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The distributions of micronekton layers in the Balearic Sea (western Mediterranean) were investigated by acoustic methods. Two multidisciplinary surveys were carried out in late autumn 2009 and summer 2010, recording acoustic, biological, and hydrographic data. We described acoustic layers, migratory behavior, sampled species, and water masses processes. Acoustic modeling of gas-bearing organisms was employed to explain differences between acoustic estimates and sampled abundances. The influence of environmental variables on the vertical distribution and migration pattern of these organisms was analyzed. The thermocline depth was related to the preferred depth for migrating myctophids, while nonmigrant species dwelled in the oxygen minimum zone of the water column both in late autumn and summer periods.Résumé : La distribution des couches de micronecton a été étudiée par méthode acoustique dans la mer des Baléares (Méditer-ranée occidentale). Deux campagnes multidisciplinaires ont été menées en automne 2009 et en été 2010, par mesures acoustique et échantillonnages biologique et hydrologique. La description des couches acoustiques, le comportement migratoire, les espèces échantillonnées et les couches d'eau sont fournies. La modélisation acoustique des organismes de type gas bearing est utilisée pour expliquer le mascage qui conduit á des différences avec les abondances mesurées. Leur préférence pour certaines gammes de variables externes dans la distribution verticale et le model de migration de ces organismes est analysée. La profondeur de la thermocline a été associée á la profondeur de migration préférée des myctophidés, de même la zone de la colonne d'eau présentant le minimum d'oxygène dissous correspond á la zone de résidence des espèces non-migrantes, tous les deux en automne et en été.

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In the last few years, there has been increasing interest in the commercial exploitation of mesopelagic fish 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 fish 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 first time at five frequencies commonly used in acoustic surveys. A high-density filter was applied to reduce the bias derived from overlapping echoes erroneously assigned to single targets. Its relationship with fish length (b20) was also determined (−65.9 ± 2, −69.2 ± 3, −69.2 ± 2, −69.5 ± 2.5 and −71.5 ± 2.5 dB at 18, 38, 70, 120 and 200 kHz, respectively). Biomass estimates of pearlside in the Bay of Biscay during the four years of study (2014–2017) are given using the 38 kHz frequency. Morphological measurements of the swimbladder were obtained from soft X-ray images and used in the backscattering simulation of a gas-filled ellipsoid. Pearlside is a physoclist species, which means that they can compensate the swimbadder volume against pressure changes. However, the best fit between the model and the experimental data showed that they lose that capacity during the trawling process, when the swimbladder volume is affected by Boyle’s law.