Active Acoustic Surveys Research Articles

Active acoustic surveys reveal coastal fish community resistance to an environmental perturbation in South Florida.

Coastal fish communities are under increasing levels of stress associated with climate variation and anthropogenic activities. However, the high degree of behavioral plasticity of many species within these communities allow them to cope with altered environmental conditions to some extent. Here, we combine meteorological information, data from hydroacoustic surveys, and recordings of goliath grouper sound production to examine the response of coastal fish communities to heavy rainfall events in South Florida, USA, that resulted in the release of excess storm water into surrounding estuaries and coastal waters. We observed a nearly 12,000% increase in water column acoustic backscatter following a heavy rainfall event of September 16th, 2015. Interestingly, estimates of school backscatter, a proxy for biomass, increased by 172% with the onset of the perturbation. Schooling fish density also increased by 182%, as did acoustically derived estimates of mean schooling fish length (21%). Following the perturbed period, school backscatter decreased by 406%, along with schooling density (272%), and mean schooling fish length (35%). Hydrophone and hydroacoustic data also revealed that goliath grouper (Epinephelus itajara) spawning aggregations were persistent in the region throughout the duration of the study and continued to exhibit courtship behavior during the perturbed period. Our observations demonstrate the high level of resistance common in coastal species but raises new questions regarding the threshold at which fish communities and reproductive activities are disrupted. As coastal land use continues to increase, and the effects of global climate change become more pronounced, more Before-After Control Impact (BACI) studies will provide improved insight into the overall response of nearshore communities to future perturbations and the cumulative effect of repeated perturbations over extended periods.

Laboratory Investigation of Hydraulic Fracture Growth in Zimbabwe Gabbro

AbstractDirect measurement of the apparent toughness evolution during hydraulic fracture growth remains challenging in rocks due to the difficulty of measuring the time evolution of the fracture geometry. We report hydraulic fracturing tests performed in 250 × 250 × 250 mm Zimbabwe gabbro under different confining stress. Using time‐lapse active acoustic surveys, we reconstruct the fracture front and fracture width using scattered and transmitted waves. After verifying that hydraulic fracture growth is dominated by the energy dissipated in fracture creation such that the fluid pressure can be considered spatially uniform in the fracture (negligible viscous flow energy dissipation), we estimate the apparent fracture toughness, using the theory of linear hydraulic fracture mechanics, by combining fracture geometry and pressure data. The apparent fracture energy is higher than that measured from semicircular bending tests. The created fractures evolve in a planar and radial geometry at the macroscopic scale, but we observe overlapping segments and bridges at the millimetric scale. This complex segmentation at a scale on par with the rock grain size is likely responsible for the observed higher fracture energy at centimetric scale. Such fracture segmentation and bridging will necessarily occur when encountering heterogeneities and result in a further increase of fracture energy at a larger scale even if the overall macroscopic geometry remains planar. The experimental data set reported here should guide modeling efforts necessary to better predict hydraulic fracture growth in rock masses.

Coastal seascape variability in the intensifying East Australian Current Southern Extension

Coastal pelagic ecosystems are highly variable in space and time, with environmental conditions and the distribution of biomass being driven by complex processes operating at multiple scales. The emergent properties of these processes and their interactive effects result in complex and dynamic environmental mosaics referred to as “seascapes”. Mechanisms that link large-scale oceanographic processes and ecological variability in coastal environments remain poorly understood, despite their importance for predicting how ecosystems will respond to climate change. Here we assessed seascape variability along the path of the rapidly intensifying East Australian Current (EAC) Southern Extension in southeast Australia, a hotspot of ocean warming and ecosystem tropicalisation. Using satellite and in situ measures of temperature, salinity and current velocity coupled with contemporaneous measurements of pelagic biomass distribution from nine boat-based active acoustic surveys in five consecutive years, we investigated relationships between the physical environment and the distribution of pelagic biomass (zooplankton and fish) at multiple timescales. Survey periods were characterised by high variability in oceanographic conditions, with variation in coastal conditions influenced by meso-to-large scale processes occurring offshore, including the position and strength of eddies. Intra-annual variability was often of a similar or greater magnitude to inter-annual variability, suggesting highly dynamic conditions with important variation occurring at scales of days to weeks. Two seascape categories were identified being characterised by (A) warmer, less saline water and (B) cooler, more saline water, with the former indicating greater influence of the EAC on coastal processes. Warmer waters were also associated with fewer, deeper and less dense biological aggregations. As the EAC continues to warm and penetrate further south, it is likely that this will have substantial effects on biological activity in coastal pelagic ecosystems, including a potential reduction in the accessibility of prey aggregations to surface-feeding predators and to fisheries. These results highlight the import role of offshore oceanographic processes in driving coastal seascape variability and biological activity in a region undergoing rapid oceanic warming and ecological change.

Distribution of acoustic fish backscatter associated with natural and artificial reefs in the Northeastern Gulf of Mexico

Artificial reefs have been deployed throughout the northern Gulf of Mexico in an effort to enhance fish production and mitigate losses from the depletion of natural habitats. Measuring the efficacy of artificial reefs in these capacities requires both fine- and large-scale direct comparisons between natural and artificial reef structures. We conducted mobile active acoustic surveys to examine intra-reef and regional spatial patterns of acoustic fish backscatter in ~2 km2 survey areas over 14 natural and 17 artificial reefs along the northwestern Florida continental shelf. Nested generalized additive models (GAMs) showed that intra-reef backscatter from fish at natural reefs was diffused throughout the survey area forming low density patches, whereas backscatter at artificial reefs was concentrated in high density hot-spots that were constricted horizontally within a mean of 116 m away from the structure. The exact distance from artificial structures at which backscatter was decreased to background levels, termed the area of influence, was found to be mediated by reef longitude, relief, area, and depth. On a regional scale, hierarchical GAMs (HGAMs) which incorporated altitude from the seafloor explained a larger proportion of deviance in backscatter than either latitude or longitude at both habitat types, indicating that the horizontal distribution of backscatter at these habitats varied throughout the water column. Trends in the horizontal distribution of backscatter were primarily driven by scatterers within 20 m of the seafloor at natural reefs and within 10 m of the surface at both natural and artificial reefs. HGAMs which examined only the horizontal distribution of backscatter showed a negative gradient of fish backscatter at natural reefs from west to east along the northwestern Florida continental shelf which was significantly influenced by reef depth. At both reef types, backscatter from fish was highest in the bottom 10 m of the water column and declined significantly with increasing altitude from the seafloor.

Seasonal variations in multi-year active acoustic surveys of fish and zooplankton in the New York Bight

Active acoustic surveys provide fine-scale measurements over broad spatial and temporal scales of organism abundance in the water column which can be useful as ecosystem indicators. As part of a long-term time series monitoring program, we conducted seasonal surveys within the New York Bight beginning in 2017. The survey comprises eight transects that extend from near-shore to the continental shelf break, and include multiple stations within each transect where net tows and CTD casts are conducted. Acoustic backscatter measurements at 38, 70, 120, and 200 kHz from the water column were integrated (NASC, m2 nmi−2) to examine the distribution and relative abundance of pelagic scatterers. Generally, NASC was highest during the summer and fall surveys, which corresponds to expected seasonal migrations of fish and squid in this area. Winter NASC distributions were highest in waters deeper than 50 m, but increased nearshore during the summer and fall surveys. Backscatter also appeared to be associated with bathymetric features, such as the shelfbreak and the Hudson Canyon, and in some cases with hydrographic fronts and gradients. These data are the first steps in a longer-term monitoring program of pelagic organisms in the New York Bight.

The first record of European free-tailed bat, Tadarida teniotis Rafinesque, 1814, and note on probable elevational movement from Nepal

Out of the four Molossidae species from South Asia, the distribution of the European free-tailed bat, T. teniotis is most poorly known. This species has been occasionally reported from Afghanistan, Bangladesh, Bhutan, and India; however, no records exist in Nepal. Here we report the first record of T. teniotis from Nepal and comment on its possible elevational movement in the Himalaya. Active acoustic surveys were conducted in the Kali Gandaki canyon during autumn and winter seasons at two elevational zones, 800–1200 m and 2100–2500 m, in three habitat types (forest, agricultural land, and human settlements). Echolocation calls of T. teniotis were easily distinguished by their low frequency, shallow frequency modulation, and long duration. During autumn, the activity was recorded only at 2100 to 2500 m and varied significantly from winter activity, while T. teniotis was observed at both elevational zones during winter. The result confirms the presence of T. teniotis from Nepal. Based on our observations of differential activity at different elevation zones in two seasons, we recommend more intensive studies to confirm seasonal migration and to understand seasonal demographics along the Kali Gandaki landscape and in the entire Himalayan range at large.

Fishery-independent observations of Atlantic menhaden abundance in the coastal waters south of New York

Atlantic menhaden (Brevoortia tyrannus) is a migratory forage fish whose geographic range extends from the Gulf of Maine to Florida. Despite the ecological and commercial importance of menhaden, few fishery-independent surveys have been conducted to quantify their distributions of abundance and biomass. Active acoustic surveys of menhaden schools were conducted using 38, 120, and 200 kHz scientific echosounders in two regions on the continental shelf south of Long Island, New York during the summer of 2014 and spring, summer, and fall of 2015. Spatial distributions of menhaden schools were considered to be clustered across all surveys where three or more schools were observed. Estimates of school volume were highly variable, ranging from 30 to 48,000 m3. Menhaden aggregations were absent in the spring (April, May, and June) and present in the summer and fall (July, August, and September) surveys. Target strength measurements at 120 kHz during the summer and fall surveys were consistent with predicted values for adult Atlantic menhaden, suggesting that fish size did not change substantially during this time. Peak menhaden abundance was observed during August 2015 (approximately 157,000 fish km−2 with a wet weight biomass density of 35,000 kg km−2). Multifrequency acoustic surveys can provide an efficient and fishery-independent method for quantifying menhaden abundance and distribution in coastal habitats.

Estimating fish abundance at spawning aggregations from courtship sound levels

Sound produced by fish spawning aggregations (FSAs) permits the use of passive acoustic methods to identify the timing and location of spawning. However, difficulties in relating sound levels to abundance have impeded the use of passive acoustics to conduct quantitative assessments of biomass. Here we show that models of measured fish sound production versus independently measured fish density can be generated to estimate abundance and biomass from sound levels at FSAs. We compared sound levels produced by spawning Gulf Corvina (Cynoscion othonopterus) with simultaneous measurements of density from active acoustic surveys in the Colorado River Delta, Mexico. During the formation of FSAs, we estimated peak abundance at 1.53 to 1.55 million fish, which equated to a biomass of 2,133 to 2,145 metric tons. Sound levels ranged from 0.02 to 12,738 Pa2, with larger measurements observed on outgoing tides. The relationship between sound levels and densities was variable across the duration of surveys but stabilized during the peak spawning period after high tide to produce a linear relationship. Our results support the use of active acoustic methods to estimate density, abundance, and biomass of fish at FSAs; using appropriately scaled empirical relationships, sound levels can be used to infer these estimates.

Acoustic quantification of abundance, biomass, and size class of Atlantic menhaden (Brevoortia tyrannus) in a shallow estuary in Long Island, New York

Atlantic menhaden (Brevoortia tyrannus) is an euryhaline forage fish commonly found on Long Island from late spring to fall and is both ecologically and economically important to fisheries along the entire eastern coast of the United States. Schools of menhaden frequently occupy the shallow (<4 m) bays and rivers of the western Peconic Estuary on Long Island, New York. These shallow habitats are difficult to sample using traditional pelagic (i.e., net trawls) or shore-adjacent (i.e., beach seines) methods due to shallow water depths and salt marsh coastline. We conducted multiple acoustic surveys using fisheries echosounders (38, 120, and 200 kHz) and sidescan sonar between May and November in 2015 and 2016 in Flanders Bay and the Peconic River. Active acoustic surveys were capable of providing estimates of abundance, biomass, and length distribution of Atlantic menhaden. Abundance and biomass estimates for the entire western Peconic Estuary were extrapolated from survey data and showed large (order of magnitude) variations in the menhaden population in these waters from spring to fall. Length distributions of menhaden differed both among seasons and between years. Data were ground-truthed using video, photographs, and morphological measurements of Atlantic menhaden.

Persistent Enhancement of Micronekton Backscatter at the Summits of Seamounts in the Azores

Knowledge of the dynamics of micronekton at seamounts is critical to understanding the ecological role of these ecosystems. Active acoustic techniques are an effective tool to monitor the distribution and movements of pelagic organisms. We carried out several day- and nighttime active acoustic surveys over a 3-year period (2009-2011) to characterize the spatial and temporal distribution of micronekton backscatter on two seamounts (Condor and Gigante) in the Azores and in the surrounding open-waters. The highest mean volume backscattering strength (MVBS) was consistently found in the water column over the seamount summits, regardless of the season and diel period. MVBS over the summits was 14 to 26 times higher than over the slopes, and 10 times higher than in open-waters. Diel variations in backscatter intensity were more pronounced in open-waters and in Gigante seamount, with higher values during the day in open-waters, and at night over the summits and slopes of Gigante. Over Condor seamount, diel changes in backscatter intensity were small, but MVBS was generally higher at night than during the day, as in Gigante. Persistence of strong acoustic backscatter over the summits of Condor and Gigante seamounts is a key finding of this study and may be explained by the presence of a seamount-associated micronekton community and by the retention of vertically migrating micronekton. The latter hypothesis is consistent with observed day-night differences in backscatter, suggesting that nocturnal migrants may be passively transported or actively swim above seamount summits and slopes. Possible physical mechanisms leading to the observed patterns in micronekton distribution are discussed. This study contributes to a better understanding of how seamounts may influence the spatial and temporal dynamics of micronekton assemblages.

Characterizing a Foraging Hotspot for Short-Finned Pilot Whales and Blainville's Beaked Whales Located off the West Side of Hawai'i Island by Using Tagging and Oceanographic Data.

Satellite tagging data for short-finned pilot whales (Globicephala macrorhynchus) and Blainville’s beaked whales (Mesoplodon densirostris) were used to identify core insular foraging regions off the Kona (west) Coast of Hawai‘i Island. Ship-based active acoustic surveys and oceanographic model output were used in generalized additive models (GAMs) and mixed models to characterize the oceanography of these regions and to examine relationships between whale density and the environment. The regions of highest density for pilot whales and Blainville’s beaked whales were located between the 1000 and 2500 m isobaths and the 250 and 2000 m isobaths, respectively. Both species were associated with slope waters, but given the topography of the area, the horizontal distribution of beaked whales was narrower and located in shallower waters than that of pilot whales. The key oceanographic parameters characterizing the foraging regions were bathymetry, temperature at depth, and a high density of midwater micronekton scattering at 70 kHz in 400–650 m depths that likely represent the island-associated deep mesopelagic boundary community and serve as prey for the prey of the whales. Thus, our results suggest that off the Kona Coast, and potentially around other main Hawaiian Islands, the deep mesopelagic boundary community is key to a food web that supports insular cetacean populations.

Passive acoustic determination of herring size.

Over the last decade, acoustic signatures of a variety of fishes have been recorded and analyzed. More recently, these vocalizations have been used to passively detect aggregations of fish, demonstrating the potential to supplement and enhance traditional active acoustic surveys. Based on previously published work on acoustic emissions of herring, this paper discusses the possibility of not merely passively detecting absence or presence of aggregations of herring but, additionally, assessing abundance and size. Theoretical considerations as well as data collected in Prince William Sound, Alaska, on Pacific herring (Clupea Pallasii) are presented.

Habitat-associated and temporal patterns of bat activity in a diverse forest landscape of southern New England, USA

The development and use of acoustic recording technology, surveys have revealed the composition, relative levels of activity, and preliminary habitat use of bat communities of various forest locations. However, detailed examinations of acoustic surveys results to investigate temporal patterns of bat activity are rare. Initial active acoustic surveys of bat activity on the Quabbin Reservoir watershed in central Massachusetts recorded high numbers of call sequences by five bat species. The results showed the importance of aquatic and open habitats for bats in this diverse forest landscape. Because the preliminary surveys were restricted in extent (habitats), number (replicates), and duration (active surveys only, limited number of repeated surveys), the authors strongly recommended more comprehensive and extensive surveys of bat activity on the area. This paper reports the results of active (manual) and passive (automated) acoustic bat activity surveys in three replicates of ten habitats, conducted three to four times per year over the course of the study between 2004 and 2006. Over the full study, 2,800 actively-recorded 37,632 passively-recorded call sequences were examined and identified to species when possible. General patterns in activity did not differ by habitat between the two survey methods. Call sequences were recorded more often in open, especially aquatic habitats than in cluttered habitats. The use of passive surveys allowed for the assessment of temporal patterns of activity and an analysis of the relationship between ambient temperature and activity.

Passive Acoustic Detection and Monitoring of Schools of Herring

Passive acoustic detection and monitoring of various marine fishes has recently received much attention in the literature. It has been recognized that passive acoustic techniques have the potential to complement traditional active acoustic surveys and to significantly increase their overall efficiency, if the acoustic signatures of the considered species are well understood. In this paper, the potential of passive acoustic techniques is explored for the specific case of Pacific herring (Clupea palassii). It is demonstrated that schools of herring can acoustically be detected by observing the sound of coordinated bubble release, triggered, e.g., by predator activity. This sound not only has identifiable features that can be exploited for determining the presence or absence by simple means, but could also carry abundance and size information. Work supported by ONR and the NMFS via the PWSSC.

Passive Acoustics as a Tool in Fisheries Science

AbstractMany fishery biologists that are interested in documenting fish habitat and following the movements and behavior of fishes use acoustic tags. Because over 700 fish species naturally produce low‐frequency, species‐specific sounds, these can be used as natural acoustic tags. Passive acoustic approaches (monitoring sound‐producing fishes with hydrophones) show great promise for gathering data in a noninvasive and continuous manner. In this special section, authors review past studies and contribute new findings based on the concept of passive acoustics, in which the sounds produced by fish are used to identify the species present and quantify their relative abundance. Fish have long been known to produce low‐frequency sounds, especially members of the families Sciaenidae, Gadidae, Ictaluridae, Cyprinidae, Batrachoididae, Haemulidae, Lutjanidae, and Serranidae. Passive acoustic methods include the use of low‐frequency hydrophones, digital recorders, autonomous recording sonobuoys and data loggers, and towed hydrophone arrays to record fish sounds. The sounds of fishes that have been recorded so far have been described in monographs, scientific papers, and online digital libraries; in most cases, the recordings are species specific and can be used to identify fish. Work is progressing in using the passive acoustic approach along with traditional fisheries sampling methods (net and active acoustic surveys) to identify habitat use, spawning areas, and relative abundances. The authors in this special section present new passive acoustics‐derived data on sciaenids, batrachoidids, and ictalurids. They outline the methods currently being used and discuss their limitations, provide examples where passive acoustics has been employed successfully, warn of pitfalls in interpreting acoustic data, and lay the groundwork for future studies.

Bat Activity in a Forest Landscape of Central Massachusetts

Nine species of bat are known to occur across the six New England states, but most aspects of their natural history, such as foraging habitat use, are poorly understood. Recent published research has documented the importance of still-water habitats as foci of bat flight activity. To better understand and document habitat use in southern New England, we used the AnaBat II acoustical monitoring system to assess species composition and relative levels of summer flight activity. Active acoustic surveys were conducted in six habitat types on the Quabbin Reservation in central Massachusetts in 2003 and 2004. Bat flight activity, as measured by numbers of echolocation call sequences, was high, with an average of 24 search-phase and 4 feeding-buzz calls per 20-minute survey period. Myotis lucifugus (little brown bat) was the most commonly recorded species. Bat flight activity was high over all still-water habitats, but greatest over large ponds. Large-bodied bats, such as Eptesicus fuscus (big brown bat), were recorded more often in open, structurally uncluttered habitats. Of the small-bodied bats, little brown bats were ubiquitous, whereas Myotis septentrionalis (northern myotis) was most common in structurally cluttered habitats of seasonal forest (vernal) pools and along forest streams. Generalized habitat associations among the bat species we recorded are similar to those reported for other New England forest sites. The Quabbin Reservation is an excellent site to continue examining bat-habitat relationships because of the abundance and diversity of aquatic habitats, in both cluttered closed-canopy and uncluttered open-canopy settings.

An 8-year cycle in krill biomass density inferred from acoustic surveys conducted in the vicinity of the South Shetland Islands during the austral summers of 1991–1992 through 2001–2002

Data from single and multi-frequency active acoustic surveys conducted annually in the vicinity of the South Shetland Islands, Antarctica were re-analyzed using updated procedures for delineating volume backscattering due to Antarctic krill, adjusting for signal contamination due to noise, and compensating for diel vertical migration of krill outside of the acoustic observation window. Intra-and inter-seasonal variations in krill biomass density and dispersion were derived from the re-processed data set for surveys conducted in the austral summers of 1991/1992 through 2001/2002. Estimated biomass density ranged from 1 to 60 g m –2, decreasing from mid-range levels in 1991/1992 to a minimum in 1992/1993–1993/1994, increasing to a peak in 1996/1997–1997/1998, and decreasing again through 2000/2001–2001/2002. Although this variability may be attributed to changes in the spatial distribution of krill relative to the survey area, comparisons with the proportion of juvenile krill in simultaneous net samples suggest that the changes in biomass density are consistent with apparent changes in reproductive success. A truncated Fourier series fit to the biomass density time series is dominated by an 8-year cycle and predicts an increase in krill biomass density in 2002/2003 and 2003/2004. This prediction is supported by an apparent association between cycles in the extent of sea ice cover and per-capita krill recruitment over the last 23 years and indications that ice cover in the winter of 2002 is seasonally early and extensive.