Abstract
Abstract Pedersen, G., and Korneliussen, R. J. 2009. The relative frequency response derived from individually separated targets of northeast Arctic cod (Gadus morhua), saithe (Pollachius virens), and Norway pout (Trisopterus esmarkii). – ICES Journal of Marine Science, 66: 1149–1154. The concept of relative frequency response r(f) of fish is an important feature used to characterize acoustic targets. It is defined as the volume-backscattering coefficient at a specific frequency f relative to that of a reference frequency. When based on volume backscattering, r(f) reliably distinguishes several acoustic categories if the insonified volumes are reasonably comparable between the frequencies, and that enough samples and targets are measured to constrain stochastic variations in the data within acceptable limits. Therefore, r(f) distinguishes different fish species with swimbladders poorly if they appear as single targets. Using target-strength (TS) data, the acoustic measurements are more spatially comparable, and averaging the TS over an echotrace of a single fish improves the ability to distinguish between different species. Frequency response was estimated using TS data from in situ measurements, collected using Simrad EK60 echosounders with split-beam transducers transmitting simultaneously at 18, 38, 70, 120, and 200 kHz. Selected series with nearly pure catches of northeast Arctic cod (Gadus morhua), saithe (Pollachius virens), and Norway pout (Trisopterus esmarkii) were analysed using a target-tracking algorithm. The frequency response of northeast Arctic cod and saithe did not differ significantly, but at high frequencies, the response of both northeast Arctic cod and saithe differed from that of Norway pout. However, in the latter case, northeast Arctic cod and saithe could be separated, because of their different TS magnitudes.
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