Test experiments of target strength of herring by comparing density indices obtained by acoustic method and purse seine catches

Abstract

The spawning stock of the Norwegian spring spawning herring has been assessed by acoustic methods using the target strength TS = 20 logL - 71.9 dB (L = mean length of fish) Comparable estimates of fish density were obtained from scattering layers of spent herring near the spawning grounds by using average area backscattering coefficient and purse seine catches. The results show that under the prevailing conditions the catch corresponded to fish density of some 4 dB below the TS-value used. The experiment indicates that the acoustic stock estimates obtained on the spawning grounds since 1988 should at least be raised by a factor of 2.0.