Continuous acoustic measurements were made with calibrated echosounders (Foote et al., 1987) Simrad EK80 connected to hull-mounted (6m below the waterline) transducers operating at six frequencies: 18, 38, 70, 120, 200 and 333 kHz. Data were acquired at a typical vessel speed of 8 knots with an average ping interval of 2.6 s offshore and 0.6 s in shallow waters. The pulse length was set at 1024 µs for all frequencies and transmit powers were of 1600, 2000, 600, 200, 90 and 40 W for the six channels at 18, 38, 70, 120, 200 and 333 kHz, respectively. The water column was sampled down to depths of 1000, 800, 400, 250, 120 and 80 m, respectively for those channels.

Acoustic data were scrutinized, corrected and analysed using the “Movies3D” software developed at the “Institut Français de Recherche pour l'Exploitation de la Mer” (Ifremer; Trenkel et al., 2009) combined with the French “Institut de Recherche pour le Développement” (IRD) open source tool “Matecho” (Perrot et al., 2018), developed in MATLAB. 

The near-field noise was removed (9 m deep from the surface) from the analysis; phantom bottom echoes were excluded and the bottom line was corrected manually. The first 100 m were removed for the 18 kHz due to electrical noise. Echograms were cleaned by eliminating unwanted signals such as one impulsive noise, transient noise due to the interferences with other electronic devices or other sources. Background noise was estimated and subtracted using methods described by De Robertis and Higginbottom (2007). The weather was very quiet during these surveys, thus no further cleaning was necessary. The acoustic nautical area scattering coefficient (sA in m2 nmi-2 or NASC), an indicator of marine organisms biomass, and the acoustic volume backscattering strength (Sv in dB re 1m-1), an indicator of marine organisms density, were calculated. Data were echo-integrated onto 1 m high layers over 0.05 nmi ESDU (elementary sampling distance unit) with a -100 dB threshold, from 9 m down to 1000 m depth. The data provided are the results of the echointegration processing