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Acoustic quantification of aquatic biomass using echosounders requires accurate calibration. With the advancing applications of deep-water echosounders involving moored, towed, profiling and autonomous instruments, calibration of echosounders at the operating depth is needed to ensure unbiased estimates of biomass and species identification. In this context, the deepwater calibration acoustic facility (DeCAF) was used to examine the depth-dependent variations in on-axis gain (G0) and equivalent two-way beam angle (W) of three different transducers, operating at 38 (Simrad ES38-DD and MSI-38) and 120 kHz (Simrad ES120-7CD) used for biomass surveys and species identification. The analyses carried out using the sphere calibration method reveal significant variations in calibrated G0 and Ψ that in combination could result in substantial systemic biases in quantitative biomass estimation and species identification. The depth-dependent G0 variations derived using the DeCAF system are in accordance with available deep-water calibration results, demonstrating reasonable repeatability of target strength measurements (±60.6 dB) over the calibration deployments. Importantly, Ψ of the transducers was calculated to be consistently lower than the values specified by the manufacturer, and for one transducer would result in 18% change in the estimated biomass. These results highlight the potential for bias if the Ψ of transducer is not independently calculated using the actual survey echosounder. [ABSTRACT FROM AUTHOR]

Echo integration is a well-established method for estimating fish biomass, but is challenging for a low target strength (TS), deep-living fish species such as orange roughy (Hoplostethus atlanticus). A novel approach has been to utilize the infrastructure of the fishing vessel's trawlnet by attaching an acoustic-optical system (AOS) to the net's headline. Deep deployment of the AOS via the trawlnet reduces uncertainties associated with hull-mounted acoustics that include the influence of weather on data quality, low resolution due to the long range to target, large acoustic dead zone on sloping seabed, and inability to identify and differentiate other fish that co-occur with the target species. The AOS system simultaneously records acoustic data at multiple frequencies (38 and 120 kHz), species composition in video and stereo imagery, and environmental data as the net collects biological samples and/or commercial catch. All data streams were considered in a multiple-lines-of-evidence approach to give improved estimates of orange roughy biomass with low error due to species uncertainty. AOS-based biomass estimates, made over a 5-year period from 11 key spawning locations in Australia and New Zealand, showed a strong correlation (r2 = 0.97, n = 39) between frequencies (38 and 120 kHz); the 38 kHz estimates were, on average, 8% higher than 120 kHz, with a standard deviation of 20%. This similarity in estimates across frequencies improves confidence in results compared with single-frequency surveys that are potentially prone to large errors resulting from unknown (mixed) species composition and target strengths, calibration, and sound absorption uncertainties. [ABSTRACT FROM AUTHOR]

Measurements of mean volume backscattering strength (Sv, dB re 1 m-1) at ocean-basin scale were made using 38-kHz hull-mounted echosounders on ships of opportunity as part of Australia's Integrated Marine Observing System. The data were collected on vessels of various designs, none of which were purposely built for collecting high-quality acoustic data. A full range of weather extremes affected the quality of the data and could cause large biases in Sv. To remove first-order biases and improve processing efficiency, a sequence of new and existing data processing filters were applied in a semi-automated procedure. These filters were designed to mitigate the effects of three types of noise: impulsive (less than one ping), transient (multiple pings), and background (hours or longer). A filter was also applied to identify signals that were attenuated by air bubbles beneath the transducer. These filters were applied to data from transits across the Southwest Pacific, Indian, and Southern Oceans to produce quality-controlled Sv datasets that are now available from a publicly accessible repository. These filters may be relevant to other open-ocean acoustic observing endeavours, and one or more could be used to mitigate bias in data from a range of acoustic applications. [ABSTRACT FROM AUTHOR]

It is often assumed that in situ target strength (TS) measurements from dispersed fish are representative of the surveyed schooling fish. For in situ TS measurements of orange roughy in deep water, it has been difficult to validate the target species, individual lengths, and tilt angles and how representative these are of schooling fish. These problems have been addressed by attaching an acoustic optical system (AOS) to a trawl net. The AOS enables in situ measurements of TS and volume backscattering strength (Sv) at 38 and 120 kHz with optical verification of species and stereo camera measurements of fish length and tilt angle. TS estimates believed representative of the schooling population were derived by (1) weighting the frequency-dependent TS values by the Sv frequency difference distribution of orange roughy schools and (2) weighting the in situ TS measurements with an assumed tilt angle distribution. The 120-kHz TS estimates were less sensitive to variations in frequency difference and tilt angle, suggesting that this frequency may be better for biomass estimates than 38 kHz, the traditional survey frequency. Computations performed with an anatomically detailed scattering model agree with measurements of TS at both frequencies over a range of tilt angles. [ABSTRACT FROM AUTHOR]

Kloser, R. J., Ryan, T. E., Young, J. W., and Lewis, M. E. 2009. Acoustic observations of micronekton fish on the scale of an ocean basin: potential and challenges. – ICES Journal of Marine Science, 66: 998–1006.Acoustic methods of characterizing micronekton communities (∼2 to 20 cm length) on the scale of an ocean basin could provide valuable inputs to ecosystem-based fishery management, marine planning, and monitoring the effects of climate change. The micronekton fish are important forage for top predators (e.g. tunas), and information on their diversity, distribution, size-structure, and abundance is needed to increase accuracy of top-predator distribution and abundance predictions. At the scale of an ocean basin, four years of Tasman Sea transects using a fishing vessel provide fine-scale maps of acoustic backscatter at 38 kHz that reveal detailed spatial patterns and structure to depths of 1200 m. Research-vessel data provide detailed biodiversity, density, size structure, and acoustic-scattering information from depth-stratified net sampling and a lowered acoustic probe. Wet-weight biomass estimates of the micronekton fish in the region vary considerably by a factor of 5–58 between acoustics (16–29 g m−2), nets (1.6 g m−2), and large spatial-scale, ecological models (0.5–3 g m−2). We demonstrate the potential and challenges of an acoustic basin-scale, fishing-vessel monitoring programme, including optical and net sensing, which could assist in characterizing the biodiversity, distribution, and biomass of the micronekton fish. [ABSTRACT FROM PUBLISHER]

Ryan, T. E., Kloser, R. J., and Macaulay, G. J. 2009. Measurement and visual verification of fish target strength using an acoustic-optical system attached to a trawlnet. – ICES Journal of Marine Science, 66: 1238–1244.It is difficult to make acoustic target-strength (TS) measurements of fish behaving naturally in deep-water habitats. The fish may avoid the acoustic instrumentation, and, if measured, there is uncertainty about their species and their orientation relative to the incident sound. To address these issues, a novel acoustic-optical system (AOS) has been developed, which combines a battery-powered, dual-frequency, split-beam acoustic system with a low-light video camera. The AOS attaches to the headline of a commercial deep-water demersal trawlnet that herds fish past the AOS and to the codend. This paper describes initial trials of the AOS to measure calibrated TS of New Zealand orange roughy, validated with video images. The fish species were visually identified, and their behaviour and orientation were approximated. The trawl catch provided associated samples for species identification and measurements of their length and other biological metrics. The combination of acoustics and optics in a net-mountable system constitutes a powerful sampling tool with broader applications in fishery research and ecosystem investigations. [ABSTRACT FROM PUBLISHER]

Micronekton constitutes the largest unexploited marine biomass worldwide. It is one of the most conspicuous and ecologically important components of the still poorly known mesopelagic ecosystem. Acoustic data were collected from both fishing and research vessels along 18 transects for a total of 47 682 linear kilometers to investigate large-scale distribution of micronekton over a long latitudinal gradient (20–50°S) and two contrasted seasons (summer and winter) in the South-Western Indian Ocean. Acoustic backscatter at 38 kHz was used as a proxy of mid-water organisms' abundance (0–800 m depth). Two consistent features were diel vertical migration of backscatters and vertical distribution of micronekton in three distinct layers, namely the surface (SL), intermediate (IL) and deep (DL) layers. Satellite remote sensing data was used to position oceanic fronts, and hence define water masses, from the tropical to low Antarctic zones. A key finding of this study was the significant correlation observed between abundance and distribution of acoustic backscatter and position relative to these front and water masses. Total backscatter peaked in the subtropical zone, with low abundances in the colder Polar Frontal Zone. The high overall abundances in subtropical waters resulted mainly from high backscatters in the IL and DL that contrasted with low SL values, especially during the day (2–11%). The warmer the waters, the higher SL backscatter was, with the highest absolute and relative (38–51% of the total abundance) values observed at night in the Tropical Zone and the lowest abundance in the Antarctic Zone. No significant seasonal pattern was found, but SL backscatters were very low in winter compared to summer in the Polar Frontal Zone. Moreover, the Northern winter shift of the fronts induced a Northern latitudinal shift of the peak in abundance from summer to winter. The present study highlights the value of building large acoustic databases collected from both research and fishing vessels. The method provides unique opportunities to gather basic information on micronekton and is an essential step to describe oceanic zones of relevant biological interest in terms of trophic ecology. [ABSTRACT FROM AUTHOR]