Understanding the relationship between the Bering Sea Cold Pool and vocal presence of odontocetes in the context of climate changea).

The Cold Pool is a subsurface layer with water temperatures below 2 °C that is formed in the eastern Bering Sea. This oceanographic feature of relatively cooler bottom temperature impacts zooplankton and forage fish dynamics, driving different energetic pathways dependent upon Bering Sea climatic regime. Odontocetes echolocate to find prey, so tracking foraging vocalizations acoustically provides information to understand the implications of climate change on Cold Pool variability influencing regional food web processes. Vocal foraging dynamics of ice-associated and seasonally migrant marine mammal species suggest that sperm whales spend more time searching for prey in warm years when the Cold Pool is reduced but are more successful at capturing prey during cold years when the Cold Pool is stronger. Beluga whale foraging vocal activity was relatively consistent across climate regimes but peaked during the warm regime. Killer whale foraging vocal activity peaked in both warm and cold regimes with indicators of different ecotypes exploiting changing prey conditions across climate regimes. Foraging activity of odontocete apex predators may serve as a sentinel indicator of future ecosystem change related to prey availability that is linked to a diminishing Cold Pool as water temperatures rise and seasonal sea ice decreases due to climate change.

Multidimensional comparison of underwater soundscapes using the soundscape codea).

The soundscape of a given habitat is a product of its physical environment, human activity, and presence of soniferous marine life, which can be used to understand ecosystem processes, habitat quality, and biodiversity. Shallow coral habitats are hotspots of biodiversity and marine life. Deep-sea coral environments, in comparison, are generally poorly understood. Four soundscapes along the U.S. Outer Continental Shelf (OCS) and one soundscape from the Great Barrier Reef were quantified to explore how differences in habitat, depth, and substrate manifest acoustically. Comparisons were made between (1) deep, cold-water and shallow, warm-water coral reefs and (2) deep-sea coral and sandy bottom habitats. Application of the soundscape code to recordings in each location seeded cluster analyses of soundscape metrics and an assessment of daily trends to quantitatively compare the soundscapes. The shallow, tropical reef soundscape differed from the deep-sea soundscapes in amplitude and impulsiveness. Differences in soundscape properties among the deep-sea soundscapes suggested cold-water coral sites produce different soundscapes than the deep sites without live hard bottom. This initial assessment of deep-sea soundscapes along the U.S. OCS provides baseline acoustic properties in a region likely to experience changes due to climate and human use.

Decadal community structure shifts with cold pool variability in the eastern Bering Sea shelf.

A characteristic feature of the eastern Bering Sea (EBS) is a subsurface layer linked to seasonal sea ice (SSI) and defined by bottom temperatures less than 2 °C, which is termed the cold pool. Cold pool variability is directly tied to regional zooplankton and fish dynamics. Multifrequency (200 and 460 kHz) acoustic backscatter data were collected remotely using upward looking echosounders along the EBS shelf from 2008 and 2018 and used as a proxy of biological abundance. Acoustic data were coupled with bottom temperature and regional SSI data from the cold (2006-2013) and warm (2014-2018) regimes to assess the relationship between biological scattering communities and cold pool variation. Acoustic backscatter was 2 orders of magnitude greater during the cold regime than during the warm regime, with multifrequency analysis indicating a shift in the warm regime frequency-dependent scattering communities. Cold pool proxy SSI was a stronger predictor for biological scattering than bottom temperature in the cold regime, while warm regime bottom temperature and SSI were equal in predictive power and resulted in improved predictive model performance. Results suggest coupled cold pool and frequency-dependent scattering dynamics are a potential regime shift indicator and may be useful for management practices in surrounding Arctic ecosystems.

Bearing Stake 1977 revisited: An understanding of ambient sound sources in the Indian Ocean.

A previous analysis of 1977 passive acoustic recordings in the Indian Ocean focused on sound pressure levels (SPLs) and showed that SPLs were slightly depth dependent and highly influenced by shipping activities [Wagstaff and Aitkenhead, IEEE J. Ocean. Eng. 30(2), 295-302 (2005)]. Consequently, SPL alone does not provide a consistent comprehensive metric to compare among sites or with contemporary recordings in the same region. Therefore, a source separation analysis was devised and applied to identify the major sound source contributions at three Indian Ocean locations. Shipping noise was a major sound contributor in all sites, while the site with the most diverse number of sources was in the central Arabian Sea.

Comparison of estimated 20-Hz pulse fin whale source levels from the tropical Pacific and Eastern North Atlantic Oceans to other recorded populations.

Passive acoustic monitoring, mitigation, animal density estimation, and comprehensive understanding of the impact of sound on marine animals all require accurate information on vocalization source level to be most effective. This study focused on examining the uncertainty related to passive sonar equation terms that ultimately contribute to the variability observed in estimated source levels of fin whale calls. Differences in hardware configuration, signal detection methods, sample size, location, and time were considered in interpreting the variability of estimated fin whale source levels. Data from Wake Island in the Pacific Ocean and off Portugal in the Atlantic Ocean provided the opportunity to generate large datasets of estimated source levels to better understand sources of uncertainty leading to the observed variability with and across years. Average seasonal source levels from the Wake Island dataset ranged from 175 to 188 dB re 1 µPa m, while the 2007-2008 seasonal average detected off Portugal was 189 dB re 1 µPa m. Owing to the large inherent variability within and across this and other studies that potentially masks true differences between populations, there is no evidence to conclude that the source level of 20-Hz fin whale calls are regionally or population specific.

Two unit analysis of Sri Lankan pygmy blue whale song over a decade.

Sri Lankan pygmy blue whale song consists of three repeated units: (1) low frequency pulsive unit, (2) frequency modulated (FM) upsweep, and (3) long tonal downsweep. The Unit 2 FM unit has up to three visible upsweeps with energy concentrated at approximately 40, 50, and 60 Hz, while the Unit 3 (∼100 Hz) tonal downsweep is the most distinct unit lasting 20-30 s. Spectral characteristics of the Units 2 and 3 song elements, along with ocean sound levels, were analyzed in the Indian Ocean from 2002 to 2013. The peak frequency of the tonal Unit 3 calls decreased from approximately 106.5 to 100.7 Hz over a decade corresponding to a 5.4% decrease. Over the same time period, the frequency content of the Unit 2 upsweeps did not change as dramatically with only a 3.1% change. Ambient sound levels in the vocalization bands did not exhibit equivalent patterns in amplitude trends. Analysis showed no increase in the ambient sound or compensated peak amplitude levels of the tonal downsweeps, eliminating the presence of a Lombard effect. Here it is proposed that each song unit may convey different information and thus may be responding to different selective pressures.

Fin whale density and distribution estimation using acoustic bearings derived from sparse arrays.

Passive acoustic monitoring of marine mammals is common, and it is now possible to estimate absolute animal density from acoustic recordings. The most appropriate density estimation method depends on how much detail about animals' locations can be derived from the recordings. Here, a method for estimating cetacean density using acoustic data is presented, where only horizontal bearings to calling animals are estimable. This method also requires knowledge of call signal-to-noise ratios, as well as auxiliary information about call source levels, sound propagation, and call production rates. Results are presented from simulations, and from a pilot study using recordings of fin whale (Balaenoptera physalus) calls from Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) hydrophones at Wake Island in the Pacific Ocean. Simulations replicating different animal distributions showed median biases in estimated call density of less than 2%. The estimated average call density during the pilot study period (December 2007-February 2008) was 0.02 calls hr-1 km2 (coefficient of variation, CV: 15%). Using a tentative call production rate, estimated average animal density was 0.54 animals/1000 km2 (CV: 52%). Calling animals showed a varied spatial distribution around the northern hydrophone array, with most detections occurring at bearings between 90 and 180 degrees.

An Empirical Mode Decomposition-based detection and classification approach for marine mammal vocal signals.

Detecting marine mammal vocalizations in underwater acoustic environments and classifying them to species level is typically an arduous manual analysis task for skilled bioacousticians. In recent years, machine learning and other automated algorithms have been explored for quickly detecting and classifying all sound sources in an ambient acoustic environment, but many of these still require a large training dataset compiled through time-intensive manual pre-processing. Here, an application of the signal decomposition technique Empirical Mode Decomposition (EMD) is presented, which does not require a priori knowledge and quickly detects all sound sources in a given recording. The EMD detection process extracts the possible signals in a dataset for minimal quality control post-processing before moving onto the second phase: the EMD classification process. The EMD classification process uniquely identifies and labels most sound sources in a given environment. Thirty-five recordings containing different marine mammal species and mooring hardware noises were tested with the new EMD detection and classification processes. Ultimately, these processes can be applied to acoustic index development and refinement.

Global Trends in Ocean Noise.

This ongoing work provides information about sound level trends from three ocean regions to compare with those of the North Pacific to determine whether increasing sound levels are a global phenomenon. Here the term soundscape is used to describe a measured physical property that can be selectively decomposed by frequency and sound level is used to provide insight relating to conditions ranging from the quietest conditions (sound floor) to the most extreme acoustic events. Acoustic time series from the Indian, South Atlantic, and Equatorial Pacific Oceans were used to quantify the rate and direction of low-frequency change over the past decade.

Is low frequency ocean sound increasing globally?

Low frequency sound has increased in the Northeast Pacific Ocean over the past 60 yr [Ross (1993) Acoust. Bull. 18, 5-8; (2005) IEEE J. Ocean. Eng. 30, 257-261; Andrew, Howe, Mercer, and Dzieciuch (2002) J. Acoust. Soc. Am. 129, 642-651; McDonald, Hildebrand, and Wiggins (2006) J. Acoust. Soc. Am. 120, 711-717; Chapman and Price (2011) J. Acoust. Soc. Am. 129, EL161-EL165] and in the Indian Ocean over the past decade, [Miksis-Olds, Bradley, and Niu (2013) J. Acoust. Soc. Am. 134, 3464-3475]. More recently, Andrew, Howe, and Mercer's [(2011) J. Acoust. Soc. Am. 129, 642-651] observations in the Northeast Pacific show a level or slightly decreasing trend in low frequency noise. It remains unclear what the low frequency trends are in other regions of the world. In this work, data from the Comprehensive Nuclear-Test Ban Treaty Organization International Monitoring System was used to examine the rate and magnitude of change in low frequency sound (5-115 Hz) over the past decade in the South Atlantic and Equatorial Pacific Oceans. The dominant source observed in the South Atlantic was seismic air gun signals, while shipping and biologic sources contributed more to the acoustic environment at the Equatorial Pacific location. Sound levels over the past 5-6 yr in the Equatorial Pacific have decreased. Decreases were also observed in the ambient sound floor in the South Atlantic Ocean. Based on these observations, it does not appear that low frequency sound levels are increasing globally.

Variation in low-frequency estimates of sound levels based on different units of analysis.

The measurement and analysis of underwater sound is a complicated process because of the variable durations of contributing sources and constantly changing water column dynamics. Because the ambient sound distribution does not always follow a Gaussian structure and may be nonstationary in time, analysis over an extended period is required to accurately characterize the data. Utilizing recordings from the Indian Ocean, the temporal variation in ambient sound including transient signals was examined using multiple processing window lengths and subsampling intervals. Results illustrate the degree of uncertainty in sound levels based on different units of analysis. The average difference between sound level estimates in the 10-30 Hz band due to subsampling was 2 dB and as high as 4 dB. The difference in the full band (5-110 Hz) was as high as 6 dB. Longer averaging windows (200 vs 60 s) resulted in larger variations over different subsampling intervals. This work demonstrates how sampling protocols within a single dataset can influence results and acknowledges that comparative studies at the same location but with different sampling protocols can be substantial if signal processing parameters are not statistically accounted for to confirm interpretation of results and observed trends.

Decadal trends in Indian Ocean ambient sound.

The increase of ocean noise documented in the North Pacific has sparked concern on whether the observed increases are a global or regional phenomenon. This work provides evidence of low frequency sound increases in the Indian Ocean. A decade (2002-2012) of recordings made off the island of Diego Garcia, UK in the Indian Ocean was parsed into time series according to frequency band and sound level. Quarterly sound level comparisons between the first and last years were also performed. The combination of time series and temporal comparison analyses over multiple measurement parameters produced results beyond those obtainable from a single parameter analysis. The ocean sound floor has increased over the past decade in the Indian Ocean. Increases were most prominent in recordings made south of Diego Garcia in the 85-105 Hz band. The highest sound level trends differed between the two sides of the island; the highest sound levels decreased in the north and increased in the south. Rate, direction, and magnitude of changes among the multiple parameters supported interpretation of source functions driving the trends. The observed sound floor increases are consistent with concurrent increases in shipping, wind speed, wave height, and blue whale abundance in the Indian Ocean.

Minimal COVID-19 quieting measured in the deep offshore waters of the U.S. Outer Continental Shelf.

Using a 2-year time series (2019-2020) of 1-min sound pressure level averages from seven sites, the extension of COVID-related quieting documented in coastal soundscapes to deep (approximately 200-900 m) waters off the southeastern United States was assessed. Sites ranged in distance to the continental shelf break and shipping lanes. Sound level decreases in 2020 were observed at sites closest to the shelf break and shipping lanes but were inconsistent with the timing of shipping changes related to a COVID-19 slowdown. These observations are consistent with increased numbers of vessel tracks in 2020 compared to 2019 at a majority of sites.

Divergence of a stereotyped call in northern resident killer whales.

Northern resident killer whale pods (Orcinus orca) have distinctive stereotyped pulsed call repertoires that can be used to distinguish groups acoustically. Repertoires are generally stable, with the same call types comprising the repertoire of a given pod over a period of years to decades. Previous studies have shown that some discrete pulsed calls can be subdivided into variants or subtypes. This study suggests that new stereotyped calls may result from the gradual modification of existing call types through subtypes. Vocalizations of individuals and small groups of killer whales were collected using a bottom-mounted hydrophone array in Johnstone Strait, British Columbia in 2006 and 2007. Discriminant analysis of slope variations of a predominant call type, N4, revealed the presence of four distinct call subtypes. Similar to previous studies, there was a divergence of the N4 call between members of different matrilines of the same pod. However, this study reveals that individual killer whales produced multiple subtypes of the N4 call, indicating that divergence in the N4 call is not the result of individual differences, but rather may indicate the gradual evolution of a new stereotyped call.

Hybrid millidecade spectra: A practical format for exchange of long-term ambient sound data.

This Letter proposes a frequency scaling for processing, storing, and sharing high-bandwidth, passive acoustic spectral data that optimizes data volume while maintaining reasonable data resolution. The format is a hybrid that uses 1 Hz resolution up to 455 Hz and millidecade frequency bands above 455 Hz. This hybrid is appropriate for many types of soundscape analysis, including detecting different types of soundscapes and regulatory applications like computing weighted sound exposure levels. Hybrid millidecade files are compressed compared to the 1 Hz equivalent such that one research center could feasibly store data from hundreds of projects for sharing among researchers globally.

Erratum: Hybrid millidecade spectra: A practical format for exchange of long-term ambient sound data [JASA Express Lett. 1(1), 011203 (2021)].

In the original paper [JASA Express Lett. 1(1), 011203 (2021)], a method for processing, storing, and sharing high-bandwidth, passive acoustic spectral data that optimizes data volume while maintaining reasonable data resolution was proposed. The format was a hybrid that uses 1-Hz resolution up to 455 Hz and millidecade frequency bands above 455 Hz. The choice of 455 Hz was based on a method of computing the edge frequencies of millidecade bands that is not compatible with summing millidecades to decidecades. This has been corrected. The new transition frequency is the first frequency with a millidecade with greater than 1 Hz, 435 Hz.

The soundscape of the Anthropocene ocean.

Oceans have become substantially noisier since the Industrial Revolution. Shipping, resource exploration, and infrastructure development have increased the anthrophony (sounds generated by human activities), whereas the biophony (sounds of biological origin) has been reduced by hunting, fishing, and habitat degradation. Climate change is affecting geophony (abiotic, natural sounds). Existing evidence shows that anthrophony affects marine animals at multiple levels, including their behavior, physiology, and, in extreme cases, survival. This should prompt management actions to deploy existing solutions to reduce noise levels in the ocean, thereby allowing marine animals to reestablish their use of ocean sound as a central ecological trait in a healthy ocean.

Manatee (Trichechus manatus) vocalization usage in relation to environmental noise levels.

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

Information theory analysis of Australian humpback whale song.

Songs produced by migrating whales were recorded off the coast of Queensland, Australia, over six consecutive weeks in 2003. Forty-eight independent song sessions were analyzed using information theory techniques. The average length of the songs estimated by correlation analysis was approximately 100 units, with song sessions lasting from 300 to over 3100 units. Song entropy, a measure of structural constraints, was estimated using three different methodologies: (1) the independently identically distributed model, (2) a first-order Markov model, and (3) the nonparametric sliding window match length (SWML) method, as described by Suzuki et al. [(2006). "Information entropy of humpback whale song," J. Acoust. Soc. Am. 119, 1849-1866]. The analysis finds that the song sequences of migrating Australian whales are consistent with the hierarchical structure proposed by Payne and McVay [(1971). "Songs of humpback whales," Science 173, 587-597], and recently supported mathematically by Suzuki et al. (2006) for singers on the Hawaiian breeding grounds. Both the SWML entropy estimates and the song lengths for the Australian singers in 2003 were lower than that reported by Suzuki et al. (2006) for Hawaiian whales in 1976-1978; however, song redundancy did not differ between these two populations separated spatially and temporally. The average total information in the sequence of units in Australian song was approximately 35 bits/song. Aberrant songs (8%) yielded entropies similar to the typical songs.