Beaked whale and dolphin tracking using a multichannel autonomous acoustic recorder

Where's Whaledo: A software toolkit for array localization of animal vocalizations

Where's Whaledo is a software toolkit that uses a combination of automated processes and user interfaces to greatly accelerate the process of reconstructing animal tracks from arrays of passive acoustic recording devices. Passive acoustic localization is a non-invasive yet powerful way to contribute to species conservation. By tracking animals through their acoustic signals, important information on diving patterns, movement behavior, habitat use, and feeding dynamics can be obtained. This method is useful for helping to understand habitat use, observe behavioral responses to noise, and develop potential mitigation strategies. Animal tracking using passive acoustic localization requires an acoustic array to detect signals of interest, associate detections on various receivers, and estimate the most likely source location by using the time difference of arrival (TDOA) of sounds on multiple receivers. Where's Whaledo combines data from two small-aperture volumetric arrays and a variable number of individual receivers. In a case study conducted in the Tanner Basin off Southern California, we demonstrate the effectiveness of Where's Whaledo in localizing groups of Ziphius cavirostris. We reconstruct the tracks of six individual animals vocalizing concurrently and identify Ziphius cavirostris tracks despite being obscured by a large pod of vocalizing dolphins.

Passive acoustic monitoring of sperm whales and anthropogenic noise using stereophonic recordings in the Mediterranean Sea, North West Pelagos Sanctuary

A total of 147 days spread over 4 years were recorded by a stereophonic sonobuoy set up in the Mediterranean sea, near the coast of Toulon, south of France. These recordings were analyzed in the scope of studying sperm whales (Physeter macrocephalus) and the impact anthropic noises may have on this species. With the use of a novel approach, which combines the use of a stereophonic antenna with a neural network, 226 sperm whales’ passages have been automatically detected in an effective range of 32 km. This dataset was then used to analyze the sperm whales’ abundance, the background noise, the influence of the background noise on the acoustic presence, and the animals’ size. The results show that sperm whales are present all year round in groups of 1–9 individuals, especially during the daytime. The estimated density is 1.69 whales/1000 km\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2$$\end{document}2. Animals were also less frequent during periods with an increased background noise due to ferries. The animal size distribution revealed the recorded sperm whales were distributed in length from about 7 to 15.5 m, and lonely whales are larger, while groups of two are composed of juvenile and mid-sized animals.

Tracking time differences of arrivals of multiple sound sources in the presence of clutter and missed detections

Acoustic line transect surveys are often used in combination with visual methods to estimate the abundance of marine mammal populations. These surveys typically use towed linear hydrophone arrays and estimate the time differences of arrival (TDOAs) of the signal of interest between the pairs of hydrophones. The signal source TDOAs or bearings are then tracked through time to estimate the animal position, often manually. The process of estimating TDOAs from data and tracking them through time can be especially challenging in the presence of multiple acoustically active sources, missed detections, and clutter (false TDOAs). This study proposes a multi-target tracking method to automate TDOA tracking. The problem formulation is based on the Gaussian mixture probability hypothesis density filter and includes multiple sources, source appearance and disappearance, missed detections, and false alarms. It is shown that by using an extended measurement model and combining measurements from broadband echolocation clicks and narrowband whistles, more information can be extracted from the acoustic encounters. The method is demonstrated on false killer whale (Pseudorca crassidens) recordings from Hawaiian waters.

Marine health of the Arabian Gulf: Drivers of pollution and assessment approaches focusing on desalination activities

The Arabian Gulf is one of the most adversely affected marine environments worldwide, which results from combined pollution drivers including climate change, oil and gas activities, and coastal anthropogenic disturbances. Desalination activities are one of the major marine pollution drivers regionally and internationally. Arabian Gulf countries represent a hotspot of desalination activities as they are responsible for nearly 50% of the global desalination capacity. Building desalination plants, up-taking seawater, and discharging untreated brine back into the sea adversely affects the biodiversity of the marine ecosystems. The present review attempted to reveal the potential negative effects of desalination plants on the Gulf's marine environments. We emphasised different conventional and innovative assessment tools used to assess the health of marine environments and evaluate the damage exerted by desalination activity in the Gulf. Finally, we suggested effective management approaches to tackle the issue including the significance of national regulations and regional cooperation.

Passive acoustic detection and estimation of the number of sources using compact arrays

The problem of estimating the number of sound-producing sources detected using a compact array of hydrophones is addressed. Closed form expressions representing the techniques of automatic detection and estimation of the number of callers are given. Their performance is evaluated on a year-long dataset (1 October 2015-6 October 2016) containing humpback whale and killer whale calls collected in the Strait of Georgia, near Vancouver, British Columbia. Manual verification of the automatic detections produced by the approach required ∼40 h.

Maximum likelihood estimators and Cramér-Rao bound for estimating azimuth and elevation angles using compact arrays

The problem of estimating the azimuth and elevation angle of a sound source using a compact array of hydrophones is addressed. The closed-form representations for several time-difference of arrival (TDOA) based estimators are given, and their accuracies are evaluated using both statistical simulations and in situ tests. Simulations demonstrated that the accuracy provided by the estimators is close to the Cramér-Rao bounds. In real conditions, the main cause of azimuth and elevation errors can be refraction, surface and bottom reflections and other unpredictable sound propagation effects resulting in large and slowly changing errors.

Passive acoustic methods for fine-scale tracking of harbour porpoises in tidal rapids

The growing interest in generating electrical power from tidal currents using tidal turbine generators raises a number of environmental concerns, including the risk that marine mammals might be injured or killed through collision with rotating turbine blades. To understand this risk, information on how marine mammals use tidal rapid habitats and in particular, their underwater movements and dive behaviour is required. Porpoises, which are the most abundant small cetacean at most European tidal sites, are difficult animals to tag, and the limited size of tidal habitats means that any telemetered animal would be likely to spend only a small proportion of time within them. Here, an alternative approach is explored, whereby passive acoustic monitoring (PAM) is used to obtain fine scale geo-referenced tracks of harbour porpoises in tidal rapid areas. Large aperture hydrophone arrays are required to obtain accurate locations of animals from PAM data and automated algorithms are necessary to process the large quantities of acoustic data collected on such systems during a typical survey. Methods to automate localisation, including a method to match porpoise detections on different hydrophones and separate different vocalising animals, and an assessment of the localisation accuracy of the large aperture hydrophone array are presented.

Delphinid echolocation click detection probability on near-seafloor sensors

The probability of detecting echolocating delphinids on a near-seafloor sensor was estimated using two Monte Carlo simulation methods. One method estimated the probability of detecting a single click (cue counting); the other estimated the probability of detecting a group of delphinids (group counting). Echolocation click beam pattern and source level assumptions strongly influenced detectability predictions by the cue counting model. Group detectability was also influenced by assumptions about group behaviors. Model results were compared to in situ recordings of encounters with Risso's dolphin (Grampus griseus) and presumed pantropical spotted dolphin (Stenella attenuata) from a near-seafloor four-channel tracking sensor deployed in the Gulf of Mexico (25.537°N 84.632°W, depth 1220 m). Horizontal detection range, received level and estimated source level distributions from localized encounters were compared with the model predictions. Agreement between in situ results and model predictions suggests that simulations can be used to estimate detection probabilities when direct distance estimation is not available.

Passive acoustic monitoring of beaked whale densities in the Gulf of Mexico

Beaked whales are deep diving elusive animals, difficult to census with conventional visual surveys. Methods are presented for the density estimation of beaked whales, using passive acoustic monitoring data collected at sites in the Gulf of Mexico (GOM) from the period during and following the Deepwater Horizon oil spill (2010-2013). Beaked whale species detected include: Gervais' (Mesoplodon europaeus), Cuvier's (Ziphius cavirostris), Blainville's (Mesoplodon densirostris) and an unknown species of Mesoplodon sp. (designated as Beaked Whale Gulf - BWG). For Gervais' and Cuvier's beaked whales, we estimated weekly animal density using two methods, one based on the number of echolocation clicks, and another based on the detection of animal groups during 5 min time-bins. Density estimates derived from these two methods were in good general agreement. At two sites in the western GOM, Gervais' beaked whales were present throughout the monitoring period, but Cuvier's beaked whales were present only seasonally, with periods of low density during the summer and higher density in the winter. At an eastern GOM site, both Gervais' and Cuvier's beaked whales had a high density throughout the monitoring period.

Three-dimensional tracking of Cuvier's beaked whales' echolocation sounds using nested hydrophone arrays

Cuvier's beaked whales (Ziphius cavirostris) were tracked using two volumetric small-aperture (∼1 m element spacing) hydrophone arrays, embedded into a large-aperture (∼1 km element spacing) seafloor hydrophone array of five nodes. This array design can reduce the minimum number of nodes that are needed to record the arrival of a strongly directional echolocation sound from 5 to 2, while providing enough time-differences of arrivals for a three-dimensional localization without depending on any additional information such as multipath arrivals. To illustrate the capabilities of this technique, six encounters of up to three Cuvier's beaked whales were tracked over a two-month recording period within an area of 20 km(2) in the Southern California Bight. Encounter periods ranged from 11 min to 33 min. Cuvier's beaked whales were found to reduce the time interval between echolocation clicks while alternating between two inter-click-interval regimes during their descent towards the seafloor. Maximum peak-to-peak source levels of 179 and 224 dB re 1 μPa @ 1 m were estimated for buzz sounds and on-axis echolocation clicks (directivity index = 30 dB), respectively. Source energy spectra of the on-axis clicks show significant frequency components between 70 and 90 kHz, in addition to their typically noted FM upsweep at 40-60 kHz.

Offshore killer whale tracking using multiple hydrophone arrays

To study delphinid near surface movements and behavior, two L-shaped hydrophone arrays and one vertical hydrophone line array were deployed at shallow depths (<125 m) from the floating instrument platform R/P FLIP, moored northwest of San Clemente Island in the Southern California Bight. A three-dimensional propagation-model based passive acoustic tracking method was developed and used to track a group of five offshore killer whales (Orcinus orca) using their emitted clicks. In addition, killer whale pulsed calls and high-frequency modulated (HFM) signals were localized using other standard techniques. Based on these tracks sound source levels for the killer whales were estimated. The peak to peak source levels for echolocation clicks vary between 170-205 dB re 1 μPa @ 1 m, for HFM calls between 185-193 dB re 1 μPa @ 1 m, and for pulsed calls between 146-158 dB re 1 μPa @ 1 m.

Range estimation of cetaceans with compact volumetric arrays

Passive acoustic monitoring is the method of choice to detect whales and dolphins that are acoustically active and to monitor their underwater behavior. The NATO Science and Technology Organization Centre for Maritime Research and Experimentation has recently implemented a compact passive acoustic monitor (CPAM), consisting of three arrays of two hydrophones each that are combined in a fixed three-dimensional arrangement and that may be towed at depths of more than 100 m. With its volumetric configuration, the CPAM is capable of estimating the three-dimensional direction vector of arriving sounds and under certain conditions on relative geometry between the whale and hydrophone array, the CPAM may also estimate the range to echolocating animals. Basic ranging methods assume constant sound speed and apply straightforward geometry to obtain depth and distance to the sound source. Alternatively, ray-tracing based methods may be employed to integrate the information provided by real sound speed profiles. Both ranging methods combine measurements of sound arrival angles and surface reflection delays and are easily implemented in real-time applications, whereby one could promote the ray-tracing approach as the preferred method because it may integrate real sound speed profiles.

Passive acoustic monitoring using a towed hydrophone array results in identification of a previously unknown beaked whale habitat

Beaked whales are diverse and species rich taxa. They spend the vast majority of their time submerged, regularly diving to depths of hundreds to thousands of meters, typically occur in small groups, and behave inconspicuously at the surface. These factors make them extremely difficult to detect using standard visual survey methods. However, recent advancements in acoustic detection capabilities have made passive acoustic monitoring (PAM) a viable alternative. Beaked whales can be discriminated from other odontocetes by the unique characteristics of their echolocation clicks. In 2009 and 2010, PAM methods using towed hydrophone arrays were tested. These methods proved highly effective for real-time detection of beaked whales in the Southern California Bight (SCB) and were subsequently implemented in 2011 to successfully detect and track beaked whales during the ongoing Southern California Behavioral Response Study. The three year field effort has resulted in (1) the successful classification and tracking of Cuvier's (Ziphius cavirostris), Baird's (Berardius bairdii), and unidentified Mesoplodon beaked whale species and (2) the identification of areas of previously unknown beaked whale habitat use. Identification of habitat use areas will contribute to a better understanding of the complex relationship between beaked whale distribution, occurrence, and preferred habitat characteristics on a relatively small spatial scale. These findings will also provide information that can be used to promote more effective management and conservation of beaked whales in the SCB, a heavily used Naval operation and training region.

Methods for tracking multiple marine mammals with wide-baseline passive acoustic arrays

Most methods used to track marine mammals with passive acoustics require that time differences of arrivals (TDOAs) are established and are associated between hydrophone pairs. Consequently, multiple animal trackers commonly apply single-animal TDOA localization methods after performing a call separation and/or TDOA association step. When a wide-baseline array is used with multiple animals that make similar calls with short inter-call-intervals, the separation/association step can be challenging and potentially rejects valid TDOAs. This paper extends a model-based TDOA method to deal with multiple-animal datasets in a way that does not require a TDOA association step; animals are separated based on position. Advantageously, false TDOAs (e.g., a direct path associated with a multipath arrival) do not need to be removed. An analogous development is also presented for a model-based time of arrival tracking method. Results from simulations and application to a multiple sperm whale dataset are used to illustrate the multiple-animal methods. Although computationally more demanding than most track-after-association methods because separation is performed in a higher-dimensional space, the methods are computationally tractable and represent a useful new tool in the suite of options available for tracking multiple animals with passive acoustics.

Tracking dolphin whistles using an autonomous acoustic recorder array

Dolphins are known to produce nearly omnidirectional whistles that can propagate several kilometers, allowing these sounds to be localized and tracked using acoustic arrays. During the fall of 2007, a km-scale array of four autonomous acoustic recorders was deployed offshore of southern California in a known dolphin habitat at ~800 m depth. Concurrently with the one-month recording, a fixed-point marine mammal visual survey was conducted from a moored research platform in the center of the array, providing daytime species and behavior visual confirmation. The recordings showed three main types of dolphin acoustic activity during distinct times: primarily whistling during daytime, whistling and clicking during early night, and primarily clicking during late night. Tracks from periods of daytime whistling typically were tightly grouped and traveled at a moderate rate. In one example with visual observations, traveling common dolphins (Delphinus sp.) were tracked for about 10 km with an average speed of ~2.5 m s(-1) (9 km h(-1)). Early night recordings had whistle localizations with wider spatial distribution and slower travel speed than daytime recordings, presumably associated with foraging behavior. Localization and tracking of dolphins over long periods has the potential to provide insight into their ecology, behavior, and potential response to stimuli.

Effective beam pattern of the Blainville's beaked whale (Mesoplodon densirostris) and implications for passive acoustic monitoring

The presence of beaked whales in mass-strandings coincident with navy maneuvers has prompted the development of methods to detect these cryptic animals. Blainville's beaked whales, Mesoplodon densirostris, produce distinctive echolocation clicks during long foraging dives making passive acoustic detection a possibility. However, performance of passive acoustic monitoring depends upon the source level, beam pattern, and clicking behavior of the whales. In this study, clicks recorded from Digital acoustic Tags (DTags) attached to four M. densirostris were linked to simultaneous recordings from an 82-hydrophone bottom-mounted array to derive the source level and beam pattern of the clicks, as steps towards estimating their detectability. The mean estimated on-axis apparent source level for the four whales was 201 dBrms97. The mean 3 dB beamwidth and directivity index, estimated from sequences of clicks directed towards the far-field hydrophones, were 13° and 23 dB, respectively. While searching for prey, Blainville's beaked whales scan their heads horizontally at a mean rate of 3.6°/s over an angular range of some +/-10°. Thus, while the DI indicates a narrow beam, the area of ensonification over a complete foraging dive is large given the combined effects of body and head movements associated with foraging.

Tracking beaked whales with a passive acoustic profiler float

Acoustic methods are frequently used to monitor endangered marine mammal species. Advantages of acoustic methods over visual ones include the ability to detect submerged animals, to work at night, and to work in any weather conditions. A relatively inexpensive and easy-to-use acoustic float, the QUEphone, was developed by converting a commercially available profiler float to a mobile platform, adding acoustic capability, and installing the ERMA cetacean click detection algorithm of Klinck and Mellinger [(2011). J. Acoust. Soc. Am. 129(4), 1807-1812] running on a high-power DSP. The QUEphone was tested at detecting Blainville's beaked whales at the Atlantic Undersea Test and Evaluation Center (AUTEC), a Navy acoustic test range in the Bahamas, in June 2010. Beaked whale were present at AUTEC, and the performance of the QUEphone was compared with the Navy's Marine Mammal Monitoring on Navy Ranges (M3R) system. The field tests provided data useful to evaluate the QUEphone's operational capability as a tool to detect beaked whales and report their presence in near-real time. The range tests demonstrated that the QUEphone's beaked whale detections were comparable to that of M3R's, and that the float is effective at detecting beaked whales.

Estimating animal population density using passive acoustics

Reliable estimation of the size or density of wild animal populations is very important for effective wildlife management, conservation and ecology. Currently, the most widely used methods for obtaining such estimates involve either sighting animals from transect lines or some form of capture-recapture on marked or uniquely identifiable individuals. However, many species are difficult to sight, and cannot be easily marked or recaptured. Some of these species produce readily identifiable sounds, providing an opportunity to use passive acoustic data to estimate animal density. In addition, even for species for which other visually based methods are feasible, passive acoustic methods offer the potential for greater detection ranges in some environments (e.g. underwater or in dense forest), and hence potentially better precision. Automated data collection means that surveys can take place at times and in places where it would be too expensive or dangerous to send human observers.

Here, we present an overview of animal density estimation using passive acoustic data, a relatively new and fast-developing field. We review the types of data and methodological approaches currently available to researchers and we provide a framework for acoustics-based density estimation, illustrated with examples from real-world case studies. We mention moving sensor platforms (e.g. towed acoustics), but then focus on methods involving sensors at fixed locations, particularly hydrophones to survey marine mammals, as acoustic-based density estimation research to date has been concentrated in this area. Primary among these are methods based on distance sampling and spatially explicit capture-recapture. The methods are also applicable to other aquatic and terrestrial sound-producing taxa.

We conclude that, despite being in its infancy, density estimation based on passive acoustic data likely will become an important method for surveying a number of diverse taxa, such as sea mammals, fish, birds, amphibians, and insects, especially in situations where inferences are required over long periods of time. There is considerable work ahead, with several potentially fruitful research areas, including the development of (i) hardware and software for data acquisition, (ii) efficient, calibrated, automated detection and classification systems, and (iii) statistical approaches optimized for this application. Further, survey design will need to be developed, and research is needed on the acoustic behaviour of target species. Fundamental research on vocalization rates and group sizes, and the relation between these and other factors such as season or behaviour state, is critical. Evaluation of the methods under known density scenarios will be important for empirically validating the approaches presented here.