Autonomous Acoustic Recorders Research Articles

Correcting gray whale (Eschrichtius robustus) call rates in San Ignacio Lagoon, using sound exposure level measurements of ambient noise

Autonomous acoustic recordings of gray whales (Eschrichtius robustus) were made in San Ignacio Lagoon, in February 2005‐2008, while animals were present to breed and raise calves. Counts were made of the gray whales' most common vocalization, type S1. A sequence of semi‐automated procedures was implemented to assist with call detection. Hourly call rates were computed for all seasons and adjusted for expected changes in detection range, caused by variations in the ambient background noise level. In this environment, the underwater acoustic background combines biological, oceanographic and man‐made sources and can present changes of 10 dB above the average base level of 96dB re 1uPa 2‐s between 350 and 750 Hz over semidiurnal scales. The relative changes in call rates in 2006 and 2008 are compared with visual survey counts conducted over the same period. The definition of SNR in the present study develops from energy flux densities or sound exposure levels (SEL). SEL were calculated experimentally through even sampling in time and individual sampling for each call. By assuming that the background masks a proportion of the detected calls, corrections were applied to determine the vocal activity within a fixed detection range.

Acoustic environments of bottlenose dolphins (Tursiops truncatus) in the Big Bend region of Florida

The researchers examined the acoustic environments of coastal bottlenose dolphins (Tursiops truncatus) in the Big Bend region of Florida, one of the most pristine coastal environments in the state, by means of remote autonomous acoustic recorders deployed at eight sites. The rates and types of dolphin vocalizations (whistles, echolocation, burst‐pulse calls, and pops) differed between the eight sites. Dolphins were not the only source of noise in their marine habitat; other biological and anthropogenic sounds occurred. Each site was found to have a unique soundscape with respect to dolphin, fish, snapping shrimp, and anthropogenic sound. Toadfish (Opsanus beta), silver perch (Bairdiella chrysoura), and sea catfish (Arius felis) were the only identifiable fish species to produce sound and each caused notable increases in sound levels at low frequencies where they vocalized. Locations exhibited different time peaks in fish produced sounds. Overall, anthropogenic noise was uncommon and only found in the form of boat noise. Surprisingly, dolphin vocalizations were not found at all locations only at Alligator Harbor, Dog Island, Carabelle River, and West Pass. The lack of dolphin vocalizations does not imply dolphins were not present in these regions only that no dolphins were vocalizing at the time of recordings.

Direct underwater visual and acoustic observations of echolocation behavior of sperm whales around a longline

Sperm whales (Physeter macrocephalus) have learned to approach and depredate fishing vessels longlining for black cod in the Gulf of Alaska. In May 2006, off Sitka, Alaska, a video camera was placed inside an underwater housing with an external hydrophone mount, attached to a longline that had been partially hauled to the surface, and lowered between depths of 90 and 120 m, with black cod (Anoploploma fimbria) attached 2 to 4 m above the camera as bait. The camera was mounted facing towards the surface to exploit the ambient light and to eliminate the need for artificial lighting. An additional hydrophone was deployed at 17-m depth during the 40–60-m deployments, and a third autonomous acoustic recorder was usually mounted 1.3 m below the camera. During one encounter a whale investigated the line, producing characteristic creak sounds that were recorded on the three hydrophones, and which were subsequently time-aligned using vessel noise as a reference, in order to permit acoustic tracking. During the second encounter a whale interacted with two fish within 4 m of the camera, but only the camera hydrophone was available. Preliminary analysis of the echolocation behavior displayed by both animals is presented. [Work sponsored by the National Geographic Society and the North Pacific Research Board.]

Temporal patterns in marine mammal sounds from long‐term broadband recordings

Recent advances in the technology for long‐term underwater acoustic recording provide new data on the temporal patterns of marine mammal sounds. Autonomous acoustic recordings are now being made with broad frequency bandwidth up to 200‐kHz sampling rates. These data allow sound recording from most marine mammal species, including, for instance, the echolocation clicks of odontocetes. Large data storage capacity up to 1280 Gbytes allow these recordings to be conducted over long time periods for study of diel and seasonal calling patterns. Examples will be presented of temporal patterns from long‐term recordings collected in four regions: the Bering Sea, offshore southern California, the Gulf of California, and the Southern Ocean. These data provide new insight on marine mammal distribution, seasonality, and behavior.

Blue Whale (<I>Balaenoptera musculus</I>) Diel Call Patterns Offshore of Southern California

Diel and seasonal calling patterns for blue whales (Balaenoptera musculus) were observed in coastal waters off southern California using seafloormounted autonomous acoustic recording packages (ARPs). Automated call counting from spectrogram cross-correlation showed peak seasonal calling in late summer/early fall. When call counts were organized by daily time intervals, calling peaks were observed during twilight periods, just after sunset and before sunrise. Minimum calling was observed during the day. Nighttime calling was greater than daytime calling, but also showed a minimum between the dusk and dawn calling peaks. These peaks correlate with the vertical migration times of krill, the blue whales’ primary prey. One hypothesis to explain these diel variations is that blue whale calling and foraging may be mutually exclusive activities. Fewer calls are produced during the day while prey are aggregated at depth and foraging is efficient. More calls are produced during the twilight time periods when prey are vertically migrating and at night when prey are dispersed near the sea surface and foraging is less efficient.

Modular autonomous array deployments using passive acoustic time synchronization

Multi-element acoustic arrays generally use wired cable to transmit signals to a central recording location. While convenient, hardwiring hydrophones together increases array fragility and field costs while decreasing deployment flexibility. It is thus difficult to integrate acoustic array operations with field activities involving marine mammals in remote environments. Economic trends in the cell-phone and consumer electronics industries, combined with recent trends in tagging technology development, have led to the existence of compact low-power autonomous acoustic recorders that store data to either flash memory or small hard drives. In this presentation it is shown how two or more autonomous recorders can be time-synchronized using passive acoustic measurements of the background ambient noise field, effectively creating coherent array processing systems of varying aperture, spacing, and deployment geometry. Configurations tested to date include short and large-aperture vertical arrays off the coasts of Australia and Alaska, bottom-mounted horizontal arrays in San Ignacio Lagoon, Baja California, and instruments installed inside a glider. [Work sponsored by ONR Acoustic Entry Level Faculty Award.]

High-frequency Acoustic Recording Package (HARP) for long-term monitoring of marine mammals

Advancements in low-power and high-data-capacity computer technology during the past decade have been adapted to autonomously record sounds from whales over long time periods. Acoustic monitoring of whales has advantages over traditional visual surveys including greater detection ranges, continuous long-term monitoring in remote locations under various weather conditions, and lower cost. One currently used tool for providing long-term acoustic monitoring of marine mammals is an autonomous acoustic recording package (ARP) which uses a tethered hydrophone above a seafloor-mounted instrument frame. Since 2000, ARPs have been deployed to record baleen whale sounds in the Bering Sea, in the Beaufort Sea, in the Gulf of Alaska, off the coast of southern California, around Antarctica and near Hawaii. ARP data have provided new information on the seasonal presence, abundance, call character and patterns of calling whales. The need for a broader-band, higher-data capacity system capable of recording odontocete whales, dolphins and porpoises for long time periods has prompted the development of a High-frequency Acoustic Recording Package (HARP). The HARP design is described and data analysis strategies are discussed using examples of HARP broad-band (sample rates up to 200 kHz) recorded data.

Acoustic observations of longline depredation by sperm whales

Sperm whales have learned to take sablefish off longline gear in the Gulf of Alaska. Over the past two years the Southeast Alaska Sperm Whale Avoidance Project (SEASWAP) has collected biopsy and photo-ID data concerning longline depredation, via collaboration with local fishermen in Sitka, AK. In 2004, in collaboration with SEASWAP, sets of autonomous acoustic recorders were attached to the anchor lines of several longline deployments, effectively converting the fishing gear into a vertical acoustic array deployed 200 m beneath the surface. The fishing vessels then left the area, leaving the acoustic instruments behind to monitor the region. Several hours later, typically the next morning, each vessel would return and begin hauling in the gear, a procedure that was also recorded by the instruments. In May 2004 an interaction of two sperm whales with the F/V Cobra was recorded. It was found that the animals started producing sounds within 20 min of the longline recovery and continued being extremely vocally active throughout the recovery. Acoustic multipath was exploited to estimate the range and depth of the whales. [Work sponsored by the North Pacific Research Board.]

Seasonality of blue and fin whale calls and the influence of sea ice in the Western Antarctic Peninsula

The calling seasonality of blue (Balaenoptera musculus) and fin (B. physalus) whales was assessed using acoustic data recorded on seven autonomous acoustic recording packages (ARPs) deployed from March 2001 to February 2003 in the Western Antarctic Peninsula. Automatic detection and acoustic power analysis methods were used for determining presence and absence of whale calls. Blue whale calls were detected year round, on average 177 days per year, with peak calling in March and April, and a secondary peak in October and November. Lowest calling rates occurred between June and September, and in December. Fin whale calling rates were seasonal with calls detected between February and June (on average 51 days/year), and peak calling in May. Sea ice formed a month later and retreated a month earlier in 2001 than in 2002 over all recording sites. During the entire deployment period, detected calls of both species of whales showed negative correlation with sea ice concentrations at all sites, suggesting an absence of blue and fin whales in areas covered with sea ice. A conservative density estimate of calling whales from the acoustic data yields 0.43 calling blue whales per 1000 n mi2 and 1.30 calling fin whales per 1000 n mi2, which is about one-third higher than the density of blue whales and approximately equal to the density of fin whales estimated from the visual surveys.

Tracking sperm whale (Physeter macrocephalus) dive profiles using a towed passive acoustic array.

A passive acoustic method is presented for tracking sperm whale dive profiles, using two or three hydrophones deployed as either a vertical or large-aperture towed array. The relative arrival times between the direct and surface-reflected acoustic paths are used to obtain the ranges and depths of animals with respect to the array, provided that the hydrophone depths are independently measured. Besides reducing the number of hydrophones required, exploiting surface reflections simplifies automation of the data processing. Experimental results are shown from 2002 and 2003 cruises in the Gulf of Mexico for two different towed array deployments. The 2002 deployment consisted of two short-aperture towed arrays separated by 170 m, while the 2003 deployment placed an autonomous acoustic recorder in tandem with a short-aperture towed array, and used ship noise to time-align the acoustic data. The resulting dive profiles were independently checked using single-hydrophone localizations, whenever multipath reflections from the ocean bottom could be exploited to effectively create a large-aperture vertical array. This technique may have applications for basic research and for real-time mitigation for seismic airgun surveys.

Autonomous Acoustic Recording Packages (ARPs) for Long-Term Monitoring of Whale Sounds

Advancements in low-power and high-data capacity computer technology during the past decade have been adapted to autonomously record acoustic data from vocalizing whales over long time periods. Acoustic monitoring of whales has advantages over traditional visual surveys including greater detection ranges, continuous long-term monitoring in remote locations and in various weather conditions, and lower cost. An autonomous acoustic recording package (ARP) is described that uses a tethered hydrophone above a seafloor-mounted instrument frame. ARPs have been deployed to record baleen whale sounds in the Bering Sea, off the coast of southern California, near the West Antarctic Peninsula, and near Hawaii. ARP data have provided new information on the seasonal presence, abundance, call character, and patterns of vocalizing whales. Current development is underway for a broader-band, higher-data capacity system capable of recording odontocete whales, dolphins, and porpoises for long time periods.

An autonomous acoustic recorder using a directional sensor for locating calling bowhead whales

A new directional acoustic recorder has been developed to assess the movement and distribution of vocalizing marine mammals. Called DASAR (Directional Autonomous Seafloor Acoustic Recorder), the instrument incorporates an omnidirectional and two orthogonal horizontal sensors from a DIFAR (DIrectional Frequency and Recording) sonobuoy, as well as a magnetic compass. With appropriate processing, the horizontal directions can be computed for received sounds. The acoustic bandwidth is 1000 Hz, appropriate for bowhead calls, and recording is continuous for 45 days. DASARs were deployed in a hexagonal array seaward of an artificial gravel island recently constructed for oil and gas production northwest of Prudhoe Bay, Alaska. Operation was during the fall migration of bowhead whales past the island. The objective was to relate calling-whale locations to the character and levels of sounds emanating from the island and from nearby tugs and barges. [Work supported by BP Exploration (Alaska).]

An autonomous acoustic recorder for shallow arctic waters

Autonomous acoustic recorders permit full-time environmental monitoring for extended periods. The device described here has been used on the ocean bottom in depths to 44 m. It uses a 4-Gbyte disk to record sounds sampled continuously at 1 kHz (500-Hz bandwidth) except that for 1 min every 14 m 24 s the sample rate is doubled. The lower sample rate is for monitoring the sounds of seismic surveys (airgun pulses) and bowhead whale calls. The higher sample rate is for ambient noise analysis. Capacity is adequate for 24 days. Retrieval is by grappling and has been used on 100 units that operated for 15 days in the Alaskan Beaufort Sea during September 1996. [Work supported by BP Exploration (Alaska), Inc.]

Near-Surface Observations of Wind and Rain-Generated Sound Using the SCANR: An Autonomous Acoustic Recorder

Recent oceanic ambient noise observations demonstrate a clear linear relation between sound pressure and both wind speed and rainfall rate. Wave whitecapping increases the sound level but the relation is still tenuous. Assessment of the true correlation of the ambient sound with its sources, a necessity for the understanding of the generating mechanisms, requires precise measurement of the surface phenomena made at the immediate location of the sound observations. Most noise records have lacked such proximate environmental measurements. Furthermore, most ambient noise has been recorded at depths of 250–4000 m and the data were low-pass filtered, both effects minimizing the acoustic response from small-scale and (or) short-term surface phenomena. Near-surface observations of ambient sound associated with rapidly changing wind and rainfall events were made using the Self Contained Ambient Noise Recorder (SCANR) system which provides autonomous digital tape records of ambient sound at selected frequencies. The SCANR was placed at 4 m depth from a pier in Narragansett Bay, Rhode Island. Wind and rainfall were monitored simultaneously with the ambient noise recordings at narrow bands cantered at 15 and 25 kHz and a broad band. A correlation of 0.97 of the 15 kHz band with wind speed was obtained for a 24 h period with winds ranging from 0.2–15 m s−1. At higher sustained winds (12–15 m s−1), a pronounced decrease occurred in sound pressure which appears due to increased absorption of the sound generated at the surface by the whitecap-produced bubble layer. Use of direct sound pressure output, in lieu of logarithmic sound levels, best showed the immediate acoustic response to changes of the wind field and rainfall rates associated with passing squalls. Rain-produced sound attained 35 dB increase within 2–3 min during a passing line squall which was tracked with an MIT weather radar at Cambridge, Massachusetts. The SCANR system has proved useful for observing the near-surface ambient sound field at both broad and narrow bands up to 30 kHz. It is easily deployed and retrieved while nearby observations are made of wind speed, whitecapping intensity, and rainfall rate.