks. This analysis is carried out in the time and frequency domains and also represented in time‐frequency distributions (Wigner‐Ville, RID). An advantage of this analysis is that it can be carried out for any arbitrary echol...

The target strength as a function of aspect angle were measured for four species of fish using dolphin-like and porpoise-like echolocation signals. The polar diagram of target strength values measured from an energy flux density perspective showed considerably less fluctuation with azimuth than would a pure tone pulse. Using detection range data obtained from dolphin and porpoise echolocation experiments, the detection ranges for the Atlantic cod by echolocating dolphins and porpoises were calculated for three aspect angles of the cod. Maximum detection ranges occurred when the fish was broadside to the odontocete and minimum detection ranges occurred when the cod was in the tail aspect. Maximum and minimum detection ranges for the bottlenose dolphin in a noise-limited environment was calculated to be 93 and 70 m, respectively. In a quiet environment, maximum and minimum detection ranges for the bottlenose dolphin were calculated to be 173 and 107 m, respectively. The detection ranges for the harbor porpoise in a quiet environment were calculated to be between 15 and 27 m. The primary reason for the large differences in detection ranges between both species was attributed to the 36 dB higher source level of the bottlenose dolphin echolocation signals.

Dolphins have been observed foraging for prey that bury themselves into a sandy bottom. The dolphins swim about 1 to 2 m above the bottom and scan in a circular motion or swim immediately off the bottom, scanning from side to side with their beams pointed approximately normal to the bottom. The dolphin has been emulated by using dolphinlike sonar signals (120‐kHz peak frequency, 39‐kHz bandwidth, 50‐μs duration) in order to classify proud and buried targets in real time. The transducer was attached to a bottom‐crawling remotely operated vehicle. Target echoes were received via a cable and digitized at 1 MHz. Short‐time Fourier transform and the Morlet wavelet were used to obtain time‐frequency representations of the echoes. Echoes were processed in a hierarchical neural network system to perform target classifications. Six targets (cast iron pot, stainless steel sphere, glass jar, concrete tile, and coral rock) were placed either on the ocean bottom or buried into the bottom. Echoes were separated into three target categories: (1) cast iron pot; (2) stainless steel sphere; and (3) remaining four objects. The neural network was able to correctly identify 74%, 97%, and 88% of the category 1, 2, and 3 test echoes.

Snapping shrimp are among the major sources of biological noise in shallow bays, harbors, and inlets, in temperate and tropical waters. Snapping shrimp sounds can severely limit the use of underwater acoustics since their snaps are very broadband with energy from several hundred Hertz to 200 kHz. Snapping shrimp noise is always present having little diurnal and seasonal variations. Peak‐to‐peak source levels can be as high as 189 dB re:1 μPa. Dolphin emissions are another common source of biological noise. Dolphin sound emissions can be categorized as whistles, burst pulses, and echolocation clicks. Whistles usually contain energy in the 5–20‐kHz range, although higher‐order harmonics can extend beyond 100 kHz. Burst pulse signals and echolocation clicks are broadband, short duration signals with frequency components beyond 150 kHz. Peak‐to‐peak source levels of echolocation signals can be as high as 220–225 dB re:1 μPa. Burst pulses are similar in shape to echolocation clicks but with about 6–10‐dB lower...

Broadband simulated dolphin echolocation signals were used to measure the ex situ backscatter properties of mesopelagic boundary community animals. The signal had a peak frequency of 100 kHz, 3‐dB bandwidth of 35 kHz, and duration of 80μs. Subjects were captured by trawling with a 2‐m‐opening Isaacs‐Kidd midwater trawl. Backscatter measurements were conducted on the deck of the ship in a 2000‐L seawater plastic tank with the transducer placed on the bottom, pointed upwards. Anesthetized animals were suspended in the transducer’s beam by nylon monofilament lines connected to a support device constructed out of dowels. Backscatter measurements were obtained in both the dorsal and lateral aspects for seven myctophids and only in the dorsal aspect for the other 13 myctophids, 6 shrimps, and 3 squids. The echoes from the myctophids and shrimps usually had two highlights, one from the surface of the animal nearest the transducer and a second probably from the signal propagating through the body of the subject a...

The sounds produced by snapping shrimp (genus Synalpheus and Alpheus) are the main contributors to the ambient sound field of shallow‐water tropical and subtropical marine habitats worldwide. Despite this, relatively little is known about the temporal and spatial variability of snapping shrimp activity. To investigate this issue further, ecological acoustic recorders (EARs) were utilized to obtain time‐series records of the ambient sound field from multiple locations in Hawaii and American Samoa. Strong diel trends marked by sharp increases in acoustic activity during crepuscular periods were observed at all sites. However, sites varied greatly both in absolute sound levels and relative fluxes between day and night. In addition, fluctuations in snapping shrimp sound levels were found to track lunar cycles, with higher levels occurring during the new moon. To further investigate the reason for the higher levels observed during crepuscular periods, individual shrimp were localized using a three‐hydrophone line array on both a coral reef and a mud and algae habitat in Kaneohe Bay, Hawaii. The density of acoustically active shrimp was found to be higher during the sunset period than during the day or at night.

A dolphin’s ability to discriminate targets may greatly depend on the relative amplitudes and the time separations of echo highlights within the received signal. Previous experiments with dolphins have varied the physical parameters of targets, but did not fully investigate how changes in these parameters corresponded with the composition of the scattered acoustic waveforms and the dolphin’s subsequent response. A novel experiment utilizes a phantom echo system to test a dolphin’s detection response of relative amplitude differences of secondary echo highlights and the time separation differences of all the echo highlights both within and outside the animal’s integration window. By electronically manipulating these echoes, the underlying acoustic classification cues can be more efficiently investigated. In the first study, the animal successfully discriminated between a standard echo signal and one with the middle highlight amplitude at −7 dB. When the middle highlight amplitude was raised to −6 dB, the a...

Mysticetes and odontocetes differ in many morphological and behavioral aspects related to feeding. Odontocetes echolocate using clicks to localize prey, while the mechanism through which mysticetes orient and find food is unknown. In this study, DTAGs were deployed on four humpback whales (Megaptera novaeangliae) observed feeding off the northeastern United States. From two of the four tagged whales, bouts of clicks produced with a temporal pattern resembling odontocete echolocation were recorded and 102 bouts of megapclicks were identified. Spectrally, megapclicks were unlike previously reported humpback sounds or odontocete clicks, and also had much lower amplitudes (peak frequency ∼1700 Hz, RL 148±6 dB Re: 1 uPa peak). Interclick intervals (ICI) were bimodally distributed (peaks at 25 and 80 ms), with shorter ICIs occurring at the end of megapclick bouts, in a pattern similar to an odontocete terminal buzz used in foraging. Behavioral sensor data supported a possible foraging function for megapclicks. All megapclick bouts occurred during nighttime hours, and the terminal buzz was associated with a sharp body roll of greater than 70 deg. Low source levels, among other characteristics, contraindicate an echolocation function; however, further analysis and research is required to distinguish among this and other hypotheses for sound function.

Acoustic studies of baleen whales are becoming increasingly common. However, a minority of studies combines acoustic data with technologies that allow sound production to be placed in a detailed behavioral context. Noninvasive digital acoustic recording tags (DTAGS) were attached to humpback whales (Megaptera novaeangliae) on the western North Atlantic’s Great South Channel feeding grounds to study foraging and acoustic behavior. Acoustic records totaling 48.4 data hours from four attachments were aurally and automatically analyzed, and occurrences of several sound types were correlated with body orientation and dive behavior of the tagged subject. Whales produced a wide variety of sound types, which differed in depth and context of production. Long, tonal groans and moans (3‐s average duration, 500‐Hz peak frequency) occurred more frequently in upper portions of the water column, in contrast to broadband paired bursts (50‐ms average duration, 300‐Hz peak frequency), which appeared associated with bottom ...

Although spatio–temporal links exist between some military active sonar testing and cetacean mass stranding events, the underlying causal mechanism(s) of the strandings are not well understood. It has been demonstrated that cetaceans may experience in vivo gas bubble/emboli development; however, the basis for cavitation formation remains unknown. Acoustically mediated in vivo cavitation formation in cetaceans exposed to high‐intensity sound sources has been proposed. The distance of these exposure effects from midfrequency sound sources can be influenced by a combination of factors affecting underwater acoustic propagation. To investigate these issues, this experiment considered rectified diffusion as a possible mechanism for acoustically mediated in vivo cavitation formation in cetaceans. Gelatin samples were placed under water and exposed to an SPL of 205 dB re: 1 μ Pa at 4.5 kHz for up to 1000 s. The sound field was generated by a Massa transducer similar to the transducer element used in the United St...

The echolocation detection range of the Atlantic bottlenose dolphin Tursiops truncates and the harbor porpoise Phocoena phocoena have been either measured or estimated in nonmasking and noise‐masking environments using human‐made targets. These experiments provide little information on the detection range for fish prey in a natural environment. Backscatter and corresponding target strength measurements were performed on four species of fish, Atlantic cod, gray mullet, pollock, and sea bass, using simulated dolphin and harbor porpoise echolocation signals. Backscatter measurements were also performed on several species of snappers using simulated bottlenose dolphin echolocation signals. The fish subject was anesthetized and placed in a monofilament net bag which was attached to a monofilament curtain attached to a rotor. The target strength and backscatter characteristics were determined as a function of angle in the lateral plane. The results of detection experiments using real targets were used to estima...

The effects of anthropogenic noise on baleen whales is a topic of intense interest not only to marine mammalogists but also to a variety of commercial interests, the military, oceanographers, and researchers that use sounds in the ocean. One of the major difficulties in setting standards for exposure levels and the effective use of anthropogenic sounds is a lack of knowledge of the auditory properties of whales. The auditory sensitivity and frequency range of hearing (audiogram) are not known, as well as how anthropogenic sounds physically affect the auditory system, the psychological well being, and the use of sounds by whales. One approach to gain some insight in this area is to study the acoustics of a baleen whale species, the humpback whale. The source levels and frequency range of humpback whale song units have been measured and can provide insights into what these whales expose each other to. The highest rms source level was 173 dB for a specific unit. The average sound pressure levels of chorusing have been measured and can provide insights into the long‐term acoustic environment of humpback whales. The highest daily average of sound pressure level for chorusing sounds was 130 dB.

A line array of three hydrophones with a video camera attached to the array was used to measure the echo location signals of wild Atlantic Spotted Dolphins. The separation distance between hydrophones in the array was 30 cm. The array was attached to a float that supported an amplifier–line driver assembly with the signals sent via a 76‐m multi‐conductor cable back to the support boat. The float and array assembly was oriented by a swimmer. The echo location signals from the hydrophone were digitized simultaneously at a sample rate of 500 kHz. Twenty files of echo location click trains were collected with the quality of the data varying from poor (files with lots of whistles and off‐axis signals) to very good. The on‐axis signals typically had a bimodal spectrum with a low‐frequency peak at 45–60 Hz and a high‐frequency peak at 120–140 kHz. Peak‐to‐peak source levels up to 210 dB re: 1 μPa were measured. The rms bandwidth varied between 32 and 46 kHz, with a cluster around 40 kHz.

Monitoring the changing state of coral reef habitats is a challenging task that is exacerbated when the reefs in question are in remote locations. Physical sensors provide a wide range of measurements of local environmental variables, but do not give an indication of biological activity. The preliminary findings of an effort to use the ambient sound field as a means of characterizing and monitoring biological activity on coral reefs and surrounding waters are reported. Moored recording systems were developed to sample the sound field of reefs on Oahu, Hawaii for 1‐min periods, at 30‐min intervals, for 10 days at a time. Snapping shrimp produce the dominant acoustic energy on the reefs examined and exhibit clear diel acoustic trends. Peaks in activity consistently occur during crepuscular periods. At frequencies below 2 kHz, many fish sounds occur, which also exhibit distinct temporal variability. Cetacean sounds are also common, indicating the occurrence of an apex predator in the area. Many sounds can be...

Songs sung by male humpback whales consist of distinct, pulsed sounds that are designated as units. Units are produced in some sequence to form a phrase, a repeated set of phrases forms a theme, and repeated themes form a song. A song can last from minutes to hours. The songs of eight humpback whales were recorded with a vertical array of five hydrophones spaced 7 m apart with the array located within 100 m of the whales. At least seven distinct units were identified aurally from this data set obtained during the 2002 winter humpback whale session in Hawaii. Four distinct recurring themes were found in the songs, and for each whale at least two themes were recorded. The average duration of each unit sampled and the silent interval following the unit were determined in order to describe the temporal characteristics of the songs. From the data the temporal consistency and cadence control of unit production by each humpback whale and between whales were determined. Understanding the temporal and spectral characteristics of units within songs and how these units vary between whales could ultimately help in the design of computer algorithms to automatically identify individual whales.

The singing behavior of male humpback whales on the winter breeding grounds is still a poorly understood phenomenon. Previous work indicates that the chorusing levels of singing whales off west Maui are higher at night than during the day. However, the cause of this variation is not known. To investigate whether more whales sing at night or whether the same number simply move closer to near‐shore recorders following sunset, the abundance and location of singing whales off Kaena Point, Oahu was examined. A bottom‐moored recording system was used to establish that the same diel pattern observed off Maui also occurs off Oahu. The location of singing whales was determined by localizing singers along a preset transect track using a towed hydrophone array. More whales were found singing along the coastline examined at night than during the day. There was no indication of a shoreward migration. These results suggest that, at night, singing displays may be a more effective behavioral tactic than direct competition in a pod. Also, more singers were found along the northern part of the coastline, which is dominated by a shallow bank, indicating that singers were selective with respect to where they chose to sing.

Numerous studies have been conducted on humpback whale song, but substantially fewer have focused on the acoustic properties of non‐song sound production (i.e., feeding and social sounds). Non‐invasive digital acoustic recording tags (DTAGS) were attached to humpback whales on the western North Atlantics Great South Channel feeding grounds during July 2004. Acoustic records totaling 48.4 data hours from four of these attachments were aurally analyzed for temporal trends in whale signal production. A custom automatic detection function was also used to identify occurrences of specific signals and evaluate their temporal consistency. Patterns in sound usage varied by stage of foraging dive and by time of day. Amount of time with signals present was greater at the bottom of dives than during surface periods, indicating that sounds are probably related to foraging at depth. For the two tags that recorded at night, signals were present during a greater proportion of daylight hours than night hours. These resul...

Despite decades of research on captive dolphins, there still is much to learn about dolphin echolocation and communication in the wild. This is mainly due to the difficulties associated with recording dolphin signals accurately in the wild, such as their broadband (with frequencies over 200 kHz) and directional nature, the difficulty of identifying vocalizing dolphins, and being confined to equipment aboard a vessel. To resolve these difficulties, an underwater portable battery‐operated digital recording device was developed that is capable of simultaneously capturing acoustic signals up to 220 kHz on 4 channels as well as video. It consists of a custom‐made underwater housing, containing two batteries, a digital camcorder, and an acoustic recording unit with signal conditioning board, 4‐channel data acquisition card, PC/104‐plus single board computer and power supply, and a notebook hard disk. Attached to the housing is a 4‐hydrophone star array. Post‐recording, acoustic signals recorded within 15 m are accurately localized in 3‐D and attributed to individual dolphins on the video. Thus, acoustic signal features are correlated with different echolocation behaviors and communicative signal exchanges among dolphins are assessed. The system has been successfully field tested with spinner dolphins (Stenella longirostris) along the leeward coast of Oahu, Hawaii.

The Biomimetic Sonar invention ensonifies submerged objects, digitizes acoustic images reflected from the ensonified objects, and classifies and stores the digitized images using electronic processing derived from that believed to be used by Tursiops truncatus , the bottlenose dolphin. The invention also provides a probable identification of an ensonified object based on comparison of an ensonified object with templates in a library of acoustic images.

Echoes from four different cylinders having the same outer diameter (3.8 cm) and length (10.2 cm) were collected at 1‐deg increments from the broadside to the end aspect and used in a neural network recognition study. The water‐filled stainless‐steel, water‐filled aluminum, and foam‐filled aluminum cylinders had the same 0.64‐cm wall thickness. The fourth cylinder consisted of coral rock pebbles embedded in degassed epoxy. A dolphinlike sonar signal with a peak frequency of 120 kHz and a duration of approximation 50 μs was used in a monostatic echo collection system. Fifty echoes digitized at a 1‐MHz rate were collected for each target at each aspect. Time‐frequency representation of the echoes along with the envelope of the matched filter response were fed to two parallel independent multilayered feedforward backpropagation neural networks and the results were fused together in another feedforward network. The network was able to achieve better than 90% correct recognition of the echoes when trained with...

In this experiment the dolphin was asked to perform a discrimination task between two simulated targets. The echoes from these targets were created using a phantom echo generator which replicated the actual echo characteristics of solid spherical stainless‐steel or brass targets. The system provided full knowledge of each click produced by the dolphin and the corresponding echo received by the dolphin. During the discrimination task the dolphin used a range of clicks that differed widely in peak frequency and bandwidth. However, the echoes that were returned to the dolphin showed significantly less variability in these characteristics. The dolphin in this study carefully maintained consistency in just a portion of the full bandwidth of its clicks, the region that was both within the dolphin’s hearing range and had the best reflectivity off the target. The rest of the click bandwidth outside this region showed a higher degree of variability. The dolphin most likely demonstrated less control over these freq...

An important requirement for understanding the function of animal signals is the ability to determine the context in which they occur. To improve our understanding of how dolphins use acoustic signals in the wild a three‐hydrophone towed array was used to investigate the spatial distribution of Hawaiian spinner dolphins (Stenella longirostris) relative to each other as they produced whistles, burst pulses and echolocation clicks. Groups of 10 to 60+ animals were recorded while they traveled and socialized in nearshore waters off Oahu, HI. Broadband recordings up to 62.5 kHz were obtained using a PC‐based multi‐channel system with simultaneous analog‐to‐digital converters. Signaling animals were localized using time of arrival difference cues on the three channels. Localized whistles produced closely in time typically originated from individuals spaced widely apart (10–100+ m), supporting the hypothesis that whistles play a role in maintaining contact between animals in a dispersed group. Multiple burst pu...

We examined the hypothesis that sounds produced by odontocetes can debilitate fish by testing the effects of three odontocete‐like pulsed signals on three individuals of each of three fish species: sea bass, cod, and herring. We used a high‐frequency click with a center frequency of 120 kHz exposing the fish to approximately 112 dB, a mid‐frequency click with a center frequency of 70 kHz and 208 dB exposure level, and a low‐frequency click with a center frequency of 40 kHz and 193 dB exposure level. Individual fish were placed in a 0.3‐m‐diam net enclosure immediately in front of a transducer. Each fish was allowed to remain in the experimental set up for at least 3 min prior to exposure to the clicks which were presented at a rate of 100 pulses/s grading to 700 pulses/s in 1.1, 2.2, and 3.3 s. Sea bass were also exposed to a constant pulse rate of 700 pulses/s for exposures of up to 30 s. No effect was observed in any of the fish for any signal type or pulse modulation rate. Based on our results, the hyp...

Tens of thousands of odontocetes are caught annually as bycatch in gillnet fisheries worldwide. Of particular interest are bottlenose dolphins and harbor porpoises, both caught in U.S. waters. One method to potentially reduce this incidental‐take is to use nets that are easier for echolocating marine mammals to detect. These acoustically enhanced gillnets are designed to reflect odontocete sonar signals better than regular nylon nets. They have shown to reduce bycatch. Whether or not this is due to increased acoustic reflectivity remains to be determined. We measured the target strength of cod and monkfish gillnets, including those enhanced with barium sulphate, an iron oxide, using dolphin‐ and porpoise‐like echolocation signals. From the target strengths we then predicted the detection distances of the nets by the respective animals. It was determined that at reasonable distances bottlenose dolphins should be able to detect all nets. But because of lower probable source levels, harbor porpoises may not be able to detect any gillnet before it becomes entangled. Stiffness of the nets was also quantified and experimental nets proved to be greater in stiffness than regular nets, thus likely a significant factor in reducing odontocete bycatch.

Brain auditory evoked potentials (AE’s) were recorded in a false killer whale Pseudorca crassidens trained to accept suction‐cup EEG electrodes and to detect targets by echolocation. AEP collection was triggered by echolocation pulses transmitted by the animal. The target strength varied from 22 to 40 dB; the distance varied from 1.5 to 6 m. All the records contained two AEP sets: the first one of a constant latency (transmission‐related AEP) and a second one with a delay proportional to the distance (echo‐related AEP). The amplitude of echo‐related AEPs was almost independent of both target strength and distance, though combined variation of these two parameters resulted in echo intensity variation within a range of 42 dB, thus indicating automatic gain control (AGC). The amplitude of transmission‐related AEPs was independent of distance but dependent on target strength: the less the target strength, the higher the amplitude. Recording of transmitted pulses has not shown variation of their intensity depe...

Groups of spinner dolphins work together to actively aggregate small animals in the deep‐scattering layer that serve as their prey. Detailed information on dolphin foraging behavior, obtained with a 200‐kHz multibeam sonar (Simrad MS2000), made it possible to correlate echolocation and foraging. Fifty‐six groups of spinner dolphins foraging at night within a midwater micronekton sound‐scattering layer were observed with the sonar. During sonar surveys, the rates of whistles and echolocation clicks were measured using four hydrophones at 6‐m depth intervals. Significant differences in click rates were found between depths and between the different stages of foraging. Groups of foraging dolphins ranged in size from 16 to 28 dolphins. Click rates were not significantly affected by the number of dolphins in a foraging group. Contrary to initial predictions, click rates were relatively low when sonar data indicated that pairs of dolphins were actively feeding. Highest echolocation rates occurred within the scattering layer, during transitions between foraging states. Whistles were only detected when dolphins were not in a foraging formation and when animals were surfacing. This suggests clicks may be used directly or indirectly to cue group movement during foraging.

The melon of odontocetes has been hypothesized to be a focusing body that channels echolocation signals produced within the nasal region of the animal’s head into the water. The acoustic field of two echolocating harbor porpoises (Phocoena phocoena) was measured at the melon’s surface with an array of four broadband hydrophones embedded in suction cups. The clicks detected by each hydrophone were simultaneously digitized at a sampling rate of 500 kHz and stored on a PC. Digital still photographs of the array were taken before each echolocation trial to measure the hydrophone’s position. The shape and dimensions of the melon were measured with a flexible shape‐retaining measuring device. The axis of the echolocation beam was found to be approximately 5.6–6.1 cm from the edge of the animal’s upper lip along the midline of the melon, which coincides with the axis of the low‐density lipid core of the melon. Click amplitudes dropped off rapidly (12–14 dB) from the maximum at hydrophones 3.5 cm apart, providing support for the melon‐focusing hypothesis. Changes in the waveform on consecutive hydrophones suggest that the porpoises can manipulate either the shape of the melon or the output of the sources within 0.086 s.

Dolphins have a very sophisticated short range sonar that surpasses all technological sonar in its capabilities to perform complex target discrimination and recognition tasks. The system that the U.S. Navy has for detecting mines buried under ocean sediment is one that uses Atlantic bottlenose dolphins. However, close examination of the dolphin sonar system will reveal that the dolphin acoustic hardware is fairly ordinary and not very special. The transmitted signals have peak‐to‐peak amplitudes as high as 225–228 dB re 1 μPa which translates to an rms value of approximately 210–213 dB. The transmit beamwidth is fairly broad at about 10o in both the horizontal and vertical planes and the receiving beamwidth is slightly broader by several degrees. The auditory filters are not very narrow with Q values of about 8.4. Despite these fairly ordinary features of the acoustic system, these animals still demonstrate very unusual and astonishing capabilities. Some of the capabilities of the dolphin sonar system will be presented and the reasons for their keen sonar capabilities will be discussed. Important features of their sonar include the broadband clicklike signals used, adaptive sonar search capabilities and large dynamic range of its auditory system.

The Hawaiian mesopelagic boundary community is an island‐associated midwater scattering layer comprised of small fishes, shrimps, and squids that undergoes diel vertical as well as horizontal migrations. It has been hypothesized that light levels are an important cue or trigger for vertical migration and presumably, horizontal migration. The migration pattern of the scattering layer was measured over complete lunar cycles while the incident light levels were recorded. Due to differences in the rise and set times of the moon and cloud cover, light and lunar cycle were not completely coupled, allowing separation of the light effects of moon phase and other cues associated with lunar cycle. Four calibrated echosounder moorings were deployed with approximately even spacing, perpendicular to the leeward coast of Oahu. Moorings were deployed for one complete lunar cycle at each of three locations, recording 10 echoes every 15 min. Light sensors measured the nocturnal light intensity at 30‐s intervals. Statistic...

To test the hypothesis that spinner dolphins in Hawaii may actively aggregate their prey through cooperative foraging, a 200‐kHz multi‐beam sonar (Simrad MS2000) was used to observe 323 groups of spinner dolphins foraging within a midwater, micronekton sound‐scattering layer off Oahu. Strong cooperation was observed in groups of 8–14 pairs of spinner dolphins. The dolphin group size was highest at midnight when the density of prey was highest and was significantly higher in Makua Beach, where the prey density was higher, than Electric Beach, where the prey density was lower. Cooperative groups of dolphins aggregated their food resources, apparently using their preys’ avoidance behavior to create distinct, high‐density patches in the prey. Prey aggregation was strongly stereo‐typed, regardless of the distribution of the scattering layer. Dolphins swam around the edge of a 28–40 m diameter circle at least 5 times, concentrating prey within this area before pairs of dolphins on opposite sides of the circle swapped positions in the circle, swimming through the high density prey ’donut’ they had formed. The hypothesis that nocturnal animals aggregate prey in midwater could not have been tested without the three‐dimensional information on prey distribution and dolphin geometry provided by the multi‐beam.

Many experiments have been performed with echolocating dolphins to determine their target detection and discrimination capabilities. Target detection experiments have been performed in a naturally noisy environment, with masking noise and with both phantom echoes and masking noise, and in reverberation. The echo energy to rms noise spectral density for the Atlantic bottlenose dolphin (Tursiops truncatus) at the 75% correct response threshold is approximately 7.5 dB whereas for the beluga whale (Delphinapterus leucas) the threshold is approximately 1 dB. The dolphin’s detection threshold in reverberation is approximately 2.5 dB vs 2 dB for the beluga. The difference in performance between species can probably be ascribed to differences in how both species perceived the task. The bottlenose dolphin may be performing a combination detection/discrimination task whereas the beluga may be performing a simple detection task. Echolocating dolphins also have the capability to make fine discriminate of target prope...

A vertical array of five hydrophones was used to measure the acoustic field of singing humpback whales. Once a singer was located, two swimmers with snorkel gear were deployed to determine the orientation of the whale and to position the boat so that the array could be deployed in front of the whale at a minimum standoff distance of 10 m. The spacing of the hydrophones was 7 m with the deepest hydrophone deployed at depth of 35 m. An 8‐channel TASCAM recorder having a bandwidth of 24 kHz was used to record the hydrophone signals. The location of the singer was determined by computing the time of arrival differences between the hydrophone signals. The maximum source level varied between individual units in a song, with values between 180 and 190 dB. The acoustic field determined by considering the relative intensity of higher frequency harmonics in the signals indicate that the sounds are projected in the horizontal direction with the singer’s head canted downward 45 to 60°. High‐frequency harmonics extended beyond 24 kHz, suggesting that humpback whales may have an upper frequency limit of hearing as high as 24 kHz.

The masked hearing threshold of a Pacific white‐sided dolphin (Lagenorhynchus obliquidens) was determined by measuring the animal’s auditory brainstem response (ABR). The dolphin was trained to wear surface‐contact electrodes embedded in suction cups and to swim into a hoop centered at 1 m below the water surface facing a sound projector 5 m away. Broadband transient signals with center frequencies of 8, 16, 32, 64, 80, and 100 kHz were used as the stimuli. ABR signals were measured by digitizing the electrode signals in 32 point blocks at a sampling rate of 20 kHz. Five hundred blocks were averaged in order to obtain an ABR. The response latency for suprathreshold threshold signals was approximately 1.9 ms with the highest peak‐to‐peak ABR amplitude of approximately 2.8 uV occurring for a signal frequency of 64 kHz. The spectrum of the ABR signal was similar to that of Tursiops truncatus, with a major peak at 1120 Hz and a secondary peak at 664 Hz. Threshold was determined by progressively reducing the amplitude of the stimulus until an evoked potential could not be detected. The energy signal‐to‐noise ratio within an integration window at threshold varied between 1 and 8 dB.

The characteristics of acoustic echoes from six species of deep-dwelling (up to 400 m) Hawaiian Lujanid snappers were determined by backscatter measurements at the surface. A broadband linear frequency-modulated signal and a short dolphinlike sonar signal were used as the incident signals. The fish were anesthetized and attached to a monofilament net that was attached to a rotor so echoes could be collected along the roll, tilt, and lateral axes of each fish. The temporal highlight structure of broadband echoes was determined by calculating the envelope of the cross-correlation function between the incident signal and the echoes. The echo waveforms were complex with many highlights and varied with the orientation of the fish. In the tilt plane, the strongest echoes occurred when the incident signal was perpendicular to the long axis of the swimbladder. The number of highlights was the fewest at this orientation. The number of echo highlights and the length of echoes increased as the fish was tilted from this orientation. The highlight structure of the echoes resulted in the transfer function being rippled, with local maxima and minima that changed with fish size and species. The echo structures in both the time and frequency domains were generally consistent within species and were easily distinguishable between species.

Acoustic data from two sperm whale neonates (Physeter macrocephalus) in rehabilitation are presented and implications for sound production and function are discussed. The clicks of neonate sperm whale are very different from usual clicks of adult specimens in that neonate clicks are of low directionality [SL anomaly (0°–90°)

The target strengths and swimbladder morphology of six snapper species were investigated using broadband sonar, x rays, and swimbladder casts. Backscatter data were obtained using a frequency-modulated sweep (60–200 kHz) and a broadband, dolphinlike click (peak frequency 120 kHz) from live fish, mounted and rotated around each of their three axes. X rays revealed species-specific differences in the shape, size, and orientation of the swimbladders. The angle between the fish’s dorsal aspect and the major axis of its swimbladder ranged from 3° to 12° and was consistent between individuals within a species. This angle had a one-to-one relationship with the angle at which the maximum dorsal aspect target strength was measured (r2=0.93), regardless of species. Maximum dorsal aspect target strength was correlated with length within species. However, the swimbladder modeled as an air-filled prolate spheroid with axes measured from the x rays of the swimbladder predicted maximum target strength significantly bett...

The Springer Handbook of Auditory Research presents a series of comprehensive and synthetic reviews of the fundamental topics in modern auditory research. It is aimed at all individuals with interests in hearing research including advanced graduate students, post-doctoral researchers, and clinical investigators. The volumes will introduce new investigators to important aspects of hearing science and help established investigators to better understand the fundamental theories and data in fields of hearing that they may not normally follow closely. Each volume is intended to present a particular topic comprehensively, and each chapter will serve as a synthetic overview and guide to the literature. As such, the chapters present neither exhaustive data reviews nor original research that has not appeared in peer-reviewed journals. The series focuses on topics that have developed a solid data and conceptual foundation rather than on those for which a literature is only beginning to develop. New research areas will be covered on a timely basis in the series as they begin to mature. Each volume in the series consists of five to eight substantial chapters on a particular topic. In some cases the topics will be ones of traditional interest for which there is a solid body of data and theory, such as auditory neuroanatomy (Vol. 1) and neurophysiology (Vol. 2).

The Hawaiian mesopelagic boundary community, an island‐associated, midwater sound‐scattering layer, undergoes diel vertical and horizontal migrations. To understand the dynamics of the community’s migration at small temporal scales and large spatial scales, five bottom‐mounted, 200‐kHz active‐acoustic mooring that transmitted ten signals every 15 min, from dusk until dawn for 5 days. Two layers within the boundary community were observed to undergo simultaneous diel vertical and horizontal migration. Vertical migration rates were measured at 0–1.7 m/min, while the horizontal rate averaged 1.67 km/h, swamping the vertical movement. The vertex of the migration pattern was observed 45 min before the midpoint between sunset and sunrise. Until the vertex, animal density increased relatively constantly as the animals migrated towards shore, with the highest animal densities found in the shallowest areas at midnight. Animal abundance estimates at the leading and trailing edge of the layer support the hypothesis that increased animal densities near shore are related to packing, as mesopelagic animals avoid the surface and the bottom. We observed high levels of biomass moving rapidly, over a great distance, into shallow waters very close to shore, providing insight into the significant link the mesopelagic boundary community provides between near‐shore and oceanic systems.

An outstanding problem in fisheries acoustics is the depth dependence of scattering characteristics, required for sonar surveys, of swimbladder‐bearing fish. The swimbladder, which is the major source of acoustic reflectivity, may become compressed due to increasing pressure with water depth, changing backscatter characteristics. To address this, echoes from deepwater snappers were obtained from a manned submersible using broadband sonar. A low‐light camera mounted next to the sonar transducer permitted simultaneous identification of species, fish size, and orientation. Data were compared to echoes from captured fish measured at the surface. The relationship between fish length and in situ target strength showed no difference from the relationship measured at the surface. No differences in the species‐specific temporal echo characteristics were observed between surface and in situ measures. This indicates that the size and shape of the fish’s swimbladder are maintained both at the surface and at depths of...

The characteristics of acoustic echoes from six species of deep dwelling (up to 350 m) Hawaiian Lutjanid snappers were determined by backscatter measurements at the surface. A broadband linear frequency modulated signal and a short dolphin‐like sonar signal were used as the incident signal. The fishes were anesthetized and attached to a monofilament net which was in turn attached to a rotor so echoes could be collected along the roll, tilt, and lateral axes. The echo waveforms were complex with many highlights and varied with the orientation of the fish. The highlight structure was determined by calculating the envelope of the cross‐correlation function between the incident signal and the echoes. In the tilt plane, the strongest echo occurred when the incident angle was perpendicular to the long axis of the swim bladder. The number of highlights was the fewest at this perpendicular orientation and increased as the fish is tilted from this orientation. The echo structures were easily distinguishable between species and were generally consistent within species. The highlight structure of the echoes resulted in the transfer function being rippled, with local maxima and minima at different frequencies.

Small cetaceans are often caught as bycatch in gillnet fisheries. One potential method that has been shown to significantly reduce harbor porpoise incidental take is the use of barium sulfate enhanced gillnets. One hypothesis states barium sulfate nets reduce bycatch because they are acoustically more reflective than monofilament nets, thus easier to detect by echolocating odontocetes. This premise remains untested. To determine acoustic reflectivity, this study compared the target strength of a barium sulfate gillnet with a comparable monofilament gillnet at 0, 10, 20, 30, and 40. At normal incidence (0) the monofilament target strength measured slightly greater than the barium sulfate net and biosonar detection ranges are estimated to be greater for the monofilament. As the angle of incidence increased from normal to 20, the barium sulfate net echo strength remained the same while the nylon net echo strength decreased significantly at 10. Biosonar detection ranges are estimated to be greater for the barium sulfate net at increased angles. Echolocating odontocetes may utilize acoustic reflections of both nets but due to the weak target strength of both nets, other mechanisms may also mitigate reduced odontocete bycatch. Current psychoacoustic research is investigating the net detection ranges of captive bottlenose dolphins.

Snapping shrimps are the major source of biological noise in shallow bays, harbors, inlets, and coral reefs in temperate and tropical waters. The sounds are impulsive with a very broad spectrum spanning from several hundred Hz to over 200 kHz. Peak frequencies are typically between 1–10 kHz with a very slow drop‐off in energy with increasing frequency. At 200 kHz, the intensity of the sound may be only 20 dB below the intensity at the peak frequency. The peak‐to‐peak source level can be higher than approximately 189 dB re 1 mPa. When a shrimp rapidly closes its snapper claw, a high‐velocity water jet that has speed exceeding cavitation conditions is produced. The sound is the result of a cavitating bubble collapsing. In a body of water, the overall level of snapping shrimp sounds can be as high as 20 dB above the noise level typical of sea state 6. The noise is present continuously, with little diurnal and seasonal variations and its broad frequency extent make it extremely difficult to filter this noise. Therefore, snapping shrimp sounds can severely limit the use of underwater bioacoustic monitoring in a coral reef environment.

During echolocation, whales and dolphins perceive faint echoes very shortly after loud ongoing pulses. How does the auditory system avoid the masking of the echo by the preceding pulse? To investigate mechanisms of release from masking, auditory brainstem evoked responses (ABR) were recorded in bottlenose dolphins and a false killer whale both during active echolocation and during paired external acoustic stimulation. When the animal ensonified a target, both the ongoing pulse and echo (2‐ms delay) evoked ABRs of comparable amplitudes, in spite of echo intensity of 40 dB relative the ongoing pulse near the animal’s head. When a phantom echo was used (emitted pulses were replayed through an electronic channel with the same delay of 2 ms), the ABR to the echo was comparable to that to the emitted pulse at the echo intensity of 20 dB, but the ABR decreased when the echo intensity diminished to 40 dB relative to the ongoing pulse. An explanation of higher efficiency of the real than the phantom echo may be a ...

A distinction is traditionally made between the echolocation clicks of the whistling members of the family Delphinidae and those of nonwhistling smaller odontocetes, which include the family Phocoenidae and members of genus Cephalorhynchus and Kogia. The larger, whistling delphinids typically emit clicks that are short (40–70 μs) and broadband (30–60 kHz) with peak frequencies ranging from 40 to 130 kHz. Clicks of the smaller nonwhistling odontocetes are generally longer (>100 μs), more narrowband (15–25 kHz) with peak frequencies between 90 and 160 kHz. Here we report findings of long (∼135 μs), narrowband (10–25 kHz) clicks produced by a whistling species of delphinid, the Hawaiian spinner dolphin (Stenella longirostris). These clicks typically have peak frequencies between 90 and 100 kHz and predominantly occur in a doublet pattern with clicks consistently spaced 13 ms apart within a doublet and 23 ms between doublets. The vast majority of free‐ranging spinner dolphin clicks conform to the more traditional delphinid form. Narrowband clicks appear to be produced rarely, having been observed on only 4 out of 25 recording sessions. It is presently not clear whether these clicks represent echo location or communication signals.

Echolocating bottlenose dolphins can discriminate among objects that vary in size, shape, and structure. Our purpose in this project is to investigate the features of echoes dolphins may use to determine each of these object properties (e.g., target strength, number, and position of highlights). A dolphin performed a cross‐modal matching task where he was presented with an object in one modality (e.g., vision) and then asked to choose the same object from among a group of three objects using another modality (e.g., echolocation). The dolphin was presented with object sets in which the objects within the set varied along one feature (size or shape or material or texture). These objects were later ensonified with dolphin‐like clicks and the object echoes were collected with a binaural (two hydrophone) system. To mimic the dolphin’s ability to scan across objects, echoes were captured as the objects were rotated. Differences in echoic features within object sets were examined along with the dolphin’s error. In addition, an artificial neural network (ANN) was used to classify objects using the object echoes. The ANN was utilized to explore the importance of spectral versus time domain features, and single versus multiple object orientations in object classification.

In order to better understand the reasons for geographic variations in the whistles of spinner dolphins (Stenella longirostris), recordings made in the Eastern Tropical Pacific will be compared to those recorded off Midway Atoll, Northwestern Hawai’ian Islands, the island of O’ahu, main Hawai’ian Islands, and Mo’orea, French Polynesia using discriminant function analysis. The values of 10 variables were extracted from the fundamental frequency (contour) of each whistle selected for analysis (begin, end, maximum, minimum and peak frequencies, center and peak times, duration, and number of turns and steps). Geographic variations in the whistles of spinner dolphins were found when comparing animals recorded in waters of Midway Atoll, off the main Hawai’ian Islands, and off Mo’orea. Greater differences were found between the Midway and Hawai’i and Mo’orea groups. The distinctiveness of Midway whistles could be due to geographic isolation and/or differences in population structure (habitat adaptation). The results indicate that geographic differences may not occur solely due to geographic isolation. Other factors, such as population structure and group size may be important when looking at differences in the whistles of spinner groups.

The evoked‐potential (EP) method is effective in studies of hearing capabilities of cetaceans. However, until now EP studies in cetaceans were performed only in conditions of passive hearing by recording EP to external stimuli. Can this method be applied to study active echolocation in odontocetes? To answer this question, auditory brainstem evoked responses (ABR) were recorded in a false killer whale while the animal echolocated a target within an experiment in which the animal reported the target present or absent. The ABR collection was triggered by echolocation clicks. In these conditions, the recorded ABR pattern contained a duplicate set of waves. A comparison of ABR wave delays recorded during echolocation with those recorded during regular external stimulation has shown that the first set of waves is a response to the emitted click whereas the second one is a response to the echo. Both responses, to the emitted click and to the echo, were of comparable amplitude in spite of the intensity differenc...

Fish eating resident killer whales that frequent the coastal waters of Vancouver Island, Canada have a strong preference for chinook salmon. The whales in Johnston Strait often forage along the steep cliffs that extend into the water, echolocating their prey. Echolocation signals were measured with a four hydrophone symmetrical star array and the signals were simultaneous digitized at a sample rate of 500 kHz using a lunch‐box PC. A portable VCR recorded the images from an underwater camera located close to the array center. Only signals emanated from close to the beam axis (1185 total) were chosen for a detailed analysis. Killer whales project very broad band echolocation signals (Q 1.3 to 1.5) that tend to have a bimodal frequency structure. Ninety seven percent of the signals had center frequencies between 45 and 80 kHz with a band‐width between 35 and 50 kHz. The peak‐to‐peak source level of the echolocation signal decreased as a function of the one way transmission loss to the array. Source levels varied between 200 and 225 dB re 1 μPa. Using a model of target strength for chinook salmons, the echo levels from the echolocation signals are estimated for different ranges between whale and salmon.