Paul G. Fernandes

Fish do not avoid survey vessels.

The precarious condition of the worlds fisheries is making ever-greater demands of the scientific assessment of fish stocks. Traditional assessments that rely on commercial catch statistics can have major shortcomings (as shown, for example, by the collapse of Canadas northern cod stock), increasing the need for more fishery-independent data. Acoustic surveys can provide such information, but ocean-going research vessels have high operating costs, and there is also widespread concern that fish avoid these vessels because of the noise they make, thereby biasing abundance estimates. Here we present new data gathered by an autonomous underwater vehicle (AUV) showing that vessel avoidance is not a significant source of bias. Our investigation also heralds the arrival of AUVs as effective survey platforms.

Autonomous underwater vehicles: future platforms for fisheries acoustics

Autonomous underwater vehicles (AUVs) are unmanned submersibles that can be pre-programmed to navigate in three dimensions under water. The technological advances required for reliable deployment, mission control, performance, and recovery of AUVs have developed considerably over the past 10 years. Currently, there are several vehicles operating successfully in the offshore industries as well as in the applied and academic oceanographic sciences. This article reviews the application of AUVs to fisheries- and plankton-acoustics research. Specifications of the main AUVs currently in operation are given. Compared to traditional platforms for acoustic instruments, AUVs can sample previously impenetrable environments such as the sea surface, the deep sea, and under-sea ice. Furthermore, AUVs are typically small, quiet, and have the potential to operate at low cost and be unconstrained by the vagaries of weather. Examples of how these traits may be utilized in fisheries-acoustics science are given with reference to previous work in the North Sea and Southern Ocean and to potential future applications. Concurrent advances in multi-beam sonar technology and species identification, using multi-frequency and broadband sonars, will further enhance the utility of AUVs for fisheries acoustics. However, before many of the more prospective applications can be accomplished, advances in power-source technology are required to increase the range of operation. The paper ends by considering developments that may turn AUVs from objects sometimes perceived as science fiction into instruments used routinely to gather scientific facts

Oceanography: Fish do not avoid survey vessels

The precarious condition of the worlds fisheries is making ever-greater demands of the scientific assessment of fish stocks. Traditional assessments that rely on commercial catch statistics can have major shortcomings (as shown, for example, by the collapse of Canadas northern cod stock), increasing the need for more fishery-independent data. Acoustic surveys can provide such information, but ocean-going research vessels have high operating costs, and there is also widespread concern that fish avoid these vessels because of the noise they make, thereby biasing abundance estimates. Here we present new data gathered by an autonomous underwater vehicle (AUV) showing that vessel avoidance is not a significant source of bias. Our investigation also heralds the arrival of AUVs as effective survey platforms.

Diving Depths of Northern Gannets: Acoustic Observations of Sula Bassana from an Autonomous Underwater Vehicle

Abstract The Autonomous Underwater Vehicle (AUV) Autosub-1 made observations of the sea surface with an upward-looking echosounder during fish surveys in the vicinity of Shetland and Orkney (North Sea) in July 1999. Echograms from the AUV contained vertical traces extending downwards from the sea surface that were caused by diving seabirds. Visual observations provided evidence that those seabirds were Northern Gannets Sula bassana. Analysis of trace extent suggests a mean dive depth of 19.7 m (n = 19, SD = 7.5). Data on gannet diving depths are sparse, but this value is somewhat deeper than that accepted for the related Cape Gannet (Morus capensis, mean 5.9 m) which has been used in foraging models for the Northern Gannet. These observations have implications for our understanding of the foraging capabilities of gannets, and the interactions of gannets with commercially targeted fish species.

An investigation of avoidance by Antarctic krill of RRS James Clark Ross using the Autosub-2 autonomous underwater vehicle

Abstract The autonomous underwater vehicle (AUV) Autosub-2 was deployed on eight missions ahead of RRS James Clark Ross in the northern Weddell Sea and in the Bransfield Strait, Southern Ocean, to assess avoidance of the research vessel by Antarctic krill Euphausia superba . The AUV was equipped with the same type of scientific echosounder as the research vessel (Simrad EK500 operating at 38 and 120xa0kHz) and measured the density of krill along transect acoustically (gxa0m −2 wet mass) prior to the ship’s arrival. We hypothesised that if krill avoided the ship, perhaps in response to radiated noise, then the ship should detect less krill than the AUV which is known to have much lower noise levels than the ship. We were unable to detect any significant difference between the density of krill detected by the ship or the AUV, either at the transect level or at finer scales within transects. We conclude, therefore, that avoidance by krill of RRS James Clark Ross will not significantly bias acoustic estimates of krill abundance by this vessel.

Addendum: Fish do not avoid survey vessels

This corrects the article DOI: 35003648

Estimating fish abundance from acoustic surveys: calculating variance due to acoustic backscatter and length distribution error

Estimation of fish abundance from acoustic surveys requires the estimation of total acoustic backscatter of the target species in the sampled region. Although the arithmetic mean of acoustic backscatter is an unbiased estimator of the mean backscatter for regular or random sampling designs, under the presence of spatial structure, its use leads to a loss of information and the estimation of its variance is not trivial. Here, we tackle these shortcomings by building a spatial model of acoustic backscatter using spline-based generalized additive models (GAMs). GAMs were used to provide local and global estimates of acoustic backscatter, and their precision was calculated by statistical simulations of the models param- eters. For a series of surveys performed off the western and southern Iberian Peninsula, GAM estimates were unbiased and more precise than the arithmetic mean estimates. Simulations of the acoustic backscatter fields were combined with resam- pling of the trawls to provide confidence intervals for fish numbers and biomass. The relative standard errors of the esti- mates were within 13% and 46% (average 22%) for numbers and within 12% and 35% (average 19%) for biomass. Acoustic sampling error was the major contributor to the overall variance.

Fish do not avoid survey vessels (addendum)

This corrects the article DOI: 35003648

Target strengths of two abundant mesopelagic fish species

Mesopelagic fish of the Myctophidae and Sternoptychidae families dominate the biomass of the oceanic deep scattering layers and, therefore, have important ecological roles within these ecosystems. Interest in the commercial exploitation of these fish is growing, so the development of techniques for estimating their abundance, distribution and, ultimately, sustainable exploitation are essential. The acoustic backscattering characteristics for two size classes of Maurolicus muelleri and Benthosema glaciale are reported here based on swimbladder morphology derived from digitized soft x-ray images, and empirical (in situ) measurements of target strength (TS) derived from an acoustic survey in a Norwegian Sea. A backscattering model based on a gas-filled prolate spheroid was used to predict the theoretical TS for both species across a frequency range between 0 and 250 kHz. Sensitivity analyses of the TS model to the modeling parameters indicate that TS is rather sensitive to the viscosity, swimbladder volume ratio, and tilt, which can result in substantial changes to the TS. Theoretical TS predictions close to the resonance frequency were in good agreement (±2u2009dB) with mean in situ TS derived from the areas acoustically surveyed that were spatially and temporally consistent with the trawl information for both species.

Observation and extraction of three‐dimensional information on fish schools

The paper describes the performance calibration and use of a 90‐deg sector scanning sonar for the collection and extraction of information on the 3D structure of fish schools. The equipment, which consists of a 455‐kHz 60‐beam sector scanning sonar linked to a PC is described briefly. The specific calibration problems of a high‐frequency instrument with multiple beams is discussed and calibration data from on‐axis and beam shape measurements are presented. The deployment of the instrument for data collection at sea and the data collection methods are described. Examples of the data collected are given. A three‐dimensional data processing algorithm is presented along with results of reconstruction from selected schools. The statistical properties of within school data are discussed along with indications of the precision of internal structures that can be evaluated using the sonar. The development of this system is supported by the European research program, AIR.