Acoustic characterization of Pacific hake (Merluccius productus) aggregations from bi-frequency data in the Gulf of California

Merluccius productus is subject of an important fishery in the North Pacific. In Mexico, this fishery is not fully developed; however, this species is considered a potential resource because of its presence in the west coast of the Baja California peninsula and in the Gulf of California. Recent efforts have been made to estimate its biomass by acoustic methods. As a first step, this work aims to acoustically characterize the aggregations of Pacific hake in the Gulf of California. For this purpose, acoustic data were collected on board R/V BIP XI in two surveys (February and June 2014). Here we report the results from the winter survey. Zig-zag transects covering the north of the gulf were conducted with an EK60 echosounder using two frequencies (38 and 120 kHz). Biological data were collected with a bottom trawl during 30-minute fishing hauls targeting the demersal aggregations detected acoustically. Nearly 95 % of the fishing hauls were positive for Pacific hake. Most targeted aggregations were observed between 100 and 250 m depth with a Sv threshold level of -70 dB. By applying bi-frequency filtering algorithms, micro- and macro-zooplankton echoes were removed from echograms and the remaining signal was described in terms of bathymetric and morphological characteristics, and acoustic energy. We found two aggregation patterns in echograms associated with positive trawls; one was a compact layer near the sea bottom, and the other, a rather scattered layer in the water column.