Automated classification of dolphin echolocation click types from the Gulf of Mexico
[['Dolphins produce echolocation clicks while socializing , foraging and traveling [1] .', 'The prevalence of echolocation clicks makes these signals useful for monitoring delphinids using passive acoustic methods; however , only a few delphinid click types can currently be classified to species .', 'Echolocation clicks have a suite of characteristics that make them challenging to classify in acoustic recordings .', 'For example , echolocation clicks are highly directional signals which can be received “on-axis” ( animal oriented in the direction of the recording sensor while clicking ) or “off-axis” ( animal oriented away from the sensor ) , leading to differences in amplitude and interference patterns [2] .', 'Dolphin echolocation click signals also significantly attenuate over relatively short distances due to their high frequency acoustic content; therefore , the orientation and proximity of a clicking animal relative to an acoustic sensor has a large effect on the frequency structure of the recorded click [3 , 4] .', 'Behaviorally , individual dolphins may vary click source levels and beam widths [5–8] .', 'Furthermore , dolphins are typically found in large , sometimes multi-species groups in which animals vocalize simultaneously .', 'All of these factors contribute to click variability and therefore complexity in classification .', 'Despite these sources of variability , echolocation clicks of a few delphinid species as well as many beaked whale species have known species-specific spectral features [9–12] .', 'Consistent features have typically been recognized by expert analysts manually reviewing large amounts of data .', 'Previously identified characteristic spectral features include mean frequency , bandwidth , and peaks or troughs in frequency spectra indicating dominant or diminished frequencies .', 'Typical inter-click interval ( ICI ) ranges also differ between beaked whale species [13] , and ICI is used to identify porpoise click trains to species [14 , 15] , although ICI may vary as a function of depth or behavior in some cases [1 , 16 , 17] .', 'A challenge in echolocation click classification is to overcome the many sources of variability to recognize underlying consistent patterns .', 'One approach is to train analysts to recognize patterns .', 'Humans are particularly adept at pattern recognition tasks: With enough training time , contextual information and training data , an analyst can distinguish within-type and between-type click variations , and develop a sense of the major click categories in a dataset .', 'However this is an iterative , time-consuming and potentially subjective process .', 'An alternative is to develop automated methods to perform echolocation click classification .', 'Within a computational framework , one approach to the click variability problem is to consider a set of clicks as a group of objects that are similar but not identical to one another .', 'In a simple example with five clicks labeled A through E , consider a case where clicks A , B and C are very similar , click D is slightly different , and click E is very different than A-C , with some similarity to D . In this case , clicks A , B and C are regarded as the most informative for classification , as they contain consistent features among them , while clicks D and E are likely outliers .', 'We might consider A , B and C to be members of a group characterized by their common feature set .', 'In practice , an actively echolocating dolphin produces multiple clicks per second .', 'Therefore , a similar but more complex case exists in which a subset of those clicks will be highly interrelated , while others are only weakly associated .', 'This approach to the variability problem can be represented as a weighted network [18] , in which clicks are represented by nodes and the lines or edges between nodes represent the strength of the similarity between them .', 'In the example above of echolocation clicks A through E , the click characteristic inter-relationships are represented by a network with larger edge weights among similar clicks A-C and lower value edge weights among clicks D and E and their neighbors which show their greater dissimilarity from clicks A-C and each other ( Fig 1 ) .', 'A network of N nodes can also be represented as an adjacency matrix G in which G ( i , j ) represents the weight of the edge between nodes i and j , for i and j ∈ the set of nodes N [19] .', 'Once the relationships between a set of clicks are represented as a network , an unsupervised learning algorithm can be used to identify clusters of highly similar clicks .', 'Here we use an agglomerative clustering routine [20] that seeks to identify structure within the network without a priori information about what that structure might be .', 'Using this method , nodes within the network are iteratively grouped together based on the strengths of the edges between them .', 'This method can converge to a single large cluster if all nodes are highly interrelated , but multiple clusters can be identified if interrelationships are not evenly spread across the network .', 'In this work , unsupervised network-based classification methods are applied to the problem of delphinid echolocation click classification in the Gulf of Mexico ( GOM ) .', 'Long-term passive acoustic monitoring efforts using autonomous near-seafloor hydrophones at five sites in the GOM have resulted in a dataset of over 52 million unlabeled dolphin echolocation clicks .', 'Thirteen delphinid species are known to inhabit the GOM , including five members of the genus Stenella , and five species belonging to the subfamily Globicephalinae ( Table 1 ) .', 'Three of these five species , Risso’s dolphin ( Grampus griseus ) , false killer whale ( Pseudorca crassidens ) and short-finned pilot whale ( Globicephala macrorynchus ) can be distinguished based on echolocation click characteristics [11 , 21]; however , few other species have been conclusively identified .', 'Our objectives are to develop a technique for recognizing candidate click types in this dataset which may be associated with species that are not yet acoustically identifiable , and to demonstrate a method for recognizing these click types automatically in novel data .', 'Further , we support the utility of this method by comparing automatically identified types with clicks recorded using towed hydrophone arrays in the presence of vocalizing animals from the western Atlantic whose species identity has been verified by trained visual observers .', 'The described click types are informative for passive acoustic delphinid population monitoring efforts , while the methods offer an approach for automated classification of variable signals in large unlabeled acoustic datasets .'], ['Long term passive acoustic recordings were collected at three continental slope sites ( sites MC , GC , and DT ) , and two shelf sites ( sites DC and MP ) .', 'Delphinid clicks were automatically detected in large numbers during all deployments at each site , with click counts ranging from 5 . 2x105 to over 8 . 1x106 analyst-confirmed detections per deployment ( between 6 , 000 and 67 , 000 clicks per day; Table 2 ) .', 'Detections were grouped into 5-minute bins marked as click-positive or negative .', 'The number of click-positive 5-minute bins per deployment varied from almost 5 , 000 to close to 12 , 000 bins ( unnormalized for recording effort ) .', 'The average number of delphinid echolocation encounters ( periods of continuous click detections bounded before and after by at least 15 minutes without click detections ) per recording day ranged from 1 . 4 to 7 . 9 across deployments .', 'Average encounter durations were generally shorter at the shelf sites MP and DC; however , encounter durations were highly variable at all sites and ranged from 1 to 640 minutes .', 'Across all deployments , between 0 . 1% and 10 . 1% of click-positive bins contained more than 5000 clicks and were sub-sampled for classification purposes .', 'The most sub-sampled site was site DT .', 'Phases 1 and 2 were run on the full training set following the exploratory analysis .', 'In Phase 1 , the average number of automatically identified clusters per time bin ranged from 1 . 02 to 1 . 14 ( CV = 0 . 14 and 0 . 35 respectively ) across sites and deployments ( Table 2 ) .', 'In Phase 2 , seven dominant and recurrent click types ( A-G ) characterized by consistent spectral shapes and modal ICIs were identified ( Table 3 , Fig 3 ) .', 'We define the modal ICI as the most frequently observed ICI during a period of clicking .', 'Click type A was identified in the training data from the three deep sites , and one shallow site .', 'Most instances came from site GC .', 'This type was characterized by a minor narrow low frequency peak near 12 kHz , dominant energy between 20 and 35 kHz , and 0 . 15 sec modal ICI .', 'Click type B was identified in the training data from all sites except site GC .', 'This click type , presumed to be Risso’s dolphin based on Soldevilla et al . [11] and has distinct narrow energy peaks at ~ 22 , 26 , and 33 kHz .', 'The ICI distribution for this type was bimodal with shorter ICIs near 0 . 12 sec at the northern sites , and longer ICIs over 0 . 23 sec at the southern site DT .', 'Click type C was identified in the training data from the deep sites only .', 'This click type had the lowest frequency content of dominant energy between ~15 and 30 kHz , and a modal ICI of 0 . 16 sec .', 'Click type D was identified in the training data from site DC , and in one bin from site MP .', 'This click type was characterized by two low frequency peaks at 12 and 18 kHz , dominant energy between 30 and 50 kHz , and a bimodal modal ICI with peaks at 0 . 03 and 0 . 09 sec .', 'Click type E was identified in the training data from all five sites and represented 22% of the training set .', 'It was particularly common at the southern site DT .', 'Click type E was characterized by minimal energy below 20 kHz , a dominant spectral peak near 30 kHz , and a modal ICI of 0 . 06 sec .', 'Spectral variability below 20 kHz may indicate the presence of multiple subtypes , or overlap with click type F . Click type F was identified in the training data from all five sites and represented 47% of the training set .', 'This type was similar to type E , had a minor energy peak at approximately 20 kHz .', 'Some examples had a third peak at 16 kHz .', 'High variability of this type in the 10–25 kHz band suggests that it may include multiple subtypes .', 'This type had a strong modal ICI at 0 . 06 sec .', 'Click type G was only identified in the training data from the two shallow sites only: Sites DC and MP .', 'It was characterized by a broad high energy band between 15 and 52 kHz , and a peak frequency of 36 kHz and a modal ICI of 0 . 03 sec .'], ['Delphinid clicks are very short duration , highly variable signals which contain limited information when considered individually .', 'The automated clustering strategy was designed to mimic a human analyst by comparing large numbers of clicks to identify persistent features .', 'Leveraging multiple sources of information such as spectral shape and ICI distributions across bins of similar clicks further facilitated pattern recognition and click type distinction .', 'The two-step training process tackled the large dataset by reducing the number of comparisons necessary through use of filtered means and modes .', 'A variety of different pruning and clustering techniques were implemented during the algorithm development process .', 'In the final implementation , edge pruning was executed using a dynamic metric in which the weakest N% of edges were pruned from each network .', 'Using this approach , networks of highly similar nodes and networks of weakly similar nodes were pruned by the same amount .', 'An alternate approach would be to prune all edges weaker than a static threshold value .', 'Using the static approach , a network of weakly interrelated nodes would be pruned more heavily than a network of strongly interrelated nodes .', 'Both approaches were tested during development of the clustering protocol , but the dynamic metric was ultimately chosen as the more conservative pruning method for preserving click types with smaller sample sizes .', 'More aggressive pruning at site MP might reduce inclusion of false positives associated with snapping shrimp and improve classification accuracy if snap spectra are more variable than click spectra .', 'A more complex , greedy clustering algorithm [modularity; 24 , 25] , preliminarily used during the development process , was not able to reliably identify clusters of different sizes .', 'The simpler CW algorithm used in the final implementation identified both small and large clusters within a network , which is essential in identifying less common click types .', 'Further click type separation may be possible however .', 'In this dataset , some click types had very different spectral shapes and ICIs from one another such as type A and B clicks , while others were similar , such as type E and F clicks .', 'This is a challenging situation for clustering purposes , because some types separate well , while others remain intermingled , as in the case of types E and F where spectral variability may represent multiple sub-types .', 'In Phase 2 , a multi-pass clustering approach in which thresholds were incrementally increased might enable better distinction between similar types such as those within type E without over-pruning highly distinct types .', 'Reduced within-cluster variability would probably also reduce classifier confusion and improve accuracy .', 'ICI and spectral similarities ( both values between 0 and 1 ) , were combined in Phase 2 of the automated classification process by simple multiplication .', 'The multiplicative rule was used because analysts typically needed both robust ICI and spectral information to make a confident classification .', 'The two metrics did not necessarily contribute equally to the overall similarity scores because although they are both values between [0 , 1] , they did not have identical distributions .', 'Transforming the distribution of either parameter prior to multiplication would modify the influence of the parameter on the Phase 2 network .', 'For example , if spectra were deemed more reliable than ICI , SSPEC could be transformed prior to Eq ( 2 ) to give it more influence on the network .', 'For classification of the test set , the multiplication method requires that both score high to achieve a high overall similarity score .', 'An earlier implementation of this algorithm used correlation distance between ICI distributions instead of distance between modal ICIs .', 'This strategy produced similar results but performance suffered when classifying bins with high click counts .', 'As the number of detections per bin increased , click trains tended to become interleaved , resulting in higher numbers of low ICIs .', 'While true ICIs from a single animal’s click train may be a species-specific feature [26] , the interval between clicks received from multiple individuals’ trains is not informative .', 'Similarly , high false positive rates associated with snapping shrimp at site MP affected ICI distributions .', 'Modal ICI , which likely represents individuals’ ICIs , was found to be less sensitive to differences in click counts per bin and more robust to false positives .', 'Modal ICI may be more difficult to detect for species that are often found in very large groups .', 'The unsupervised click classification routine identified seven distinct delphinid click types in the training data across five sites in the Gulf of Mexico based on frequency content and modal ICI .', 'All types were identified at a minimum of two sites , and over half were identified at four or more sites .', 'One hypothesis of what is driving the persistent features leading to the click type clusters is site-specific propagation and noise conditions; however , a number of features demonstrated here are inconsistent with this explanation .', 'First , site-specific noise and propagation do not explain why multiple click types were found at each site , often within the same day or in overlapping encounters , nor do they explain why the same click types were found at multiple sites , despite differences in noise , site depth , and site location .', 'Second , site-specific propagation and noise would be expected to affect all clicks in the same way; therefore , they do not explain why some click types have complex spectra with peaks and troughs , or why frequency distributions differ between types under similar noise conditions .', 'Third , site-specific conditions do not offer an explanation for the consistent relationships between click type spectral shape and ICI distributions across deployments spanning multiple years , or why ICI distributions have consistent modal values .', 'Alternative hypotheses are that the distinct click types identified in this dataset represent different dolphin species or echolocation clicks used in different contexts [e . g . 27] .', 'Species differences may explain these observations .', 'Echolocation click frequency content and click rates have been shown to differ between odontocetes such as sperm whales , beaked whales , dolphins , and porpoises [e . g . 11 , 12 , 13 , 28]; therefore , it is reasonable to expect that these features may also differ between delphinid genera and/or species .', 'Consistent ICIs have been reported for beaked whale species [e . g . 13] and similar consistency may be typical of some delphinids [29] .', 'Spectral content may vary depending on target prey [9] , and ICI may be related to click source level , frequency content , and/or prey search distance [e . g . 30 , 31] .', 'Low frequency , high amplitude clicks have the potential to propagate farther than high frequency or low amplitude clicks .', 'This may result in a longer two-way travel time for each click .', 'Delphinids may compensate with a longer ICI to allow for the longer travel times .', 'The majority of clicks detected at the three deepest sites were associated with types E and F which had similar spectral shapes and modal ICIs .', 'According to the most recent NOAA stock assessments [22 , 23] based on summer visual surveys , approximately 80% of offshore delphinids in the GOM are members of the Stenella genus , of which spinner and pantropical spotted dolphins are the most common species .', 'Two additional Stenellid species , striped and Clymene dolphins , are also found offshore , although population estimates vary widely between surveys .', 'A fifth species , Atlantic spotted dolphin , is found primarily on the continental shelf .', 'Based on the high abundance of Stenellids as a proportion of GOM delphinids , Stenellid dolphins are the most likely match for type E and F clicks .', 'Considerable variability below 20 kHz within sites in the type E and F clusters suggests that they may include multiple subtypes , possibly representing different species .', 'Towed hydrophone array recordings made in the presence of pantropical and Atlantic spotted dolphins revealed ICIs that were consistent with type E and F clicks .', 'Distributions of the various Stenellid species differ in the GOM [32] , and this may account for the different ratios of these types across sites .', 'Based on visual survey data , species composition and abundance is expected to differ between the three deeper slope sites ( GC , MC , and DT ) and two shallower shelf sites ( MP and DC ) .', 'Primary species at the shallow sites include Atlantic spotted dolphin ( also a member of the genus Stenella ) and bottlenose dolphin [32] .', 'Rough-toothed dolphins have also been observed near site DC , although in much lower numbers .', 'Click type G which was common at the two shallow sites but was not identified at deeper locations , and click type D which was predominantly identified at site DC , are likely associated with some of these species .', 'Snapping shrimp snaps were a common source of false positives at site MP , where click type G was primarily detected .', 'Distributions associated with this click type may have been contaminated by snap signals .', 'In future work , click train tracking could be used to improve ICI estimates in noisy , shallow water environments , and encounters with very high click counts .', 'Click Type B likely represents Risso’s dolphin clicks as it contains the consistent peaks and notches described for Risso’s dolphins in the Southern California Bight , and further matches the peak structure documented here from a towed array recording of visually-verified Risso’s dolphins from the western Atlantic .', 'Modal ICI differed between the three northern sites ( MC , DC , and MP ) and the southern site ( DT ) , suggesting possible behavioral or population differences .', 'Click type A may represent short-finned pilot whale clicks as it is similar to Atlantic pilot whale ( presumed short-finned ) recordings collected using towed hydrophone arrays .', 'However , it differs from a recent description of Pacific short-finned pilot whale clicks which found spectral peaks at 12 and 18 kHz collected in the Hawaiian Islands [21] .', 'Click type A was most common at site GC in this dataset , which is consistent with short-finned pilot whales’ predominantly eastern GOM distribution based on visual surveys [32] .', 'The narrower bandwidth of click type C centered at lower frequencies is consistent with published descriptions of false killer whale ( Pseudorca crassidens ) echolocation clicks [9 , 21] from the Eastern Pacific .', 'However , there are no published estimates of modal ICI for false killer whales .', 'Across all sites , 1 . 3% of bins were classified as Type C . The most recent stock assessment estimates place false killer whales as approximately 1% of offshore GOM delphinids .', 'Melon-headed whales are expected in low densities the GOM , but information regarding distinguishing features of these clicks is limited [12] , and no clear match was identified .', 'Killer whale , pygmy killer whale and Fraser’s dolphin , although present in the GOM , may be too rare at these sites to be identified using these methods [23] .', 'Use of a larger training set with a multi-pass strategy in which dominant types , such as E and F , were identified and removed could facilitate recognition of rare types .', 'A subset of the identified click types had characteristics in common with clicks recorded in the presence of visually-identified species recorded using the towed hydrophone array .', 'Unfortunately , with the exception of the pantropical spotted dolphin data , these recordings were collected in the Atlantic and can only be tentatively compared with GOM click types .', 'Towed array hydrophones are typically much shallower than seafloor instruments , therefore the effect of acoustic propagation on recorded signals differs .', 'Further work will seek to solidify and extend comparisons between seafloor sensor types and towed array recordings of known species , with an emphasis on collecting recordings of visually identified species in the GOM .', 'The towed array environment is different from that of the seafloor sensor .', 'Towed array recordings are much more affected by vessel , ship-based electronic and wind-generated sea-surface noise , and shallow sound-speed profiles than autonomous seafloor recordings .', 'The orientations of animals relative to the sensors differ between the two types of recordings , for example , during a ship survey , dolphins are often oriented toward the bow , while the sensor is towed behind the vessel; whereas seafloor instruments are located below dolphins traveling near the sea surface , and do not typically influence dolphin orientations .', 'Animal behaviors likely differ as well because marine mammal surveys require daylight for visual marine mammal identification , but seafloor sensor recordings typically show that most delphinid clicks are detected at night [29] .', 'In addition , comparisons of simultaneous towed array and HARP recordings in the GOM have shown that towed array detection ranges may be limited by signal refraction associated with warm surface mixed layer [33] .', 'Towed array delphinid click recordings were often characterized by short encounters and highly variable click amplitudes .', 'When animals were close enough to the towed array to be detectable , both on-axis ( transmission beam oriented directly toward the sensor ) and off-axis clicks were likely received , and on-axis clicks could be clipped due to high amplitudes at close range .', 'In contrast , delphinid encounters recorded by near-seafloor HARPs were often longer in duration due to larger detection ranges .', 'Click amplitudes tended to be lower , because delphinids were farther from the sensor , and off-axis clicks were less detectable according to click propagation simulations [34] .', 'Several improvements could be made to the automated classification approach in future work .', 'First , different distance metrics could be evaluated .', 'In this study , a correlation distance metric was used to assess similarity between spectra as it was found to capture shape similarities more effectively than a simpler Euclidean distance .', 'However , the correlation distance used assigns equal weight to all frequencies in the spectra , while high frequency amplitudes are expected to vary more than low frequencies because of acoustic attenuation .', 'To account for this expectation , a weighted distance metric could be used that emphasizes low frequency shape .', 'Alternatively , click shapes could be summarized as cepstra ( inverse FFT of spectra , e . g . [28] ) to emphasize particular aspects of overall shape .', 'Preliminary experiments using cepstra and perceptual weighting were conducted as part of this study , however clustering results were poor .', 'Nonetheless , more complex weighting strategies might be useful in future work .', 'Another improvement that could be considered is to evaluate the impact of pre-filtering spectra prior to classification .', 'In this implementation , frequencies below 10 kHz were removed by a bandpass filter .', 'Future classification efforts might benefit from including lower frequency spectral content .', 'Recent work by Finneran et al . [4] suggests that delphinid clicks may have consistent spectral features below 10 kHz which may be useful for click classification [e . g . 21] .', 'Improvements could also focus on using different metrics to capture persistent features of ICIs .', 'In this study , clear modal ICI peaks were associated with the click types , and ICI previously has been found to be useful for classifying beaked whale clicks [13] .', 'While delphinids have been shown to vary their ICI in wild and captive studies [1 , 16] , this typically occurs during terminal buzzes [35] which are much lower amplitude and occur less frequently than regular clicks [35 , 36] and therefore , are much less likely to be detected in wild recordings [34] .', 'Deep seafloor instruments ( at depths of roughly 80 m or more ) often receive only a single animal’s click train at a given time due to the animals’ narrow transmission beam patterns and distance from seafloor sensors; therefore ICI often is accurately calculated and modal ICI is representative of persistent features .', 'On occasions when a group of animals is large and/or close to the sensor , multiple click trains will overlap and modal ICI values may become subject to saturation .', 'Click train tracking [37] could be used to improve modal ICI estimates in saturated cases and in noisy or shallow environments .', 'Additional improvements could be made to fully automate the classification process .', 'For example , false positives were manually removed from this dataset prior to classification .', 'However , many sources of false positives , including beaked whales , sperm whales , and ships , have their own spectral and ICI characteristics .', 'A similar approach to that described here could be used to build template clusters for false positive sources , allowing them to be identified and excluded automatically during classification .', 'In addition to accelerating the analysis process , this approach could improve the removal of false positives within bouts of true detections ( such as at shallow sites ) , facilitate false positive rate calculations , and provide certainty scores for removed detections .', 'Future work will likely seek to combine clustering with deep learning methods as a possible refinement for improved classification .', 'Finally , future improvements should focus on evaluating sources of variability within click types and on linking distinct click types with delphinid species identity or behavior states .', 'This work focused on identifying distinct click types , however , more work needs to be done to describe within-type variability .', 'Delphinids have been shown to vary their clicks depending on context [e . g . 6 , 16 , 27] .', 'The types described here are broad groupings , and are not meant to indicate a lack of variability within each type .', 'Concurrent visual identifications with HARP recordings are needed to conclusively validate potential species associations .', 'Future steps should include applying this method to a labeled dataset associated with visually-identified species to ground truth the approach .', 'Continued development of unsupervised learning strategies for identifying consistent dolphin click types will advance marine mammal monitoring programs by facilitating delphinid and toothed whale species identification in data collected using autonomous passive acoustic sensors .'], ['Long-term autonomous datasets were collected using High-frequency Acoustic Recording Packages ( HARPs ) at three continental slope and two shelf locations in the GOM between 2010 and 2012 ( Fig 6 ) .', 'Details of each HARP deployment are presented in Table 2 .', 'HARPs are autonomous bottom-mounted acoustic recorders containing a hydrophone , data logger , battery power supply , ballast weights , acoustic release system , and flotation [39] .', 'All of the seafloor recording instruments used in this study were of the same type with equivalent hardware and software .', 'Each instrument used a calibrated hydrophone ( Channel Group Technologies , Santa Barbara , CA , ITC-1042 ) buoyed approximately 10 m above the seafloor .', 'Hydrophones had an approximately flat ( ±2 dB ) sensitivity from 10 to 100 kHz of -200 dB re V/μPa .', 'Preamplifier calibrations were performed at Scripps Institution of Oceanography and at the U . S . Navy’s Transducer Evaluation Center facility in San Diego , California [38] .', 'All HARPs sampled continuously at 200 kHz throughout each deployment .', 'Towed hydrophone array recordings were collected in 2011 and 2012 ( Table 6 ) during National Oceanographic and Atmospheric Administration’s ( NOAA ) National Marine Fisheries Service ( NMFS ) Southeast Fisheries Science Center ( SEFSC ) marine mammal abundance surveys aboard the R/V Gordon Gunter , conducted in the eastern GOM and within the southeastern U . S . Atlantic coastal exclusive economic zone ( EEZ ) .', 'A team of visual observers identified dolphins to species whenever possible , thereby providing ground-truthed species identifications which acousticians could associate with concurrent array recordings .', 'A five-element hydrophone array was towed 274 m behind the ship , at an estimated depth of 15 to 18 m at typical survey speed ( 10 kn ) .', 'Hydrophone elements consisted of custom-built preamplifiers , with band-pass filters set for 3 dB roll-off at 1 kHz and 200 kHz , connected to an omni-directional spherical hydrophone ( HS-150 Sonar Research and Development , Ltd . , Beverley , UK ) .', 'Two hydrophones separated by 2 . 12 m were sampled continuously at 500 kHz using a data acquisition board ( NI USB 6251 , National Instruments Corporation , Austin , TX ) and recorded directly to hard disk drives using Logger 2000 ( International Fund for Animal Welfare , IFAW , Yarmouth Port , MA ) .', 'The towed array recording setup differs considerably from the seafloor sensors , therefore any comparisons are considered tentative .', 'The set of summary nodes identified using in the training set were used to automatically classify clicks in the test dataset ( Table 2 ) .', 'As in the classifier training , Phase 1 of the automated clustering routine was executed on click-positive bins from test data to produce a
['Introduction', 'Results', 'Discussion', 'Methods']
['Delphinids produce large numbers of short duration , broadband echolocation clicks which may be useful for species classification in passive acoustic monitoring efforts .', 'A challenge in echolocation click classification is to overcome the many sources of variability to recognize underlying patterns across many detections .', 'An automated unsupervised network-based classification method was developed to simulate the approach a human analyst uses when categorizing click types: Clusters of similar clicks were identified by incorporating multiple click characteristics ( spectral shape and inter-click interval distributions ) to distinguish within-type from between-type variation , and identify distinct , persistent click types .', 'Once click types were established , an algorithm for classifying novel detections using existing clusters was tested .', 'The automated classification method was applied to a dataset of 52 million clicks detected across five monitoring sites over two years in the Gulf of Mexico ( GOM ) .', 'Seven distinct click types were identified , one of which is known to be associated with an acoustically identifiable delphinid ( Risso’s dolphin ) and six of which are not yet identified .', 'All types occurred at multiple monitoring locations , but the relative occurrence of types varied , particularly between continental shelf and slope locations .', 'Automatically-identified click types from autonomous seafloor recorders without verifiable species identification were compared with clicks detected on sea-surface towed hydrophone arrays in the presence of visually identified delphinid species .', 'These comparisons suggest potential species identities for the animals producing some echolocation click types .', 'The network-based classification method presented here is effective for rapid , unsupervised delphinid click classification across large datasets in which the click types may not be known a priori .']
['Health of marine mammal populations is often considered an indicator of overall marine ecosystem health and resilience , particularly in highly-impacted regions such as the Gulf of Mexico .', 'Marine mammal populations are difficult to monitor given the many challenges of observing animals at sea ( e . g . weather , limited daylight , ocean conditions , and expense ) .', 'An increasingly common approach is the use of underwater acoustic sensors capable of recording marine mammal calls at remote locations for months at a time .', 'Acoustic sensors generate large datasets in which dolphin echolocation clicks are commonly present .', 'Dolphins are the most diverse family of marine mammals , and distinguishing click characteristics have only been described for a small subset of species .', 'We developed a workflow to automatically identify distinct dolphin click types within large datasets without prior knowledge of their distinguishing features .', 'Our algorithm then recognizes these click types in novel recording data across a range of monitoring locations .', 'Known species-specific click types emerge from the data using this approach , as well as new click types potentially associated with additional species .', 'This technique is a key step toward determining species identification for passive acoustic monitoring of offshore populations of dolphins and other toothed whales under a big data paradigm .']
['gulf of mexico', 'acoustics', 'medicine and health sciences', 'vertebrates', 'neuroscience', 'marine biology', 'animals', 'mammals', 'animal signaling and communication', 'animal behavior', 'pilot whales', 'bodies of water', 'zoology', 'echolocation', 'sensory physiology', 'behavior', 'marine and aquatic sciences', 'bioacoustics', 'beaked whales', 'marine mammals', 'killer whales', 'physics', 'auditory system', 'animal migration', 'eukaryota', 'whales', 'animal navigation', 'earth sciences', 'physiology', 'biology and life sciences', 'sensory systems', 'physical sciences', 'gulfs', 'amniotes', 'organisms', 'dolphins']