Identification of echoes from demersal aggregations of hake, Merluccius productus, in the Gulf of California

Abstract The distribution of Pacific hake, Merluccius productus, extends from Northern Canada to Southern Baja California and into the Gulf of California (GC), Mexico. In an emergent fishery in the Northern GC, hake dominates finfish catches from February to May. Developing management strategies aiming to make this fishery sustainable requires estimates of abundance, which can be provided by the acoustic-trawl method. To identify echoes from daytime demersal aggregations of hake, we use ranges of the sum and difference of mean volume backscattering strength (SV) measured at 38 and 120 kHz. More than 90% of the summed 38-kHz volume backscattering coefficient (sV38) associated with hake-only catches are retained using −4.2 SV120 – SV38 0.9 and −122.0 SV38 + SV120 −110.4 dB. These ranges mostly differ for cusk-eel, family Ophidiidae, and scorpionfishes, family Scorpaenidae, which are frequently caught with hake in the Northern GC, but overlap completely for sharks, family Triakidae. To estimate the accuracy of the putative hake backscatter, we present a novel method that accounts for the spatial distribution of area backscattering coefficient, the sensitivity and specificity of the hake-identification ranges, and the acoustic-proportions of hake and the other species.