Development of Multi-Beam Sonar as a Tool for Fisheries Stock Assessment

Fisheries acoustic techniques can overcome some of the limitations and sampling biases of traditional survey methods, and provide important biological information on fish abundance, spatial distribution, and behavior. The most advanced of these acoustic techniques, multi-beam sonar, is currently used almost exclusively for seafloor mapping, but it has great potential for identification of essential fish habitats (EFH), characterization of marine protected areas (MPAs), study of the spatial distribution of fish schools, and stock assessment surveys. The importance of developing multi-beam sonar as a fisheries survey tool is warranted because multi-beam sonar has (1) a larger sample area and volume than single- or split-beam echosounders and trawls; (2) better spatial description; and (3) potentially fewer behavior-related sampling biases (e.g. diel vertical migrations, vessel avoidance) than traditional surveys. Before multi-beam sonar can be used for fisheries acoustic surveys, the feasibility needs to be investigated, and the relationship between acoustic backscatter and fish biology needs to be better understood.

The three overall objectives of the proposed research are the following:

1. Determine optimal sonar and cage configurations for detection of captive Atlantic cod by multi-beam sonar

• Determine effective sonar configurations for acoustic measurements of caged fish.
• Evaluate acoustic properties and effects of a surface and submerged cage.
• Calibrate sonar systems for acoustic detection and measurement of caged fish.
• Determine optimal equipment settings for cod detection and acoustic measurements.

2. Compare acoustic information from multi-beam sonar to reality

• Relate acoustic backscatter to abundance of cod stocked in cages at several densities.
• Evaluate density-dependent effects on density and biomass estimation.
• Evaluate effects of depth, diel period, and behavior on acoustic measurements and abundance estimates.
• Evaluate size-dependent effects on density and biomass estimation.
• Test the effect of the presence of other species (e.g. Atlantic herring) on acoustic results and develop adjustments or identify limitations for species discrimination.
• Determine errors and measures of uncertainty for acoustic measures.

3. Compare multi-beam sonar data to the standard acoustic technique of split-beam sonar

• Compare acoustic results between multi- and a split-beam sonar using encaged fish.

To achieve these objectives, a series of experiments, using captive Atlantic cod in sea cages, will be conducted. In these, we will identify the acoustic “signature” of cod, and study the relationship between acoustic backscatter and elements of cod biology, including their abundance, diel and foraging behavior, depth, size, and co-occurrence of other species.

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