Estimates of variability of goldband snapper target strength and biomass in three fishing regions within the Northern Demersal Scalefish Fishery (Western Australia)

Goldband snapper (Pristipomoides multidens) is an ecologically and economically important species in the Northern Demersal Scalefish Fishery (NDSF). The Carolina M, a trap fishing vessel operating in the NDSF, was equipped with Simrad ES70 echosounders, operated at 38 and 120 kHz. In 2014 acoustic data, in combination with optical recordings of the catch, were opportunistically collected during routine fishing operations. In December 2014 pure, low density goldband snapper schools were observed on the echograms. In situ target strength (TS) estimates were derived and linked to length distributions of catch information with the curve fitting method. Estimated TS-Length (L) at 38 kHz was 20.1 log10(L)-70.5 and 16.4 log10(L)-77 at 120 kHz. Three fishing grounds, where near simultaneously recorded acoustic and optical information was available were selected. Fish school densities observed within the 38 kHz acoustic data were disaggregated according to catch proportions using kriging. Goldband snapper density estimates ranged between 9518 individuals per nmi2 in the high-density fishing region and 2512 and 945 individuals per nmi2 in the two low density fishing regions. Sampling variance was estimated using geostatistics (coefficient of variance, CV = 10-20.9%). Other errors considered were signal-to-noise ratio (CV < 1%), variation in the acoustic signal due to fluctuations in temperature and salinity (CV = 0.5-1.15%), effects of diurnal vertical migration and variability of catch information (CV = 1.2-2%). A total CV of 28.2-50.6% was estimated for all considered sources, for the three fishing regions.