Test experiments of target strength of herring by comparing density indices obtained by acoustic method and purse seine catches
Original version
This report is not to be cited without prior reference to the authorsAbstract
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.