Digital Acoustic Recording Tags Research Articles

Comparison of visual and passive acoustic estimates of beaked whale density off El Hierro, Canary Islands

Passive acoustic monitoring (PAM) offers considerable potential for density estimation of cryptic cetaceans, such as beaked whales. However, comparative studies on the accuracy of PAM density estimates from these species are lacking. Concurrent, low-cost drifting PAM, with SoundTraps suspended at 200 m depth, and land-based sightings, were conducted off the Canary Islands. Beaked whale density was estimated using a cue-count method, with click production rate and the probability of click detection derived from digital acoustic recording tags (DTags), and distance sampling techniques, adapted to fixed-point visual surveys. Of 32 870 detections obtained throughout 206 h of PAM recordings, 68% were classified as "certain" beaked whale clicks. Acoustic detection probability was 0.15 [coefficient variation (CV) 0.24] and click production rate was 0.46 clicks s - 1 (CV 0.05). PAM density estimates were in the range of 21.5 or 48.6 whales per 1000 km2 [CV 0.50 or 0.44, 95% confidence interval (CI) 20.7-22.4 or 47-50.9), depending on whether "uncertain" clicks were considered. Density estimates from concurrent sightings resulted in 33.7 whales per 1000 km2 (CV 0.77, 95% CI 8.9-50.5). Cue-count PAM methods under application provide reliable estimates of beaked whale density, over relatively long time periods and in realistic scenarios, as these match the concurrent density estimates obtained from visual observations.

Effects of vessels and their sounds on the foraging behavior of endangered killer whales (Orcinus orca)

Anthropogenic activities that have negative consequences on foraging outcomes warrant special concern in endangered species. Prey availability and vessel disturbance are identified risk factors of endangered Southern Resident killer whales (SRKW) as vessels and associated sounds can mask echolocation signals used for foraging and/or disrupt foraging behavior with implications for energy acquisition. To investigate vessel effects on foraging behavior, we utilized multi-sensor, digital acoustic recording tags (Dtags) that were suction cup-attached to SRKW. We tested a number of explanatory variables related to nearby vessels and associated sounds on the probability of prey capture and prey capture dive parameters. Tested variables included echosounder presence/absence, vessel counts, vessel distance, and vessel speed, and several competing received noise level measurements (absence of flow noise). Although noise level was not a significant explanatory variable on the probability of prey capture, flow noise limited available sample size. Furthermore, the probability of prey capture decreased as vessel speed increased which correlates with vessel noise. We also found a significant effect of echosounder presence on the duration and rate of descent of prey capture dives, indicating further consequences on foraging behavior. These results inform conservation and management efforts to preserve SRKW foraging opportunities and mitigate vessel disturbance.

The impact of temperature at depth on estimates of thermal habitat for short‐finned pilot whales

AbstractShort‐finned pilot whales (Globicephala macrorhynchus) experience dramatic changes in temperature during deep dives, but studies of pilot whale habitat use typically rely solely on surface temperature measurements. We quantified vertically integrated thermal habitat for short‐finned pilot whales using a novel metric, degree‐hours, developed using data from digital acoustic recording tags (DTAGs) deployed off Cape Hatteras along with interpolated temperature profiles at depth from the Met Office Hadley Centre EN4 oceanographic data set. We then compared estimates of thermal habitat calculated from surface waters with estimates of vertically integrated thermal habitat calculated using EN4 data collected along the eastern seaboard of the U.S. to understand how available thermal habitat is influenced by seasonal and spatial variability in water temperature. Estimates of vertically integrated thermal habitat were typically lower than estimates produced using surface temperatures, and the difference was greatest at intermediate latitudes and in warmer seasons, where and when there is a high degree of variability between surface and bottom temperatures. Our work highlights the importance of considering temperature at depth to accurately assess the thermal habitat of deep‐diving marine vertebrates, and presents a means of quantifying thermal habitat that will be useful for understanding the thermal ranges of these species.

How acoustics informs understanding of foraging behavior and effects of vessels and noise on killer whales

Foraging in toothed whales and dolphins is fundamentally tied to the use of sound. Resident-type killer whales (Orcinus orca) use echolocation to locate and capture fast-moving salmon and other fish prey. In addition to prey availability, disturbance from vessels and noise is a threat to the endangered Southern Resident killer whale population given considerable levels of commercial shipping, fishing, whale-watching and recreational vessel traffic in urban waterways that the whales use for feeding. In this study, we utilized suction cup-attached digital acoustic recording tags (DTAGs) to (1) describe whale acoustic and movement behavior during different phases of foraging that can be differentiated from other behaviors, (2) investigate vessel and noise effects on behavior and foraging outcomes in the endangered population, (3) compare foraging behavior between the endangered population that is struggling with population recovery and another population (Northern Resident killer whales) that is increasing in numbers, and (4) characterize diel patterns of foraging and other behaviors to describe their full activity budget and inform management of vessel traffic and noise during urban expansion along the Pacific Northwest coast of North America. This presentation will highlight results to date and implications for the conservation and management of marine protected species.

Effects of vessels and noise on the subsurface behavior of endangered killer whales (Orcinus orca)

Prey availability and disturbance from vessels and noise are identified threats to endangered Southern Resident killer whales. Vessel noise can mask echolocation signals used for hunting and/or disrupt foraging with implications for energy acquisition in a likely prey-limited population. We utilized suction cup-attached digital acoustic recording tags (DTAGs), to measure received noise levels, understand killer whales’ use of sound, and determine effects of vessels/noise on subsurface behavior. During the 28 tag deployments, we collected vessel data concurrently along with opportunistic predation observations to validate feeding. Broadband received levels (dB re 1μPa) were significantly different across years. Of the vessel factors considered, both vessel count and speed were significant explanatory variables of received levels. Vessels emitting echosounder signals were commonly received by the DTAGs and overlapped with the echolocation frequencies use to hunt fish. Additionally, different phases of foraging were differentiated from the acoustic record, including the detection of crunching sounds after fish kills. Together with movement data analysis, these results allow the identification of different whale activities, including prey capture dives, to test hypotheses of vessel/noise effects on behavior. This work, along with a comparative investigation involving Northern Resident DTAG data, inform killer whale conservation and management measures.

Manatee behavioral response to boats

AbstractThe long‐term viability of the Florida manatee is threatened in part by mortality from boat collisions. This study investigated manatee behavior during boat approaches to better understand factors that lead to manatee–boat collisions. Digital acoustic recording tags (DTAGs) and Argos‐linked GPS tags were deployed on 18 manatees in southwest Florida, and boat traffic around them was mapped. Suites of manatee behavioral, environmental, and boat‐related factors were recorded during opportunistic boat passes. We built generalized linear mixed models to determine which factors accounted for variation in occurrence, number, and timing of manatee behavioral changes during boat passes. Manatees responded to boats, changing their orientation, depth, and fluking behavior most often when a boat approached closely (<10 m). Manatees were also more likely to change their depth when not on a seagrass bed and when actively fluking before a boat pass. Boat speed did not affect the occurrence or intensity of manatee response. Compared to fast approaches, however, slower passes allowed the manatee more time to respond, and behavioral change occurred earlier relative to the time of the boat's closest point of approach. We conclude that faster boats likely pose a greater risk of collision with manatees than do slower boats.

Individual, ecological, and anthropogenic influences on activity budgets of long‐finned pilot whales

AbstractTime allocation to different activities and habitats enables individuals to modulate their perceived risks and access to resources and can reveal important trade‐offs between fitness‐enhancing activities (e.g., feeding vs. social behavior). Species with long reproductive cycles and high parental investment, such as marine mammals, rely on such behavioral plasticity to cope with rapid environmental change, including anthropogenic stressors. We quantified activity budgets of free‐ranging long‐finned pilot whales in order to assess individual time trade‐offs between foraging and other behaviors in different individual and ecological contexts, and during experimental sound exposures. The experiments included 1–2 and 6–7 kHz naval sonar exposures (a potential anthropogenic stressor), playback of killer whale (a potential predator/competitor) vocalizations, and negative controls. We combined multiple time series data from digital acoustic recording tags (DTAG) as well as group‐level social behavior data from visual observations of tagged whales at the surface. The data were classified into near‐surface behaviors and dive types (using a hidden Markov model for dive transitions) and aggregated into time budgets. On average, individuals (N = 19) spent most of their time (69%) resting and transiting near surface, 21% in shallow dives (depth <40 m), and only 10% of their time in deep foraging dives, of which 65% reached a depth 10 m from the sea bottom. Individuals in the largest of three body size classes or accompanied by calves tended to spend more time foraging than others. Simultaneous tagging of pairs of individuals showed that up to 50% of the activity budget was synchronized between conspecifics with decreased synchrony during foraging periods. Individuals spent less time foraging when forming larger non‐vocal aggregations of individuals in late afternoons, and more time foraging when in the mid‐range of water depths (300–400 m) available in the study area (50–700 m). Individuals reduced foraging time by 83% (29–96%) during their first exposure to sonar, but not during killer whale sound playbacks. A relative increase in foraging during repeat sonar exposures indicated habituation or change in response tactic. We discuss the possible adaptive value of these trade‐offs in time allocation to reduce individual conflict while maintaining benefits of group living.

Two‐component calls in short‐finned pilot whales (Globicephala macrorhynchus)

AbstractShort‐finned pilot whales (Globicephala macrorhynchus) have complex vocal repertoires that include calls with two time‐frequency contours known as two‐component calls. We attached digital acoustic recording tags (DTAGs) to 23 short‐finned pilot whales off Cape Hatteras, North Carolina, and assessed the similarity of two‐component calls within and among tags. Two‐component calls made up <3% of the total number of calls on 19 of the 23 tag records. For the remaining four tags, two‐component calls comprised 9%, 23%, 24%, and 57% of the total calls recorded. Measurements of six acoustic parameters for both the low and high frequency components of all two‐component calls from the five tags were compared using a generalized linear model. There were significant differences in the acoustic parameters of two‐component calls between tags, verifying that acoustic parameters were more similar for two‐component calls recorded on the same tag than for calls between tags. Spectrograms of all two‐component calls from the five tags were visually graded and independently categorized by five observers. A test of inter‐rater reliability showed substantial agreement, suggesting that each tag contained a predominant two‐component call type that was not shared across tags.

A day in the life of a dolphin: Using bio‐logging tags for improved animal health and well‐being

AbstractLittle quantitative information on the behavior, health, and activity level of managed marine mammals is currently collected, though it has the potential to significantly contribute to management and welfare of these animals. To address this, high‐resolution motion‐sensing digital acoustic recording tags (DTAGs) collected data from animals under human care (n = 5) during their daily routine, and classification algorithms were used for gait analysis and event detection. We collected and examined ~57 h of data from five bottlenose dolphins (Tursiops truncatus). Day‐scale changes in behavior and activity level were observed and diurnal changes were detected with lower activity at night (n = 1). During the day, animals spent about 70% of their time swimming. The deepest part of the lagoon is ~3 m and individual dives were typically shallow (~1 m) with the dolphins tending to utilize a fluke and glide gait pattern. Activity level was quantified using overall dynamic body acceleration. A significant relationship between normalized activity level and glide duration during different portions of the dive was measured; animals fluked more during descent and glided more during ascent. This could indicate that even during very shallow dives the dolphins use their positive buoyancy to improve energy economy.

Characterizing Chilean blue whale vocalizations with digital acoustic recording tags: A test of using tag accelerometers for caller identification

Vocal behavior of blue whales (Balaenoptera musculus) in the Gulf of Corcovado, Chile, was analyzed using digital acoustic recording tags (DTAGs). We report the occurrence of Southeast Pacific type 2 (SEP2) calls, which exhibit peak frequencies, durations, and timing consistent with previous reports. We also offer the first description of tonal downswept (D) calls for this population. Since being able to accurately assign vocalizations to individual whales is fundamental for studying communication and for estimating population densities from call rates, we further examine the feasibility of using DTAG accelerometers to identify low-frequency calls produced by tagged whales. We cross-correlated acoustic signals with simultaneous tri-axial accelerometer readings in order to analyze the phase match as well as the amplitude of accelerometer signals associated with low-frequency calls, which provides a reliable method of determining if a call is associated with a detectable acceleration. Our results suggest that vocalizations from nearby individuals are also capable of registering accelerations in the tagged whale’s DTAG record. We cross-correlate acceleration vectors between calls to explore the possibility of using signature acceleration patterns associated with sounds produced within the tagged whale as a new method of identifying which accelerometer-detectable calls originate from the tagged animal.

Effects of a scientific echo sounder on the behavior of short-finned pilot whales (Globicephala macrorhynchus)

Active echo sounding devices are often employed for commercial or scientific purposes in the foraging habitats of marine mammals. We conducted an experiment off Cape Hatteras, North Carolina, USA, to assess whether the behavior of short-finned pilot whales (Globicephala macrorhynchus) changed when exposed to an EK60 scientific echo sounder. We attached digital acoustic recording tags (DTAGs) to nine individuals, five of which were exposed. A hidden Markov model to characterize diving states with and without exposure provided no evidence for a change in foraging behavior. However, generalized estimating equations to model changes in heading variance over the entire tag record under all experimental conditions showed a consistent increase in heading variance during exposure over all values of depth and pitch. This suggests that regardless of behavioral state, the whales changed their heading more frequently when the echo sounder was active. This response could represent increased vigilance in which whales maintained awareness of echo sounder location by increasing their heading variance and provides the first quantitative analysis on reactions of cetaceans to a scientific echo sounder.

Hidden Markov models reveal complexity in the diving behaviour of short-finned pilot whales

Diving behaviour of short-finned pilot whales is often described by two states; deep foraging and shallow, non-foraging dives. However, this simple classification system ignores much of the variation that occurs during subsurface periods. We used multi-state hidden Markov models (HMM) to characterize states of diving behaviour and the transitions between states in short-finned pilot whales. We used three parameters (number of buzzes, maximum dive depth and duration) measured in 259 dives by digital acoustic recording tags (DTAGs) deployed on 20 individual whales off Cape Hatteras, North Carolina, USA. The HMM identified a four-state model as the best descriptor of diving behaviour. The state-dependent distributions for the diving parameters showed variation between states, indicative of different diving behaviours. Transition probabilities were considerably higher for state persistence than state switching, indicating that dive types occurred in bouts. Our results indicate that subsurface behaviour in short-finned pilot whales is more complex than a simple dichotomy of deep and shallow diving states, and labelling all subsurface behaviour as deep dives or shallow dives discounts a significant amount of important variation. We discuss potential drivers of these patterns, including variation in foraging success, prey availability and selection, bathymetry, physiological constraints and socially mediated behaviour.

Characterizing Chilean blue whale vocalizations with DTAGs: a test of using tag accelerometers for caller identification.

Vocal behavior of blue whales (Balaenoptera musculus) in the Gulf of Corcovado, Chile, was analysed using both audio and accelerometer data from digital acoustic recording tags (DTAGs). Over the course of three austral summers (2014, 2015 and 2016), seventeen tags were deployed, yielding 124 h of data. We report the occurrence of Southeast Pacific type 2 (SEP2) calls, which exhibit peak frequencies, durations and timing consistent with previous recordings made using towed and moored hydrophones. We also describe tonal downswept (D) calls, which have not been previously described for this population. As being able to accurately assign vocalizations to individual whales is fundamental for studying communication and for estimating population densities from call rates, we further examine the feasibility of using high-resolution DTAG accelerometers to identify low-frequency calls produced by tagged blue whales. We cross-correlated acoustic signals with simultaneous tri-axial accelerometer readings in order to analyse the phase match as well as the amplitude of accelerometer signals associated with low-frequency calls, which provides a quantitative method of determining if a call is associated with a detectable acceleration signal. Our results suggest that vocalizations from nearby individuals are also capable of registering accelerometer signals in the tagged whale's DTAG record. We cross-correlate acceleration vectors between calls to explore the possibility of using signature acceleration patterns associated with sounds produced within the tagged whale as a new method of identifying which accelerometer-detectable calls originate from the tagged animal.

Echolocation behavior of endangered fish-eating killer whales (Orcinus orca) recorded from digital acoustic recording tags (DTAGs): Insight into subsurface foraging activity

Killer whales are apex predators with diet specializations that vary among ecotypes. Resident killer whales use broadband echolocation clicks to detect and capture fish prey in their underwater environment. Here, we describe the echolocation behavior of endangered Southern Resident killer whales using DTAGs to determine subsurface foraging activity and to assess the effects of vessel and noise on foraging behavior. We deployed 29 DTAGs on individually-identified killer whales and collected complimentary field data over four seasons in summer habitat. DTAGs had two hydrophones that each recorded sound at sampling rates of 192 or 240 kHz, and other sensors to reconstruct whale movement. Prey remains were opportunistically collected during tag deployments to validate feeding. Echolocation signals of the tagged whale were inferred from spectral content and the angle of arrival that corresponded to tag placement. Preliminary results reveal that individuals produced steady click trains during shallow dives then dove to deeper depths while clicking at higher repetition rates that graded into bottom-associated low-level buzzes before ascent occurred. These results, together with movement data, are reliable subsurface foraging cues in this endangered population that can be used to assess vessel effects on foraging behavior.

Whistle characteristics and daytime dive behavior in pantropical spotted dolphins (Stenella attenuata) in Hawai'i measured using digital acoustic recording tags (DTAGs).

This study characterizes daytime acoustic and dive behavior of pantropical spotted dolphins (Stenella attenuata) in Hawai'i using 14.58 h of data collected from five deployments of digital acoustic recording tags (DTAG3) in 2013. For each tagged animal, the number of whistles, foraging buzzes, dive profiles, and dive statistics were calculated. Start, end, minimum, and maximum frequencies, number of inflection points and duration were measured from 746 whistles. Whistles ranged in frequency from 9.7 ± 2.8 to 19.8 ± 4.2 kHz, had a mean duration of 0.7 ± 0.5 s and a mean of 1.2 ± 1.2 inflection points. Thirteen foraging buzzes were recorded across all tags. Mean dive depth and duration were 16 ± 9 m and 1.9 ± 1.0 min, respectively. Tagged animals spent the majority of time in the upper 10 m (76.9% ± 16.1%) of the water column. Both whistle frequency characteristics and dive statistics measured here were similar to previously reported values for spotted dolphins in Hawai'i. Shallow, short dive profiles combined with few foraging buzzes provide evidence that little spotted dolphin feeding behavior occurs during daytime hours. This work represents one of the first successful DTAG3 studies of small pelagic delphinids, providing rare insights into baseline bioacoustics and dive behavior.

Effect of prey type on the fine-scale feeding behaviour of migrating east Australian humpback whales

For terrestrial migrants, feeding at migratory stopover sites is important, with prey quality linked to future survival and reproductive success. In contrast, the importance of this behaviour to marine species is unknown. The humpback whale Megaptera novaeangliae is a marine migrant that is thought to fast while migrating; however, recent studies suggest that feeding may occur during this time. The aims of this study were to determine how the prey type available on a migratory route off Eden, New South Wales, Australia, influenced whether whales fed or not, and to study the fine-scale behaviour of the whales. Digital acoustic recording tags (DTAGs) and focal follows were used to record whale behaviour. A larger proportion of groups were determined to be feeding when krill was observed at the surface. Whales encountering fish spent a small percentage (28%) of time feeding, behaving similarly to non-feeding whales on migration, with small groups, a higher proportion of males sampled, and relatively straight tracks. In contrast, whales encountering surface swarms of krill spent significantly more time feeding (92%) and behaved similarly to whales on feeding grounds, with larger groups, more females sampled, and tracks with high turning angles. Therefore, changes in the available prey type in - fluenced the amount of time spent feeding and the social dynamics of groups. Given the link between the amount of feeding completed on migration and the future survival and reproductive success of individuals in terrestrial species, the impact of such fluctuations on marine species such as the humpback whale deserves more attention.

The Relationship between Vessel Traffic and Noise Levels Received by Killer Whales (Orcinus orca).

Whale watching has become increasingly popular as an ecotourism activity around the globe and is beneficial for environmental education and local economies. Southern Resident killer whales (Orcinus orca) comprise an endangered population that is frequently observed by a large whale watching fleet in the inland waters of Washington state and British Columbia. One of the factors identified as a risk to recovery for the population is the effect of vessels and associated noise. An examination of the effects of vessels and associated noise on whale behavior utilized novel equipment to address limitations of previous studies. Digital acoustic recording tags (DTAGs) measured the noise levels the tagged whales received while laser positioning systems allowed collection of geo-referenced data for tagged whales and all vessels within 1000 m of the tagged whale. The objective of the current study was to compare vessel data and DTAG recordings to relate vessel traffic to the ambient noise received by tagged whales. Two analyses were conducted, one including all recording intervals, and one that excluded intervals when only the research vessel was present. For all data, significant predictors of noise levels were length (inverse relationship), number of propellers, and vessel speed, but only 15% of the variation in noise was explained by this model. When research-vessel-only intervals were excluded, vessel speed was the only significant predictor of noise levels, and explained 42% of the variation. Simple linear regressions (ignoring covariates) found that average vessel speed and number of propellers were the only significant correlates with noise levels. We conclude that vessel speed is the most important predictor of noise levels received by whales in this study. Thus, measures that reduce vessel speed in the vicinity of killer whales would reduce noise exposure in this population.

Exploring the vocal ontogeny of North Atlantic right whales

The vocal ontogeny of species can provide insight into their physical and social development and capacity for social learning. North Atlantic right whales (Eubalaena glacialis) are an endangered species with a complex range of vocalizations used in a wide range of social contexts. In this study, we systematically characterize developmental changes in sound production from birth through adulthood. Calls have been recorded as part of ongoing North Atlantic right whale research projects spanning 2001 through 2015, and include data from calves as young as 1 month of age. Data included recordings from single hydrophones, hydrophone arrays, and non-invasive digital acoustic recording tags. Only calls which could be confidently attributed to a specific whale of known age were used in the analysis. Developmental periods consisted of 0–3 months, 3–6 months, 6–12 months, and then discrete ages by year. Calls of both male and female calves were included in the analysis. Calls were also classified to age categories u...

The relationship between vessel traffic and noise levels received by killer whales

Cetaceans that rely on their acoustic environment for key life history strategies are susceptible to noise effects from anthropogenic use such as ecotourism. Endangered Southern Resident killer whales (SRKW) are the primary target for vessel-based whale-watching in the Salish Sea. Vessel interactions and associated noise have been identified as potential stressors for SRKW. Previous research has indicated that both stressors negatively impact SRKW; however, there is a missing link between vessel characteristics/behavior and noise levels actually received by individual whales. To investigate this relationship, data were collected concurrently using mobile remote sensing survey equipment packages and digital acoustic recording tags. This allowed us to obtain precise geo-referenced vessel data and noise levels received by the whales. We used linear regression to summarize patterns in vessel characteristics and relate them to received noise levels. Received noise levels (RNL) were correlated with the number of vessels. RNL also increased when larger vessels were present or when vessels were traveling at relatively high speed. These findings facilitate improved understanding of the contributions of vessel characteristics to the noise levels received by individual cetaceans. Results from this study can be used to refine existing vessel regulations in order to better manage SRKW to recovery.

Repeated call types in short‐finned pilot whales, Globicephala macrorhynchus

AbstractFour short‐finned pilot whales, Globicephala macrorhynchus, were tagged with digital acoustic recording tags (DTAGs) for a total of 30 h in the Bahamas during 2007. Spectrograms were made of all audible sounds, which were independently categorized by three observers. Of 4,098 calls, 1,737 (42%) were placed into 173 call types, which were defined as calls that occurred more than once. Of the 173 call types, 51 contained at least 10 calls (= 24), and were termed predominant call types (PCTs), which comprised 1,219 (70%) of categorized calls. PCTs tended to occur in sequences of the same call, which appeared to be produced by a single animal. However, matching interactions consisting of adjacent or overlapping calls of the same type were also observed, and some call types were recorded on more than one tag, suggesting that at least some calls are shared by members of a group or subgroup. These results emphasize the importance of categorizing calls before attempting to draw conclusions about call usage and possible effects of noise on vocal behavior.