Tyack Peter L.

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Tyack

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Peter L.

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Reference database marine mammal literature

(Woods Hole Oceanographic Institution, 1988-01) Watkins, William A. ; Bird, James E. ; Moore, Karen E. ; Tyack, Peter L.

A comprehensive Reference Database has been designed for the marine mammal literature. The system uses INMAGIC programming (Cambridge, MA) to file, store, search, retrieve, and format the data records. The database was organized to be complementary to features developed by William E. Schevill for his library of older cetacean literature, and it uses direct association of species with some 300 indexed subjects, observation dates, locations, etc. Every component and detail of the references and annotations are available for rapid search by a wide variety of simple and complex strategies. In addition, separately indexed fields provide immediate retrieval of author, editor, year, journal, type of publication, language, genus/species (searchable by order/suborder and family as well), major subject, subject, picture, observation date, geographic location (including area name and latitude/longitude), as well as the location and library call numbers of the document referred to. Codes have been adapted for ease in identifying and searching species, subjects, journals, languages, and geographic areas. These codes may be used separately or in connection with the associated terms and texts. It is anticipated that the Reference Database will be a continuing resource for marine mammal research.
Sound Cut, Record Number 93570008

(Woods Hole Oceanographic Institution, 1993-11-10) Tyack, Peter L.

Recording of 3-4 sperm whales made by AG-6400, Benthos towed array. Tagged whale. This is the complete coda exchange.
An optical telemetry device to identify which dolphin produces a sound

(Acoustical Society of America, 1985-11) Tyack, Peter L.

A small telemetry device, called a "vocalight," was designed for attachment to a dolphin's head using a suction cup. The vocalight lights up a variable number of light-emitting diodes depending upon the loudness of sounds received at a hydrophone within the suction cup. If vocalights matched for sensitivity are put on each dolphin within a captive group, observers can identify which dolphin produces a vocalization. Use of vocalights indicates that source levels of whistles from captive bottlenosed dolphins, Tursiops truncatus, range from approximately 125 to over 140 dB re: 1 µPa at 1 m.
Dynamic biosonar adjustment strategies in deep-diving Risso's dolphins driven partly by prey evasion

(Company of Biologists, 2019-12-10) Jensen, Frants H. ; Keller, Onno A. ; Tyack, Peter L. ; Visser, Fleur

Toothed whales have evolved flexible biosonar systems to find, track and capture prey in diverse habitats. Delphinids, phocoenids and iniids adjust inter-click intervals and source levels gradually while approaching prey. In contrast, deep-diving beaked and sperm whales maintain relatively constant inter-click intervals and apparent output levels during the approach followed by a rapid transition into the foraging buzz, presumably to maintain a long-range acoustic scene in a multi-target environment. However, it remains unknown whether this rapid biosonar adjustment strategy is shared by delphinids foraging in deep waters. To test this, we investigated biosonar adjustments of a deep-diving delphinid, the Risso's dolphin (Grampus griseus). We analyzed inter-click interval and apparent output level adjustments recorded from sound recording tags to quantify in situ sensory adjustment during prey capture attempts. Risso's dolphins did not follow typical (20logR) biosonar adjustment patterns seen in shallow-water species, but instead maintained stable repetition rates and output levels up to the foraging buzz. Our results suggest that maintaining a long-range acoustic scene to exploit complex, multi-target prey layers is a common strategy amongst deep-diving toothed whales. Risso's dolphins transitioned rapidly into the foraging buzz just like beaked whales during most foraging attempts, but employed a more gradual biosonar adjustment in a subset (19%) of prey approaches. These were characterized by higher speeds and minimum specific acceleration, indicating higher prey capture efforts associated with evasive prey. Thus, tracking and capturing evasive prey using biosonar may require a more gradual switch between multi-target echolocation and single-target tracking.
Manatee (Trichechus manatus) vocalization usage in relation to environmental noise levels

(Acoustical Society of America, 2009-03) Miksis-Olds, Jennifer L. ; Tyack, Peter L.

Noise can interfere with acoustic communication by masking signals that contain biologically important information. Communication theory recognizes several ways a sender can modify its acoustic signal to compensate for noise, including increasing the source level of a signal, its repetition, its duration, shifting frequency outside that of the noise band, or shifting the timing of signal emission outside of noise periods. The extent to which animals would be expected to use these compensation mechanisms depends on the benefit of successful communication, risk of failure, and the cost of compensation. Here we study whether a coastal marine mammal, the manatee, can modify vocalizations as a function of behavioral context and ambient noise level. To investigate whether and how manatees modify their vocalizations, natural vocalization usage and structure were examined in terms of vocalization rate, duration, frequency, and source level. Vocalizations were classified into two call types, chirps and squeaks, which were analyzed independently. In conditions of elevated noise levels, call rates decreased during feeding and social behaviors, and the duration of each call type was differently influenced by the presence of calves. These results suggest that ambient noise levels do have a detectable effect on manatee communication and that manatees modify their vocalizations as a function of noise in specific behavioral contexts.
Diving behavior and fine-scale kinematics of free-ranging Risso's dolphins foraging in shallow and deep-water habitats

(Frontiers Media, 2019-03-12) Arranz, Patricia ; Benoit-Bird, Kelly J. ; Friedlaender, Ari S. ; Hazen, Elliott L. ; Goldbogen, Jeremy A. ; Stimpert, Alison K. ; DeRuiter, Stacy L. ; Calambokidis, John ; Southall, Brandon L. ; Fahlman, Andreas ; Tyack, Peter L.

Air-breathing marine predators must balance the conflicting demands of oxygen conservation during breath-hold and the cost of diving and locomotion to capture prey. However, it remains poorly understood how predators modulate foraging performance when feeding at different depths and in response to changes in prey distribution and type. Here, we used high-resolution multi-sensor tags attached to Risso's dolphins (Grampus griseus) and concurrent prey surveys to quantify their foraging performance over a range of depths and prey types. Dolphins (N = 33) foraged in shallow and deep habitats [seabed depths less or more than 560 m, respectively] and within the deep habitat, in vertically stratified prey features occurring at several aggregation levels. Generalized linear mixed-effects models indicated that dive kinematics were driven by foraging depth rather than habitat. Bottom-phase duration and number of buzzes (attempts to capture prey) per dive increased with depth. In deep dives, dolphins were gliding for >50% of descent and adopted higher pitch angles both during descent and ascents, which was likely to reduce energetic cost of longer transits. This lower cost of transit was counteracted by the record of highest vertical swim speeds, rolling maneuvers and stroke rates at depth, together with a 4-fold increase in the inter-buzz interval (IBI), suggesting higher costs of pursuing, and handling prey compared to shallow-water feeding. In spite of the increased capture effort at depth, dolphins managed to keep their estimated overall metabolic rate comparable across dive types. This indicates that adjustments in swimming modes may enable energy balance in deeper dives. If we think of the surface as a central place where divers return to breathe, our data match predictions that central place foragers should increase the number and likely quality of prey items at greater distances. These dolphins forage efficiently from near-shore benthic communities to depth-stratified scattering layers, enabling them to maximize their fitness.
Vessel noise affects beaked whale behavior : results of a dedicated acoustic response study

(Public Library of Science, 2012-08-03) Pirotta, Enrico ; Milor, Rachael ; Quick, Nicola ; Moretti, David J. ; DiMarzio, Nancy A. ; Tyack, Peter L. ; Boyd, Ian L. ; Hastie, Gordon

Some beaked whale species are susceptible to the detrimental effects of anthropogenic noise. Most studies have concentrated on the effects of military sonar, but other forms of acoustic disturbance (e.g. shipping noise) may disrupt behavior. An experiment involving the exposure of target whale groups to intense vessel-generated noise tested how these exposures influenced the foraging behavior of Blainville’s beaked whales (Mesoplodon densirostris) in the Tongue of the Ocean (Bahamas). A military array of bottom-mounted hydrophones was used to measure the response based upon changes in the spatial and temporal pattern of vocalizations. The archived acoustic data were used to compute metrics of the echolocation-based foraging behavior for 16 targeted groups, 10 groups further away on the range, and 26 nonexposed groups. The duration of foraging bouts was not significantly affected by the exposure. Changes in the hydrophone over which the group was most frequently detected occurred as the animals moved around within a foraging bout, and their number was significantly less the closer the whales were to the sound source. Non-exposed groups also had significantly more changes in the primary hydrophone than exposed groups irrespective of distance. Our results suggested that broadband ship noise caused a significant change in beaked whale behavior up to at least 5.2 kilometers away from the vessel. The observed change could potentially correspond to a restriction in the movement of groups, a period of more directional travel, a reduction in the number of individuals clicking within the group, or a response to changes in prey movement.
Clicking for calamari : toothed whales can echolocate squid Loligo pealeii

(Inter-Research, 2007-11-27) Madsen, Peter T. ; Wilson, M. ; Johnson, Mark P. ; Hanlon, Roger T. ; Bocconcelli, Alessandro ; Aguilar De Soto, Natacha ; Tyack, Peter L.

Squid play an important role in biomass turnover in marine ecosystems and constitute a food source for ~90% of all echolocating toothed whale species. Nonetheless, it has been hypothesized that the soft bodies of squid provide echoes too weak to be detected by toothed whale biosonars, and that only the few hard parts of the squid body may generate significant backscatter. We measured the acoustic backscatter from the common squid Loligo pealeii for signals similar to toothed whale echolocation clicks using an energy detector to mimic the mammalian auditory system. We show that the dorsal target strengths of L. pealeii with mantle lengths between 23 and 26 cm fall in the range from –38 to –44 dB, and that the pen, beak and lenses do not contribute significantly to the backscatter. Thus, the muscular mantle and fins of L. pealeii constitute a sufficient sonar target for individual biosonar detection by toothed whales at ranges between 25 and 325 m, depending on squid size, noise levels, click source levels, and orientation of the ensonified squid. While epipelagic squid must be fast and muscular to catch prey and avoid visual predators, it is hypothesized that some deep-water squid may have adopted passive acoustic crypsis, with a body of low muscle mass and low metabolism that will render them less conspicuous to echolocating predators.
Convergence of calls as animals form social bonds, active compensation for noisy communication channels, and the evolution of vocal learning in mammals

( 2007-09-12) Tyack, Peter L.

The classic evidence for vocal production learning involves imitation of novel, often anthropogenic sounds. Among mammals, this has been reported for African elephants, harbor seals, and dolphins. A broader taxonomic distribution has been reported for vocal convergence, where the acoustic properties of calls from different individuals converge when they are housed together in captivity or form social bonds in the wild. This kind of vocal convergence has been demonstrated for animals as diverse as songbirds, parakeets, bats, elephants, cetaceans, and primates. For most of these species, call convergence is thought to reflect a group-distinctive identifier, with shared calls reflecting and strengthening social bonds. Pooling data on vocal imitation and vocal convergence suggests a wider taxonomic distribution of vocal production learning among mammals than generally appreciated. The wide taxonomic distribution of this evidence for vocal production learning suggests that perhaps more of the neural underpinnings for vocal production learning are in place in mammals than is usually imagined. One ubiquitous function for vocal production learning that is starting to receive attention involves modifying signals to improve communication in a noisy channel.
Comparing call-based versus subunit-based methods for categorizing Norwegian killer whale, Orcinus orca, vocalizations

( 2010-08-13) Shapiro, Ari D. ; Tyack, Peter L. ; Seneff, Stephanie

Students of animal communication face significant challenges when deciding how to categorise calls into subunits, calls, and call series. Here, we use algorithms designed to parse human speech to test different approaches for categorising calls of killer whales. Killer whale vocalisations have traditionally been categorised by humans into discrete call types. These calls often contain internal spectral shifts, periods of silence, and synchronously produced low and high frequency components, suggesting that they may be composed of subunits. We describe and compare three different approaches for modelling Norwegian killer whale calls. The first method considered the whole call as the basic unit of analysis. Inspired by human speech processing techniques, the second and third methods represented the calls in terms of subunits. Subunits may provide a more parsimonious approach to modelling the vocal stream since (1) there were fewer subunits than call types; (2) nearly 75% of all call types shared at least one subunit. We show that contour traces from stereotyped Norwegian killer whale calls yielded similar automatic classification performance using either whole calls or subunits. We also demonstrate that subunits derived from Norwegian stereotyped calls were detected in some Norwegian variable (non-stereotyped) calls as well as the stereotyped calls of other killer whale populations. Further work is required to test whether killer whales use subunits to generate and categorize their vocal repertoire.
Human-generated sound and marine mammals

(American Institute of Physics, 2009-11) Tyack, Peter L.

Most species of large whales are endangered because for centuries whaling fleets have decimated their populations. In the late 1960s, marine-mammal biologists discovered that fishermen setting nets for tuna in the Pacific Ocean were killing more than 100,000 dolphins a year. The cause of marine-mammal conservation became so popular at the dawn of the environmental movement that one of the first environmental accomplishments of the US Congress was to enact the Marine Mammal Protection Act of 1972, which prohibits the killing or injuring of marine mammals. Today, small remnant populations of whales, such as the North Atlantic right whale, are threatened by entanglement in fishing gear and collisions by ships. Indeed, marine biologists have estimated that hundreds of thousands of marine mammals are killed each year in fishing gear. Inadvertent effects of human activities can pose a serious risk to coastal populations, as evidenced by the recent extinction of the Chinese river dolphin due to fishing, pollution, and overdevelopment of the Yangtze River. A few decades ago, conservation efforts focused on reducing the intentional hunting of marine mammals. Nowadays, when hunts for marine mammals are better controlled, the slow degradation of habitat from a combination of sources may have a bigger impact. For example, biologists have documented cases in which the effects of coastal development—including noise, pollution, and dredging—have caused marine mammals to abandon critical breeding habitat. Noise in particular is at issue in legal actions that have been brought against the US Navy for sonar exercises that may have caused whales to strand and die.
Postpartum whistle production in bottlenose dolphins

( 2007-11) Fripp, Deborah R. ; Tyack, Peter L.

Despite much research on bottlenose dolphin signature whistles, few have investigated the role of maternal whistles in early calf development. We investigated maternal whistle use in the first weeks postpartum for captive dolphins. The overall whistling rate increased by a factor of ten when the calves were born and then decreased again in the third week of the one surviving calf. Adult whistles were distinguished from calf whistles based on the extent of frequency modulation and were further classified into signature and non-signature whistles by comparison to a dictionary of known whistles. The average rate of maternal signature whistle production increased significantly from 0.02 whistles per dolphin-minute before the calves were born to 0.2 and 0.3 whistles in weeks 1 and 2, decreasing again to 0.06 in week 3 for the mother of the surviving calf. Percent maternal signature whistles changed similarly. Signature whistle production by non-mothers did not change when the calves were born. A likely function of this increase in maternal signature whistle production is that it enables the calf to learn to identify the mother in the first weeks of life.
Understanding the combined effects of multiple stressors: a new perspective on a longstanding challenge

(Elsevier, 2022-01-29) Pirotta, Enrico ; Thomas, Len ; Costa, Daniel P. ; Hall, Ailsa J. ; Harris, Catriona M. ; Harwood, John ; Kraus, Scott D. ; Miller, Patrick J. O. ; Moore, Michael J. ; Photopoulou, Theoni ; Rolland, Rosalind M. ; Schwacke, Lori ; Simmons, Samantha E. ; Southall, Brandon L. ; Tyack, Peter L.

Wildlife populations and their habitats are exposed to an expanding diversity and intensity of stressors caused by human activities, within the broader context of natural processes and increasing pressure from climate change. Estimating how these multiple stressors affect individuals, populations, and ecosystems is thus of growing importance. However, their combined effects often cannot be predicted reliably from the individual effects of each stressor, and we lack the mechanistic understanding and analytical tools to predict their joint outcomes. We review the science of multiple stressors and present a conceptual framework that captures and reconciles the variety of existing approaches for assessing combined effects. Specifically, we show that all approaches lie along a spectrum, reflecting increasing assumptions about the mechanisms that regulate the action of single stressors and their combined effects. An emphasis on mechanisms improves analytical precision and predictive power but could introduce bias if the underlying assumptions are incorrect. A purely empirical approach has less risk of bias but requires adequate data on the effects of the full range of anticipated combinations of stressor types and magnitudes. We illustrate how this spectrum can be formalised into specific analytical methods, using an example of North Atlantic right whales feeding on limited prey resources while simultaneously being affected by entanglement in fishing gear. In practice, case-specific management needs and data availability will guide the exploration of the stressor combinations of interest and the selection of a suitable trade-off between precision and bias. We argue that the primary goal for adaptive management should be to identify the most practical and effective ways to remove or reduce specific combinations of stressors, bringing the risk of adverse impacts on populations and ecosystems below acceptable thresholds.
Signal-specific amplitude adjustment to noise in common bottlenose dolphins (Tursiops truncatus)

(Company of Biologists, 2019-11-08) Kragh, Ida M. ; McHugh, Katherine ; Wells, Randall S. ; Sayigh, Laela S. ; Janik, Vincent M. ; Tyack, Peter L. ; Jensen, Frants H.

Anthropogenic underwater noise has increased over the past century, raising concern about the impact on cetaceans that rely on sound for communication, navigation and locating prey and predators. Many terrestrial animals increase the amplitude of their acoustic signals to partially compensate for the masking effect of noise (the Lombard response), but it has been suggested that cetaceans almost fully compensate with amplitude adjustments for increasing noise levels. Here, we used sound-recording DTAGs on pairs of free-ranging common bottlenose dolphins (Tursiops truncatus) to test (i) whether dolphins increase signal amplitude to compensate for increasing ambient noise and (ii) whether adjustments are identical for different signal types. We present evidence of a Lombard response in the range 0.1–0.3 dB per 1 dB increase in ambient noise, which is similar to that of terrestrial animals, but much lower than the response reported for other cetaceans. We found that signature whistles tended to be louder and with a lower degree of amplitude adjustment to noise compared with non-signature whistles, suggesting that signature whistles may be selected for higher output levels and may have a smaller scope for amplitude adjustment to noise. The consequence of the limited degree of vocal amplitude compensation is a loss of active space during periods of increased noise, with potential consequences for group cohesion, conspecific encounter rates and mate attraction.
Auditory oddball responses in Tursiops truncatus

(Acoustical Society of America, 2021-08-27) Schalles, Matt D. ; Mulsow, Jason ; Houser, Dorian S. ; Finneran, James J. ; Tyack, Peter L. ; Shinn-Cunningham, Barbara

Two previous studies suggest that bottlenose dolphins exhibit an “oddball” auditory evoked potential (AEP) to stimulus trains where one of two stimuli has a low probability of occurrence relative to another. However, they reported oddball AEPs at widely different latency ranges (50 vs 500 ms). The present work revisited this experiment in a single dolphin to report the AEPs in response to two tones each assigned probabilities of 0.2, 0.8, and 1 across sessions. The AEP was further isolated from background EEG using independent component analysis, and showed condition effects in the 40–60 ms latency range.
Passive acoustic detection of deep-diving beaked whales

(Acoustical Society of America, 2008-11) Zimmer, Walter M. X. ; Harwood, John ; Tyack, Peter L. ; Johnson, Mark P. ; Madsen, Peter T.

Beaked whales can remain submerged for an hour or more and are difficult to sight when they come to the surface to breathe. Passive acoustic detection (PAD) not only complements traditional visual-based methods for detecting these species but also can be more effective because beaked whales produce clicks regularly to echolocate on prey during deep foraging dives. The effectiveness of PAD for beaked whales depends not only on the acoustic behavior and output of the animals but also on environmental conditions and the quality of the passive sonar implemented. A primary constraint on the range at which beaked whale clicks can be detected involves their high frequencies, which attenuate rapidly, resulting in limited ranges of detection, especially in adverse environmental conditions. Given current knowledge of source parameters and in good conditions, for example, with a wind speed of 2 m/s, a receiver close to the surface should be able to detect acoustically Cuvier's beaked whales with a high probability at distances up to 0.7 km, provided the listening duration exceeds the deep dive interval, about 2.5 h on average. Detection ranges beyond 4 km are unlikely and would require low ambient noise or special sound propagation conditions.
Estimating cetacean population density using fixed passive acoustic sensors : an example with Blainville's beaked whales

(Acoustical Society of America, 2009-04) Marques, Tiago A. ; Thomas, Len ; Ward, Jessica ; DiMarzio, Nancy A. ; Tyack, Peter L.

Methods are developed for estimating the size/density of cetacean populations using data from a set of fixed passive acoustic sensors. The methods convert the number of detected acoustic cues into animal density by accounting for (i) the probability of detecting cues, (ii) the rate at which animals produce cues, and (iii) the proportion of false positive detections. Additional information is often required for estimation of these quantities, for example, from an acoustic tag applied to a sample of animals. Methods are illustrated with a case study: estimation of Blainville's beaked whale density over a 6 day period in spring 2005, using an 82 hydrophone wide-baseline array located in the Tongue of the Ocean, Bahamas. To estimate the required quantities, additional data are used from digital acoustic tags, attached to five whales over 21 deep dives, where cues recorded on some of the dives are associated with those received on the fixed hydrophones. Estimated density was 25.3 or 22.5 animals/1000 km2, depending on assumptions about false positive detections, with 95% confidence intervals 17.3–36.9 and 15.4–32.9. These methods are potentially applicable to a wide variety of marine and terrestrial species that are hard to survey using conventional visual methods.
Beaked whales echolocate on prey

(Royal Society, 2004-12-07) Johnson, Mark P. ; Madsen, Peter T. ; Zimmer, Walter M. X. ; Aguilar De Soto, Natacha ; Tyack, Peter L.

Beaked whales (Cetacea: Ziphiidea) of the genera Ziphius and Mesoplodon are so difficult to study that they are mostly known from strandings. How these elusive toothed whales use and react to sound is of concern because they mass strand during naval sonar exercises. A new non-invasive acoustic recording tag was attached to four beaked whales (two Mesoplodon densirostris and two Ziphius cavirostris) and recorded high-frequency clicks during deep dives. The tagged whales only clicked at depths below 200 m, down to a maximum depth of 1267 m. Both species produced a large number of short, directional, ultrasonic clicks with no significant energy below 20 kHz. The tags recorded echoes from prey items; to our knowledge, a first for any animal echolocating in the wild. As far as we are aware, these echoes provide the first direct evidence on how free-ranging toothed whales use echolocation in foraging. The strength of these echoes suggests that the source level of Mesoplodon clicks is in the range of 200-220 dB re 1 μPa at 1 m. This paper presents conclusive data on the normal vocalizations of these beaked whale species, which may enable acoustic monitoring to mitigate exposure to sounds intense enough to harm them.
Noise level correlates with manatee use of foraging habitats

(Acoustical Society of America, 2007-05) Miksis-Olds, Jennifer L. ; Donaghay, Percy L. ; Miller, James H. ; Tyack, Peter L. ; Nystuen, Jeffrey A.

The introduction of anthropogenic sound to coastal waters is a negative side effect of population growth. As noise from boats, marine construction, and coastal dredging increases, environmental and behavioral monitoring is needed to directly assess the effect these phenomena have on marine animals. Acoustic recordings, providing information on ambient noise levels and transient noise sources, were made in two manatee habitats: grassbeds and dredged habitats. Recordings were made over two 6-month periods from April to September in 2003 and 2004. Noise levels were calculated in one-third octave bands at nine center frequencies ranging from 250 Hz to 64 kHz. Manatee habitat usage, as a function of noise level, was examined during four time periods: morning, noon, afternoon, and night. Analysis of sightings data in a variety of grassbeds of equal species composition and density indicate that manatees select grassbeds with lower ambient noise for frequencies below 1 kHz. Additionally, grassbed usage was negatively correlated with concentrated boat presence in the morning hours; no correlation was observed during noon and afternoon hours. This suggests that morning boat presence and its associated noise may affect the use of foraging habitat on a daily time scale.
Classification of broadband echoes from prey of a foraging Blainville's beaked whale

(Acoustical Society of America, 2008-03) Jones, Benjamin A. ; Stanton, Timothy K. ; Lavery, Andone C. ; Johnson, Mark P. ; Madsen, Peter T. ; Tyack, Peter L.

Blainville's beaked whales (Mesoplodon densirostris) use broadband, ultrasonic echolocation signals with a −10 dB bandwidth from 26 to 51 kHz to search for, localize, and approach prey that generally consist of mid-water and deep-water fishes and squid. Although it is well known that the spectral characteristics of broadband echoes from marine organisms vary as a function of size, shape, orientation, and anatomical group, there is little evidence as to whether or not free-ranging toothed whales use spectral cues in discriminating between prey and nonprey. In order to study the prey-classification process, a stereo acoustic tag was deployed on a Blainville's beaked whale so that emitted clicks and the corresponding echoes from targets in the water could be recorded. A comparison of echoes from targets apparently selected by the whale and those from a sample of scatterers that were not selected suggests that spectral features of the echoes, target strengths, or both may have been used by the whale to discriminate between echoes. Specifically, the whale appears to favor targets with one or more nulls in the echo spectra and to seek prey with higher target strengths at deeper depths.

Tyack Peter L.

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