In situ target strength estimates of optically verified southern blue whiting (Micromesistius australis)

Estimates of the acoustic target strength (TS) of southern blue whiting (Micromesistius australis) at 38 kHz were obtained using an autonomous acoustic–optical system (AOS) mounted on a demersal trawl. Data were collected from aggregations of spawning adult [mean fork length (FL) 34.4 cm] and immature (mean FL 24.6 cm) southern blue whiting south of New Zealand. Mean TS was estimated from 162 tracks containing 695 echoes from targets identified from video as southern blue whiting. The mean TS was –37.9 dB with a 95 % confidence interval (CI) of –39.7 to –36.6 dB for 21 immature fish and –34.6 dB (95 % CI –35.4 to –34.0 dB) for 141 adults. A logarithmic fit through the mean TS values produced a TS–fork length (FL) relationship from optically verified targets of TS 22.06 log10FL – 68.54. This new relationship gives TS values within 1 dB of those estimated using the relation-ship recently adopted by ICES for blue whiting (Micromesistius poutassou) of TS 20 log10TL – 65.2 (where TL is total length) obtained from in situ measurements, but higher values than those estimated from the previous relationship for southern blue whiting of TS 38 log10FL – 97, which was based on swimbladder modelling