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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.

An APH-22 vertical-takeoff-and-landing hexacopter was used to collect aerial images of schools and individuals of juvenile Atlantic bluefin tuna (ABFT; Thunnus thynnus) at the sea surface in the southern Gulf of Maine. Quantitative measures of fish length, width, and inter-fish spacing were obtained from these images by applying calibration settings and performance measures from calibrating, testing, and evaluating the onboard motion and altimeter sensors and the digital camera and lenses. The accuracy and precision of the onboard motion sensors, camera, and lens calibrations were sufficient to provide length measurements to sub-centimeter precision, but the altimeter performance was least reliable and required additional information, such as images of known-sized objects during each flight, to provide measurements at the accuracy and precision needed for data to be incorporated in fisheries management. The APH-22 was ideal for acquiring images of ABFT individuals and schools and may be a useful tool for remotely monitoring the behavior and body condition of these elusive animals.

In fisheries acoustics, the target strength (TS; dB re m2) is used to compute biological metrics such as fish biomass and density. The TS is challenging to characterize because of its stochastic relationship with fish physiology, orientation, depth, species assemblage, and size distributions. These challenges were addressed by using acoustic and physical samples of fish from trawls in the Penobscot River Estuary, Maine. The pelagic species assemblage was dominated by clupeids and osmerids. The TS was measured from individual fish using single target detection and echo tracking algorithms. An expectation–maximization algorithm was applied to identify the components of the TS and total length (TL; cm) distributions for the mixed species assemblages. Regressions were used to estimate the parameters of TS = α log10(TL) + β. The parameters, α = 31.2 [standard error (SE) 0.87] and β = –79.6 (SE 0.93), were similar to published studies from these species, but our slope and intercept were higher than those in studies from freshwater and lower than those from marine systems. These results suggest that acoustic surveys in estuaries with mixed species assemblages should carefully consider alternatives to “standard” TS-fish length equations. These results will provide necessary parameters to allow for interpretation of acoustic survey data from systems with a similar composition of pelagic species.

Large volumes of water column sonar data have been generated from acoustic surveys of living marine resources. They provide valuable information about marine ecosystems. To leverage them for acoustic target identification, scarcity of annotations is usually an issue. While some annotations have been generated by scientists via manual scrutiny or limited automation, they are limited to certain species. To fill in this gap, we propose a spatio-temporal contrastive learning approach for acoustic data. This unsupervised deep learning technique leverages both acoustic and non-acoustic (spatial and temporal) information for acoustic target classification. We firstly employ the Simple Linear Iterative Clustering (SLIC) algorithm to extract superpixels from acoustic data. Then the spatial and temporal similarity between two superpixels is computed, and the proposed spatio-temporal contrastive learning approach is applied to learn a semantically meaningful representation for superpixels. Finally, the learned representations of superpixels are used in acoustic target classification. We demonstrate that this approach outperforms previous methods with acoustic data collected by the NOAA National Marine Fisheries Service in the Gulf of Maine and Georges Bank region over 10 years. This dataset is archived at NOAA National Centers for Environmental Information and accessed for free through AWS and NOAA Big Data Program.

A network of sound sources, hydrophone arrays and physical oceanography and biological survey equipment was established at the southern edge of the New England Shelf in May 2021 to investigate how the oceanic processes at shelfbreak regions affect underwater sound propagation. The ocean processes of particular interest include shelfbreak fronts, shelf water streamers, thermohaline intrusions, and internal waves along with other significant marine geological features and biological factors, such as seafloor slopes, submarine canyons, variable seabed properties, and fish schooling and shoaling. With fixed propagation paths, acoustic fluctuations can be correlated with environmental variations between the network nodes. The temporal scale of the acoustic measurements ranges from minutes to weeks, and the spatial coverage is up to 20–30 km. Adaptive sampling and tracking of acoustic sensitivity “hot spots” was also conducted during the experiment to assist real-time joint ocean acoustics and circulation modeling. This presentation will review the design concept of this ocean acoustic network experiment and provide an overview the preliminary results. [Work supported by the Office of Naval Research.]

The New England Shelf Break is a dynamic region where warm salty slope water meets with colder less saline shelf water to form a distinct front. The position and shape of the front is modified by a range of processes on differing temporal and spatial scales. While seasonal changes are well documented, the influence of shorter time scale processes throughout the water column are less well understood. Of interest is the influence of meso-scale physical oceanographic processes such as meanders in the Gulf Stream and warm core rings and eddies. From March to June 2021 five cruises to the New England Shelf Break were conducted as part of the ONR Task Force Oceans New England Shelf Break Acoustics Experiments. The shipboard broadband echo sounders on the R/V Neil Armstrong were used to ensonify the front during each of these five cruises. Significant changes were observed, especially during the transit of a warm core eddy through the study area. Groups of scatterers in the study area were targeted by Deep-See (a broadband tow sled) and a broadband echo sounder equipped REMUS 600 to enable ensonification of individual targets to aide in acoustic identification of biology at the shelf break front.

This paper is not subject to U.S. copyright. The definitive version was published in ICES Journal of Marine Science 74 (2017): 2249–2261, doi:10.1093/icesjms/fsx050.

© International Council for the Exploration of the Sea, 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in ICES Journal of Marine Science 74 (2017): 2262–2275, doi:10.1093/icesjms/fsx155.

The article of record as published may be found at https://doi.org/10.1121/1.4983446 For horizontal-looking sonar systems operating at mid-frequencies (1–10 kHz), scattering by fish with resonant gas-filled swimbladders can dominate seafloor and surface reverberation at long-ranges (i.e., distances much greater than the water depth). This source of scattering, which can be difficult to distinguish from other sources of scattering in the water column or at the boundaries, can add spatio-temporal variability to an already complex acoustic record. Sparsely distributed, spatially compact fish aggregations were measured in the Gulf of Maine using a long-range broadband sonar with continuous spectral coverage from 1.5 to 5 kHz. Observed echoes, that are at least 15 decibels above background levels in the horizontal-looking sonar data, are classified spectrally by the resonance features as due to swimbladder-bearing fish. Contemporaneous multi-frequency echosounder measurements (18, 38, and 120 kHz) and net samples are used in conjunction with physics-based acoustic models to validate this approach. Furthermore, the fish aggregations are statistically characterized in the long-range data by highly non-Rayleigh distributions of the echo magnitudes. These distributions are accurately predicted by a computationally efficient, physics-based model. The model accounts for beam-pattern and waveguide effects as well as the scattering response of aggregations of fish. This research was supported by the U.S. Office of Naval Research, the National Oceanographic Partnership Program, NOAA, WHOI, and the Oceanographer of the U.S. Navy.

Collection of multifrequency fisheries acoustic data began at the NOAA Northeast Fisheries Science Center (NEFSC) in 1998. Over the past 20 + years, data have been collected on the NOAA fisheries survey vessels as well as commercial vessels during numerous surveys and research cruises. Many lessons in cooperation, initiative, persistence, and compromise have been learned, resulting in data sets with plenty of potential. Some of these data have been rejected and then resuscitated for stock assessments, used to estimate biomass of undersampled biota, and used to study catchability of commercial species. Until recently, data were limited to those at the NEFSC with sufficient expertise and access to the data—i.e., me, which has restricted application of the data. But now the data are searchable and available to the world via NOAA’s National Center for Environmental Information (NCEI). The evolution of acoustic technology and software from costly and proprietary to inexpensive and open-source has expanded the utility of these, and other long-term, data sets to the broader scientific community. An overarching goal has been to collect consistently high-quality data, yet only through incorporating innovation can long-term data collection survive in a dynamic environment and ever-changing scientific landscape.

A debate has developed over the ecosystem consequences following the collapse of Atlantic cod throughout the coastal waters of eastern Canada. The explosive increase in pelagic fish abundance in scientific bottom-trawl catches on the eastern Scotian Shelf has been interpreted as being due to either (i) a “pelagic outburst” of forage fish abundance resulting from predator release or conversely (ii) a change in pelagic fish vertical distribution leading to a “suprabenthic habitat occupation” thereby increasing their availability to bottom trawls. These two interpretations have diametrically opposing ecological consequences and suggest different management strategies for these important forage fish species. We argue that an objective evaluation of the available evidence supports the hypothesis that the abundance of forage fish has not increased in response to the demise of cod and other top predators, and the reliance on a single sampling gear with low catchability has biased and will continue to bias the interpretation of demographic trends of pelagic fish populations. We advocate that multiple sampling technologies providing alternative perspectives are needed for the monitoring and management of the various trophic levels if we are to achieve a balanced and objective understanding of marine ecosystems.

We describe a unique survey method that is able to cover a wide spatial and temporal range at a low cost. We utilised 10 individual small fishing vessels (lobster vessels) as acoustic research platforms to systematically survey a coastal population of Atlantic herring ( Clupea harengus) in the Gulf of Maine. We examined 38 transects spanning more than 200 miles of coastline. Due to the low cost of chartering the vessels, the survey was repeated weekly for nine weeks, resulting in over 2300 nautical miles of on-transect data. We calibrated single beam systems using a split beam guiding transducer to increase our confidence in the accuracy of the data and allow inter-ship comparisons. Although only one year of data has been analysed, the large spatial and temporal coverage of this survey provides preliminary information on herring population dynamics including spatial distribution throughout the survey area, timing of spawning and habitat associations of spawning aggregations. This survey will provide the template for longer term monitoring of herring spawning dynamics in Coastal Maine and how this may respond to climate and ocean variability.

The National Oceanographic and Atmospheric Administration (NOAA) uses water column sonar data to assess physical and biological characteristics from the ocean surface to the seabed. Acoustic surveys produce large volumes of data that can deliver valuable information beyond their original collection purpose if the data are properly managed, discoverable, and accessible to the public. NOAA's National Centers for Environmental Information, in partnership with NOAA's National Marine Fisheries Service and the University of Colorado, have created a national archive for water column sonar data to help achieve these goals. Through these efforts, over 21 TB of sonar data are now publicly available. Raw sonar files are difficult to interpret due to their size, complexity, and proprietary format. In order for users to understand the quality and composition of large volumes of archived data more easily, several visualization products were explored. Three processing methods were applied to multifrequency single-beam data (Simrad EK60) collected off the US northwest coast between 2007 and 2013. One method illustrates these complex data in a single image using a novel colour scale [multifrequency single-beam imaging (MFSBI)], another examines the nautical area scattering coefficients between two frequencies (ΔNASC), and the third indices the data into acoustic classifications [multifrequency indicator (MFI)]. The ability to apply the algorithms efficiently to multiyear datasets was explored. MFSBI proved effective at conveying the composition of the data and was easily adaptable to automated processing. ΔNASC, which required manual seabed corrections, illustrated a generalized pattern for changes in the water column across the shelf. MFI provided an empirically based statistical approach but will require more effort in the near term to evaluate and assess the accuracy and precision of each classification. Overall, spatio-temporal patterns of the acoustic backscatter identified large interannual variations in composition with the continental shelf break often playing a key role in attracting biological assemblages.

Vocalizations were recorded for over eight distinct whale species as they converged on a shoal of herring to feed; the predators divided the shoal into overlapping but species-specific foraging sectors and the activities of the whales changed between day and night. Using a combination of passive and active ocean acoustic waveguide remote sensing in an important northwestern Atlantic marine mammal autumn foraging ground, Purnima Ratilal and colleagues have mapped the movement and distribution of more than ten marine mammal species simultaneously with that of their fish prey. The vocalizations of humpback, blue, fin, minke and other marine mammal species were recorded across an area of roughly 100,000 square kilometres as they converged onto dense herring shoals, dividing them into overlapping but species-specific foraging sectors, which were maintained for more than two weeks. This snapshot of marine life reveals how predators moved through the huge shoal, indicating how the activities of the predators varied as day moved into night. The results will increase understanding of marine-mammal behaviour and inform conservation efforts. Observing marine mammal (MM) populations continuously in time and space over the immense ocean areas they inhabit is challenging but essential for gathering an unambiguous record of their distribution, as well as understanding their behaviour and interaction with prey species1,2,3,4,5,6. Here we use passive ocean acoustic waveguide remote sensing (POAWRS)7,8 in an important North Atlantic feeding ground9,10 to instantaneously detect, localize and classify MM vocalizations from diverse species over an approximately 100,000 km2 region. More than eight species of vocal MMs are found to spatially converge on fish spawning areas containing massive densely populated herring shoals at night-time11,12,13,14,15,16 and diffuse herring distributions during daytime. We find the vocal MMs divide the enormous fish prey field into species-specific foraging areas with varying degrees of spatial overlap, maintained for at least two weeks of the herring spawning period. The recorded vocalization rates are diel (24 h)-dependent for all MM species, with some significantly more vocal at night and others more vocal during the day. The four key baleen whale species of the region: fin, humpback, blue and minke have vocalization rate trends that are highly correlated to trends in fish shoaling density and to each other over the diel cycle. These results reveal the temporospatial dynamics of combined multi-species MM foraging activities in the vicinity of an extensive fish prey field that forms a massive ecological hotspot, and would be unattainable with conventional methodologies. Understanding MM behaviour and distributions is essential for management of marine ecosystems and for accessing anthropogenic impacts on these protected marine species1,2,3,4,5,17,18.

75 kHz ADCPs stationed near the 450-meter isobath within the Ocean Observatories Initiative Coastal Pioneer Array capture consistent patterns indicative of daily vertical migration (DVM) in depth profiles of both mean volume backscatter (MVB) and the vertical component of velocity (w). While the presence of DVM patterns is consistent throughout 2018 and 2019, attributes of MVB and w profiles are subject to variability within and across different seasons and years. This variability may be influenced by abiotic factors characteristic of the Mid-Atlantic Bight region such as the disruption of typical spring time temperature-salinity structures via frontal instability or the episodic introduction of warm-core rings. Additionally, biological community composition in the region may be reflected in MVB and w profiles given zooplankton survey data and knowledge of seasonal biophysical dynamics in the region, along with measured variability of other environmental properties observed by the Pioneer Array including dissolved oxygen concentration, chlorophyll-a concentration, and photosynthetically available radiation. Linking all of these factors, driven by long-term, stationary acoustical observations, can provide a useful framework for more directly assessing the impacts of aberrational physical phenomena on regional biological patterns.

An acoustic imaging microtome system (AIMS) was constructed to map the internal structure of fish. The system consists of two pairs of high-frequency (4.5 MHz) transmit-and-receive planar arrays, with one pair in the vertical plane and the other in the horizontal plane. AIMS provides a series of acoustic images, analogous to microtome slices, along the length of a fish by combining electronically controlled sequential acoustic transmissions and receptions over 224 channels and a computer-controlled mechanical device that moves the acoustic arrays lengthwise along the fish. AIMS measures the acoustic attenuation (extinction) along the direct ray paths of each transmit-receive channel pair that penetrates the fish body in a bistatic configuration. This results in a 2-D image of the interior morphology with 4- to 5-mm resolution. The images can provide morphological information on the fish's cheekbone, backbone, rib bone, skull, fins, stomach, and swimbladder, which is important for accurately modeling fish acoustic target strength, especially for non-swimbladder-bearing fish species.

Atlantic herring (Clupea harengus) in the Gulf of Maine were surveyed using fisheries-independent systematic and stratified-random surveys conducted annually by the Northeast Fisheries Science Center from 1998 to 2012 during autumn. The systematic surveys were dedicated to surveying Atlantic herring and were designed to be synchronous with pre-spawning, whereas the stratified-random surveys were broad-scale, multi-species surveys. Both types of surveys collected multi-frequency acoustic and trawl data. These surveys were analyzed to evaluate their individual effectiveness as surveys, whether results could be combined, or whether either survey could be used alone to estimate Atlantic herring abundance. With the exception of two years, more Atlantic herring in the water column were observed in the Georges Bank region during the dedicated systematic survey than later in the season by the stratified-random survey in the Georges Bank and northern Gulf of Maine regions. These observations suggest that the systematic survey was not out of synchrony with Atlantic herring spawning in the Georges Bank area, and the majority of spawning in the off-shore Gulf of Maine region occurred in the Georges Bank area. Additionally, the area backscattering time series showed no dramatic changes among surveys conducted with different vessels, suggesting that the Atlantic herring did not react substantially to changes in vessels and/or reacted similarly to the presence of any of the four vessels used.

Repeated acoustic and trawl surveys were performed in the Gulf of Maine cod spawning protection area (GOMCSPA) to: (a) describe the spatial and temporal distribution of spring-spawning Atlantic cod ( Gadus morhua ); (b) estimate their abundance and biomass; and (c) evaluate precision of the survey methods. A fishing vessel equipped with 38- and 120-kHz split-beam echo sounders surveyed once monthly from dusk to dawn along ten parallel transects that covered a 80.8 km 2 area during April–July 2011. During each survey, two bottom trawl vessels (one with a small mesh net and one with a large mesh net) each made ten tows in parallel behind the acoustic survey vessel. Cod abundance and biomass was derived from acoustic backscatter by a combination of methods: (1) species apportionment based on trawl catch vs. echo classification; (2) in situ vs. predicted target strength; (3) size of elementary distance sampling unit (EDSU) and statistical approach; and (4) with and without dead zone correction. The mean cod density based on echo classification and a 100-m EDSU resulted in a substantially lower coefficient of variation when the variance was estimated by geostatistics compared to any other method used. Based on echo classification, semivariogram modeling revealed that 67–77% of the variance in cod biomass density was explained by a spatial structural component at a range (correlation length) of 2.0–2.4 km. Density maps, produced by ordinary kriging, showed cod were relatively widespread in the survey area in May, but congregated at higher densities in areas adjacent to two elevated bathymetric features. Most cod converged to a single location in June, and were at a higher concentration compared to the highest densities observed in May. This congregation decreased in size and density in July. The survey estimates of cod biomass were 184–494 mt in May, 138–617 mt in June, and 39–135 mt in July. Based on echo classification, the biomass for the GOMCSPA, extrapolated from these survey estimates, were 260–466 mt in May, 196–513 mt in June, and 91–198 mt in July. These results provide some evidence that adult Atlantic cod in spawning condition congregated within the GOMCSPA during the seasonal fishing closure, and that the biomass being protected by the closure may have represented 4–5% of the GOM cod spawning stock biomass at the time of the study.

Stockwell, J. D., Weber, T. C., Baukus, A. J., and Jech, J. M. 2013. On the use of omnidirectional sonars and downwards-looking echosounders to assess pelagic fish distributions during and after midwater trawling. – ICES Journal of Marine Science, 70:196–203. Small pelagic fish can play an important role in the structure and function of ecosystems, and there is increasing interest in their non-market value. At the scale of fish aggregations, however, the impact of fishing has received relatively little attention, with most effort devoted to impacts of vessel and gear avoidance on stock size estimates. We used concurrent deployment of a downwards-looking echosounder (Simrad ES60 system) and an omnidirectional sonar (Simrad SP90 system) during commercial pairtrawling operations for Atlantic herring (Clupea harengus) in the Gulf of Maine to examine their potential for studying the impacts of fishing on herring aggregations. We compared a number of aggregation metrics to illustrate similarities and differences between the two systems, and then qualitatively examined their properties during and after pairtrawling events to illustrate potential applications. Our results suggest that using both downwards-looking and omnidirectional systems provides complementary information on fish aggregation metrics. Future applications of these systems in before–after–control-impact (BACI) designs may help inform management agencies when evaluating potential impacts of fishing at the time and space scales of pelagic fish aggregations.

Recently developed broadband acoustic methods were used to study mixed assemblages of fish spanning a wide range of lengths and species. Through a combination of resonance classification and pulse-compression signal processing, which provides for high-range resolution, a modified commercial broadband echosounder was demonstrated to provide quantitative information on the spatial distribution of the individual size classes within an assemblage. In essence, this system spectrally resolves the different size classes of fish that are otherwise not resolved spatially. This method reveals new insights into biological processes, such as predator–prey interactions, that are not obtainable through the use of a conventional narrowband high-frequency echosounder or previous broadband systems. A recent study at sea with this system revealed aggregations containing bladdered fish 15–30 cm in length (Atlantic herring ( Clupea harengus ) and silver hake ( Merluccius bilinearis )) and a variety of species of smaller fish 2–5 cm in length. These observations infer that the smaller 2–5 cm fish can be colocated in the same aggregations as their predator, the larger silver hake, as well as pre-spawning herring. While this technological advancement provides more information, there remain challenges in interpreting the echo spectra in terms of meaningful biological quantities such as size distribution and species composition.

Atlantic herring (Clupea harengus ) in the offshore regions of the Gulf of Maine migrate each fall from their feeding grounds to the northern portion of Georges Bank to spawn. The Northeast Fisheries Science Center’s (NEFSC) herring acoustic survey has taken advantage of this behavior by conducting systematic surveys of the pre-spawning fish each year since 1999. Multi-frequency acoustic and midwater trawl data were collected along transects oriented perpendicular to bathymetric contours. Acoustic backscatter was analyzed to describe the aggregative patterns (e.g., size, location in the water column, and spatial and temporal distribution) of Atlantic herring during these surveys and regression trees were used to examine the aggregation characteristics. The positional variables of distance to spawning grounds and vertical location in the water column were the primary characteristics for describing pre-spawning aggregations. Secondary to these were the temporal variables of diel and survey timing, and the morphological characteristic of aggregation area. Lower numbers of aggregations were observed close to the herring spawning grounds but with higher acoustic energy than larger numbers of aggregations observed further from the spawning grounds but smaller in size and lower in acoustic energy. Most aggregations were in the lower portion of the water column, but those that were in the upper portion of the water column had higher acoustic energy. Consistently throughout the decade, 90% or more of herring aggregations were located within 40 nautical miles of their spawning grounds. The regression tree method provided valuable insight to the data series where it highlighted spatial and temporal patterns and was an effective way to quantitatively summarize relationships.

Water-column echosounder data are widely used for a diversity of research objectives from fisheries abundance to identifying methane seeps to characterization of the seafloor. However, these data are voluminous, complex, and recorded in instrument-specific binary file formats. Tools to process these data are limited to a few commercial applications or custom programs developed by researchers, leaving potential users without sufficient financial resources or programming knowledge to use the data. To address this problem, NOAA Fisheries, University of Colorado Boulder’s Cooperative Institute for Research in Environmental Sciences, and NOAA National Centers for Environmental Information (NCEI) scientists are developing PyEcholab. PyEcholab is an open-source, python-based system for reading, processing, and visualizing water-column echosounder files. Currently, the system is being developed to meet existing NCEI processing and visualization needs but PyEcholab’s base classes and open architecture provide a framework for developing new file readers, processing algorithms, and visualization techniques that can be modularized into the system. The long-term vision is to engage the community in an open-source effort that continually grows PyEcholab’s capabilities and expands the accessibility of water-column echosounder data.

Stanton, T. K., Chu, D., Jech, J. M., and Irish, J. D. 2010. New broadband methods for resonance classification and high-resolution imagery of fish with swimbladders using a modified commercial broadband echosounder. – ICES Journal of Marine Science, 67: 365–378. A commercial acoustic system, originally designed for seafloor applications, has been adapted for studying fish with swimbladders. The towed system contains broadband acoustic channels collectively spanning the frequency range 1.7–100 kHz, with some gaps. Using a pulse-compression technique, the range resolution of the echoes is ∼20 and 3 cm in the lower and upper ranges of the frequencies, respectively, allowing high-resolution imaging of patches and resolving fish near the seafloor. Measuring the swimbladder resonance at the lower frequencies eliminates major ambiguities normally associated with the interpretation of fish echo data: (i) the resonance frequency can be used to estimate the volume of the swimbladder (inferring the size of fish), and (ii) signals at the lower frequencies do not depend strongly on the orientation of the fish. At-sea studies of Atlantic herring demonstrate the potential for routine measurements of fish size and density, with significant improvements in accuracy over traditional high-frequency narrowband echosounders. The system also detected patches of scatterers, presumably zooplankton, at the higher frequencies. New techniques for quantitative use of broadband systems are presented, including broadband calibration and relating target strength and volume-scattering strength to quantities associated with broadband signal processing.

Weber, T. C., Peña, H., and Jech, J. M. 2009. Consecutive acoustic observations of an Atlantic herring school in the Northwest Atlantic. – ICES Journal of Marine Science, 66: 1270–1277. Several successive images of the same school of Atlantic herring (Clupea harengus) were collected over the course of ∼1 h just north of Georges Bank in the Northwest Atlantic. Although the fish may not have been in their natural, undisturbed state, we observed what appeared to be the fish school fragmenting and dispersing, using a split-beam and a multibeam echosounder. Calibrated, 38 kHz, split-beam echosounder (Simrad EK60) and trawl-catch data provided accurate measures of the fish density beneath the vessel. Uncalibrated, 400 kHz, multibeam-echosounder (Reson 7125) data provided synoptic observations of the fish school including estimates of the school volume, morphology, and behaviour. Observations of the angular dependence in the multibeam-echosounder measurements of backscatter from fish allow investigation of the efficacy of extrapolating fish-school densities measured by the split-beam echosounder to the entire school.

Gurshin, C. W. D., Jech, J. M., Howell, W. H., Weber, T. C., and Mayer, L. A. 2009. Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders. – ICES Journal of Marine Science, 66: 1303–1309. Effective management strategies for Atlantic cod (Gadus morhua) in the Gulf of Maine require stock assessments based on accurate estimates of its abundance and distribution. If multibeam echosounders are to provide data for such estimates, the relationship between acoustic backscatter and fish biology must be better understood. Working towards this goal, a series of acoustic measurements was made using a 120 kHz, split-beam echosounder (Simrad EK60) and a 300 kHz, multibeam echosounder (Kongsberg EM3002). The transducers from both systems were fixed to a platform over a submerged 98 m3 cage made of 5 cm stretched-nylon mesh. After standard-sphere calibrations, the cage was stocked with live, mature Atlantic cod, with a mean total length of 80.7 cm (range: 51.5–105.0 cm). The echosounders synchronously collected acoustic data, while the cod were monitored with two underwater video cameras. Cod were incrementally removed from the cage to provide a time-series of acoustic backscatter at four densities (n = 128, 116, 66, and 23). Backscatter measurements of cod are compared between echosounders and over time, and the factors affecting the acoustically derived density estimates are discussed. The benefits and limitations of the EM3002 are highlighted.

Similarities in the behavior of diverse animal species that form large groups have motivated attempts to establish general principles governing animal group behavior. It has been difficult, however, to make quantitative measurements of the temporal and spatial behavior of extensive animal groups in the wild, such as bird flocks, fish shoals, and locust swarms. By quantifying the formation processes of vast oceanic fish shoals during spawning, we show that (i) a rapid transition from disordered to highly synchronized behavior occurs as population density reaches a critical value; (ii) organized group migration occurs after this transition; and (iii) small sets of leaders significantly influence the actions of much larger groups. Each of these findings confirms general theoretical predictions believed to apply in nature irrespective of animal species.

The article of record as published may be found at https://doi.org/10.1121/1.4937607 Analytical and numerical scatteringmodels with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m2) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not converge This work was supported by the NOAA Fisheries Advanced Sampling Technologies Working Group, the Office of Naval Research, and the National Oceanic Partnership Program. Josiah S. Renfree

Acoustic surveys have been conducted on Georges Bank from 1998 to present to estimate Atlantic herring (Clupea harengus) population abundance. Acoustic data were collected with a 12 or 18, 38, and 120 kHz Simrad EK500 scientific echo sounder. A pelagic trawl and underwater video images were used to collect biological information and to verify the species composition of acoustic backscatter. A multifrequency classification method was developed to improve the efficiency and accuracy of classifying species from acoustic echograms. In this method, a volume backscatter (Sv) threshold was applied equivalently to all echograms, and then a composite echogram was created based on which frequencies had Sv greater than or less than the Sv threshold. The results of this method were compared with the standard method of visually scrutinizing regions, and metrics were developed to evaluate the accuracy of classification algorithms relative to current methods, as well as to assess the effects of classification methods on population abundance estimates. In general, this method matched visually scrutinized Atlantic herring regions, but with consistent biases in classifying 38 kHz backscatter. The metrics highlighted spatial and temporal changes in the acoustic landscape, which may be indicative of intra- and inter-annual biological changes.

The EK500 has been the state-of-the-art scientific echosounder for surveying marine fish stocks for over a decade; the EK60 is its successor. Ensuring comparability in performance is vital during the transition from the EK500 to the EK60. To quantify the respective performances, each echosounder was calibrated in tandem by the standard-target method using the same 38-kHz, 12° beam width, split-beam transducer, with alternating pinging by means of an external triggering-and-switching system. The principal measurements comprised split-beam-determined angle and target strength, on-axis sensitivity, and directionality in the plane normal to the acoustic axis, as measured with a 60-mm-diameter copper sphere. Ambient noise, including volumetric reverberation, was also measured. Principal comparisons included those of the time-series and histograms of split-beam-determined target strength; respective alongship and athwartship angles as determined by the split-beam system; and as expected, difference in the split-beam-determined and experimental target-strength values in the plane normal to the acoustic axis. The mean absolute difference in off-axis angle values was also compared. While the performance of the two echosounders is generally similar, systematic differences exist. For the particular calibration measurements, the time variability in measurements of on-axis target strength was of the order of 1 dB for the EK500 and 2 dB for the EK60. The target-strength distribution for measurements made with the EK500 was normal, with standard deviation 0.2–0.3 dB, whereas for the EK60, the target-strength distribution was distinctly skewed and the standard deviation varied over 0.3–0.5 dB. Differences were found between the split-beam and physical-angle measurements. They were noticeably larger in the case of the EK60. Differences in performance between the two echosounders suggest refinements to the new system that will help realize its full potential in scientific work.

Acoustic measurements at 1.5–5 kHz on fish in the Gulf of Maine showed a swimbladder-resonance peak near 2.5 kHz at 160–190-m depth. Midwater trawls confirmed that the fish were likely to be Atlantic herring (Clupea harengus) of 19–29 cm length. Calculation using a model of swimbladder resonance gives swimbladder volumes of 1.2% of fish weight at 160–190 m. Extrapolation of this volume of gas using Boyle's Law suggests that at the sea surface, these herring would need to inflate their swimbladders by up to five to six times the volume required for neutral buoyancy. If these fish were to maintain this volume of gas with surface “gulping”, they would need to submerge from the sea surface with a 30% excess buoyancy. In general, swimbladders of the Clupeidae may have greater volumes of gas than if the fish were neutrally buoyant at the sea surface and the interpretation of HF-echosounder surveys may be additionally complex when the volume of gas and swimbladder volume are difficult to predict. Mechanisms of how herring obtain additional swimbladder gas are discussed.

Acoustic surveys of aquatic organisms are notorious for large data sets. Density distribution results from these surveys are traditionally graphed as two-dimensional plots. Increasing information content through wider acoustic frequency ranges or multiple angular perspectives has increased the amount and complexity of acoustic data. As humans are visually oriented, our ability to assimilate and understand information is limited until it is displayed. Computer visualization has extended acoustic data presentation beyond two dimensions but an ongoing challenge is to coherently summarize complex data. Our goal is to develop visualizations that portray frequency- and behavior-dependent backscatter of individual fish within aggregations. Incorporating individual fish behavior illustrates group dynamics and provides insight on the resulting acoustic backscatter. Object-oriented applications are used to visualize fish bodies and swimbladders, predicted Kirchhoff-ray mode (KRM) backscatter amplitudes, and fish swimming trajectories in three spatial dimensions over time. Through the visualization of empirical and simulated data, our goal is to understand how fish anatomy and behavior influence acoustic backscatter and to incorporate this information in acoustic data analyses.

The volume scattering strength of swim-bladdered fish is very sensitive to fish size, shape, and orientation. However, the appropriate geometrical and behavioral parameters necessary to determine these parameters are not always available. Inadequate knowledge of these parameters limits our ability to correctly estimate lengths and abundances of fish with acoustic technology. A method for extracting geometrical and behavioral parameters of individual fish using a scattering model-based echo-trace-analysis (ETA) is presented. The scattering model used in the ETA is based on the Kirchhoff approximation and a prolate spheroid is used to approximate the shape of the swimbladder. The geometric shape of a resolved echo from an individual fish is used in a least-squares algorithm to infer information about the swimming speed, direction, and orientation of the fish. The acoustic intensity distribution of an individual fish along a resolved echo trace is used in a least-squares algorithm to infer information about the size, aspect ratio, and orientation of the swimbladder. Results from the ETA were more sensitive to variations in vertical swimming speed and orientation of the swimbladder than to variations in horizontal swimming speed and aspect ratio of the swimbladder. The proposed ETA was applied to field data, a 38 kHz echogram of a school of Atlantic cod recorded with a Simrad EK500 echo sounder. The inferred sizes and orientations of a number of the resolved echo traces for individual fish were consistent with available observation data. One of the advantages of the ETA method presented here is that it can be easily extended to a dual beam, a split-beam, or a multi-beam acoustic system.

A new method has been developed for inferring the orientation of fish through the use of broadband-acoustic signals. The method takes advantage of the high range resolution of these signals, once temporally compressed through cross-correlation. The temporal resolution of these compressed signals is inversely proportional to the bandwidth, thus the greater the bandwidth the higher the resolution. This process has been applied to broadband-chirp signals spanning the frequency range 40–95 kHz to obtain a range resolution of approximately 2 cm from the original, unprocessed resolution of about 50 cm. With such high resolution, individual scattering features along the fish have been resolved, especially for angles well off normal incidence. The overall duration of the compressed echo from live, individual alewife, as measured in a laboratory tank, is shown to increase monotonically with orientation angle relative to normal incidence. The increase is due to the greater range separation relative to the transducer between the echoes from the head and tail of the fish. The results of this study show that with a priori knowledge of the length of the fish, the orientation could be estimated from the duration of a single, compressed broadband echo. This method applies to individual, acoustically resolved fish. It has advantages over previous approaches because it derives the orientation from a single ping and it does not use a formal, mathematical scattering model. Design parameters for applications in the ocean are given for a range of conditions and fish size. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights

The National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI) has developed archives for the long-term stewardship of active and passive acoustic data. Water column sonar data have been collected for fisheries and habitat characterization over large spatial and temporal scales around the world, and archived at NCEI since 2013. Protocols for archiving passive acoustic data are currently being established in support of the NOAA Ocean Noise Reference Station Network project, and monitoring marine mammals and fish. Archives maintain data, but access to these data is a core mission of NCEI that allows users to discover, query, and analyze the data in new and innovative ways. Visualization products continue to be developed and integrated into the data access portal so that researchers of varying backgrounds can easily understand the quality and content of these complex data. Spatially and temporally contemporary oceanographic and bathymetric data are also lin...

Euphausiids are a key link between primary production and higher-level predators in the Gulf of Maine, but are not well sampled during standard fisheries surveys. Multifrequency acoustic data may provide useful estimates of euphausiid distribution and biomass, as long as automated classification of acoustic backscatter is reliable and robust. Estimates of euphausiid biomass in the Georges Bank region of the Gulf of Maine were derived from annual acoustic/midwater trawl surveys from 1999 through 2012. Acoustic data were collected continuously with Simrad EK500 and EK60 echo sounders operating at 18, 38, and 120 kHz. Four different methods were used to classify euphausiids from the acoustic data: multifrequency single beam imaging, “dB-differencing” of 120- and 38-kHz volume backscatter, multifrequency z-score, and a multifrequency index. Scattering model predictions of euphausiid target strength and biological metrics were used to scale acoustic data to biomass for each classification method. Biomass estim...

One goal of acoustic-based abundance estimates is to accurately preserve spatial distributions of organism density and size within survey data. We simulated spatially­ random and spatially-autocorrelated fish density and cr 0, distributions to quantify variance in density, abundance. and backscattering cross-sectional a rea estimates, and to examine the sensitivity of abundance estimates to organism spatial distributions and methods of estimating acoustic size. Our results show that it is difficult to simul­ taneously estimate fish density and maintain accurate crbs· frequency distributions. Among our acoustic backscatter estimation methods. a weighted-mean from a local search window provided optimal estimates of density, abundance and crbs· Other methods tended to bias either crh, or density estimates. This analysis identifies the relative importance of variance sources when estimating organism density using spatial ly-i ndexed acoustic data.

The low-frequency target strength of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during Autumn 2006 spawning season is estimated from experimental data acquired simultaneously at multiple frequencies in the 300-1200 Hz range using (1) a low-frequency ocean acoustic waveguide remote sensing (OAWRS) system, (2) areal population density calibration with several conventional fish finding sonar (CFFS) systems, and (3) low-frequency transmission loss measurements. The OAWRS system's instantaneous imaging diameter of 100 km and regular updating enabled unaliased monitoring of fish populations over ecosystem scales including shoals of Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl and CFFS sampling. High spatial-temporal coregistration was found between herring shoals imaged by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to consistently exhibit a strong, roughly 20 dB/octave roll-off with decreasing frequency in the range of the OAWRS survey over all days of the roughly 2-week experiment, consistent with the steep roll-offs expected for sub-resonance scattering from fish with air-filled swimbladders.

Two important goals in studying the biology of fishes are to detect and enumerate the fish and to define where the fish are to be found. Locating and counting fish is difficult, but defining and mapping a fish’s habitat can be even more daunting. A fish’s habitat is the physical, chemical, geological and biological environment in which it resides or migrates through and includes the pelagic (open water), benthic (on or in the sea floor), and demersal (on or near the sea floor) realms. With the continuing loss of estuarine and coastal habitats it is especially critical to seek out the waters and substrates that are necessary as spawning, nursery and feeding areas for fishes. In the United States, the Magnuson-Stevens Fishery Conservation and Management Act, Public Law 94-265, as amended through October 11, 1996 calls for direct action to stop or reverse the loss of fish habitat and requires the identification of “essential fish habitat” (Section 305 of the Act). In a wider context, the wish to promote conservation through the establishment of marine protected areas also requires the identification of habitats of managed, threatened, and endangered species. Investigating the distribution of fish is especially difficult because fish can rarely be seen and counted underwater. Fisheries trawl or net surveys can provide an overall picture of fish distribution, but are destructive of the species being surveyed. One of the greatest challenges to the study of fish populations is the ability to collect data over large spatial scales and to study behavior for long periods of time, without intruding upon the lives of these animals. Two uses of acoustics have been developed for studying fish populations and behavior. Active acoustics uses sound generated actively by transducers and the acoustic scattering properties of fish to image individual fishes and populations of fishes. Passive acoustics relies on listening to the sounds produced by fishes with a hydrophone to infer their distribution and behavior. For passive acoustics to be useful a fish must make a sound, thus this technique is limited to species that produce sounds and to the times and places where they produce them. These techniques have typically been used independently, depending on the situation and goals of the study. This chapter reviews each of these technologies and

Journal home page: http://scitation.aip.org/jasa/ Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned.

Broadband acoustic backscattering measurements, advanced high-resolution imaging of fish morphology using CT scans and phase-contrast x rays (in addition to traditional x rays), and associated scattering modeling using the images have been conducted involving alewife (Alosa pseudoharengus), a swimbladder-bearing fish. A greater-than-octave bandwidth (40-95 kHz) signal was used to insonify live, individual, adult alewife that were tethered while being rotated in 1-deg increments over all angles in two planes of rotation (lateral and dorsal/ventral). These data, in addition to providing the orientation dependence of the scattering over a continuous band of frequencies, were also used (after pulse compression) to identify dominant scattering features of the fish (including the skull and swimbladder). The x-ray and CT scan images of the swimbladder were digitized and incorporated into two scattering models: (1) Kirchhoff-ray mode (KRM) model [Clay and Horne, J. Acoust. Soc. Am. 96, 1661-1668 (1994)] and (2) conformal-mapping-based Fourier matching method (FMM), which has recently been extended to finite-length bodies [Reeder and Stanton, J. Acoust. Soc. Am. 116. 729-746 (2004)]. Comparisons between the scattering predictions and data demonstrate the utility of the CT scan imagery for use in scattering models, as it provided a means for rapidly and noninvasively measuring the fish morphology in three dimensions and at high resolution. In addition to further validation of the KRM model, the potential of the new FMM formulation was demonstrated, which is a versatile approach, valid over a wide range of shapes, all frequencies and all angles of orientation.

Two broadband active acoustic systems, in concert with traditional narrowband systems and nets, were used to study distributions of fish in three regions within the Gulf of Maine. The long-range multi-beam broadband system detected fish out to 15 km range and the downward-looking short-range broadband system detected fish throughout the water column close behind the ship. The multi-year (2007–2011) study revealed distinct spatial patterns of fish and corresponding echo statistics in each region—diffusely distributed, sparsely distributed compact patches, and long (continuous) shoals. The broadband capabilities of the sonar systems (each spanning 1.5—6 + kHz) uniquely allow observations of resonance phenomena of the local swimbladder-bearing fish. The observed resonances were consistent with the fish species, sizes, and depths that were concurrently sampled in each area from a second research vessel. Spectral peak analysis also interestingly revealed the presence of distinct modes, which may be useful indicators of mixed-species and/or mixed-sized (e.g., juvenile and adult) assemblages of fish. [Work supported by Office of Naval Research.]

Fisheries resource management is in a state of transition from managing populations at the species level to managing living marine resources at the ecosystem level. This transition will require changes in the way data are collected, analyzed, integrated, and finally utilized in management decisions. C. S. Clay “Clay” was a pioneer in underwater acoustics, but my first experiences and interactions with him were as a graduate student biologist learning to observe and understand the underwater environment in new and innovative ways. Clay’s collaborations with biologists, ecologists, and oceanographers spawned novel methods of integrating and analyzing disparate data sets and many of these methods are being used today. While Clay’s influence on fisheries acoustics has been monumental, his approaches to understanding the ocean environment may be most valuable to ecosystem-based management strategies. I will highlight examples of Clay’s innovative approaches that have been used and ways they could be applied to ecosystem-based management and research.