Target Strength and swimbladder morphology of Muellers pearlside (Maurolicus muelleri)

Abstract

In the last few years, there has been increasing interest in the commercial exploitation of mesopelagic fsh and a trawl-acoustic methodology has been recommended to make estimates of abundance of these resources. This study provides relevant information on the scattering properties of a key mesopelagic fsh species in the Bay of Biscay, Mueller’s pearlside (Maurolicus muelleri), necessary to convert the acoustic density into numerical abundance. The target strength (TS) of pearlside was estimated for the frst time at fve frequencies commonly used in acoustic surveys. A high-density flter was applied to reduce the bias derived from overlapping echoes erroneously assigned to single targets. Its relationship with fsh length (b20) was also determined (−65.9±2, −69.2±3, −69.2±2, −69.5±2.5 and −71.5±2.5dB at 18, 38, 70, 120 and 200kHz, respectively). Biomass estimates of pearlside in the Bay of Biscay during the four years of study (2014–2017) are given using the 38kHz frequency. Morphological measurements of the swimbladder were obtained from soft X-ray images and used in the backscattering simulation of a gas-flled ellipsoid. Pearlside is a physoclist species, which means that they can compensate the swimbadder volume against pressure changes. However, the best ft between the model and the experimental data showed that they lose that capacity during the trawling process, when the swimbladder volume is afected by Boyle’s law.