Estimation of pelagic trawl efficiency in a combined acoustic- trawl survey, with reference to demersal fish spatial distribution.

Few analyses have been performed to estimate the efficiency of trawls targeting demersal fish using the ratio of catches and acoustic densities. In the summer of 2006, acoustic and fishing data have been collected simultaneously during three days by three fishing vessels equipped with identical semipelagic trawls during a scientific survey (CHAPAUV’06) in the Bay of Biscay. These data have been used to compute diel trawl efficiency estimates for selected demersal species and to relate them to the fish assemblage spatial structure. Acoustic back-scattering densities expressed as Nautical Areal Scattering Coefficients (NASC) recorded in the trawled layer were compared to Equivalent NASC (ENASC) calculated from the species composition in the trawl, their length structure and available target strength-length relationships. Trawl efficiency estimates were computed using log-linear or generalised linear models, as the slope, Q, of the relationship: ENASC=Q*NASC^b, for hakedominated trawls and for the whole demersal community at day and night, and for horse mackereldominated trawls at day. No significant horizontal spatial autocorrelation was found in the acoustic data at the haul scale (~4 km).