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Total-scattering cross-sections (σt) of Antarctic krill (Euphausia superba) were measured over a broad bandwidth (36–202 kHz) using a new technique based on acoustical reverberation in a cavity. From 18 February to 9 March 2002, mean total target strengths (TTS=10 log(σt/4π)), were measured from groups of 57–1169 krill (average standard length=31.6 mm; standard deviation=6.6 mm) at the Cape Shirreff field station, Livingston Island, Antarctica, and aboard RV “Yuzhmorgeologiya”. Chirp pulses were transmitted sequentially by an omni-directional emitter into one of three glass carboys containing groups of krill swimming in 9.3, 19.3, or 45.9 liters of seawater (0.6°C≤temperature≤4.0°C). Between each pulse the krill moved within the fixed-boundary tank and the modulated reverberations were sensed bi-statically with three omni-directional receivers. At each center frequency (fc), the coherent energy in 200-pulse ensembles identified sound scattered by the tank. The incoherent energy described total sound scattering from the krill. Thus, the TTS at each fc was extracted from a correlation analysis of energy reverberated in the tank. Measurement bias was determined to be ±0.4 dB from an experiment using metal sphere reference targets, and the precision was estimated as ±0.8 dB from the variability in the krill TTS (fc) measurements. The empirical estimates of mean σt corroborated a krill-scattering model based on the distorted-wave Born approximation (DWBA), enhanced by the authors to account for the stochastic nature of sound scattering (SDWBA), integrated over all scattering angles and averaged over all incident angles (SDWBATTS). The SDWBA, solved for target strength of Antarctic krill, may be the best predictor of backscatter for this important species and may also provide backscattering spectra for improving their acoustic identification. These advances may help to reduce uncertainty in krill-biomass estimation using multi-frequency echosounder data and echo-integration methods. [Copyright &y& Elsevier]

In December 2000 the US National Oceanic and Atmospheric Administration''s Fish Lidar (Light Detection And Ranging) system was operated from an airplane off the west coast of Florida. Schools of fish were located and their volume-backscattering coefficients, β(π), measured at the lidar wavelength of 532 nm. Concurrently, a 208 kHz echosounder was deployed from a small boat to measure the acoustic volume-backscattering coefficients, sv, of the same schools. Seven schools were characterized with both the lidar and the echosounder. The correlation between these seven pairs of β(π) and sv measurements was 0.994. A linear regression of β(π) versus sv had a negative y-intercept, which supports aerial observations of some degree of avoidance reaction of fish to the passing survey boat. The slope was slightly greater than unity, in agreement with previous calculations that the acoustic backscatter of similar fish is slightly greater than the lidar backscatter. The results of this study indicate that lidar is a suitable tool for surveying rapidly the distributions and abundances of epipelagic fish stocks in the shallow waters off the west coast of Florida. Aerial lidar surveys do not have the biases of fish-avoidance reaction potentially affecting acoustic and trawl surveys. [Copyright &y& Elsevier]

An area of high krill (Euphausia superba Dana) density was continuously monitored with down-looking and side-looking sonars over a period of 24 h. Measurements of volume backscattering strength were used to describe the density of krill with depth and the vertical movement of krill over time. In situ measurements were made of dorsal aspect target strength (TS) and, as krill moved into the near-surface layer (0–15 m), in situ measurements were made of lateral-aspect TS. The probability density function (PDF) of TS measurements made with the down-looking transducer had a mode at approximately −73 dB. The PDF of TS measurements made with the side-looking transducer was broader with a mode at approximately −67 dB. Sampled krill had a bi-modal length distribution with a major mode at 44 mm and a minor mode at approximately 30 mm. [ABSTRACT FROM PUBLISHER]

Deals with the distribution of volume backscattering strength near Elephant Island in Antarctica in the summer of 1995. Function of the echo integration methodology; Objectives for conducting acoustic surveys; Factor that contributed to the majority of the scatter; Differences in patchiness and apparent densities.

Presents the result of a study on the distribution and abundance of krill around Elephant Island, Antarctica, conducted in the 1993 austral summer. Investigation of the possible survey biases caused by vertical movement of krill in and out of the acoustic observation window; Description of the dominant horizontal and vertical scales of krill aggregation patterns.

Determines the distribution and abundance of krill near Elephant Island, Antarctica during the 1992 austral summer. Relation of feeding ecology and reproductive success of krill predators to aspects of the prey field; Use of bio-acoustic methodology to map the distribution of krill and to estimate biomass.

Presents observations on the distribution and number of krill near Elephant Island, Antarctica for the summer of 1993 and 1994. Mesoscale features of krill population density and distribution; Calculations of population estimates; Statistics of average density.