The structure and spatial distribution of pelagic fish schools in multispecies clusters: an acoustic study

Abstract

Two acoustic surveys were carried out in the Bay of Biscay, in 1993 and 1994, to study the distribution and abundance of pelagic stocks. Acoustic data were systematically stored using the INES/MOVIES acquisition and processing system along isobath perpendicular transects, from 25 m to 200 m bottom depth. Species identifications were obtained from midwater trawl catches. From the acoustic data, sections of cruise track in the vicinity of the trawl catches were selected for analysis. Three species, anchovy (Engraulis encrasicolusL.), sprat (Sprattus sprattusL.), and horse mackerel (Trachurussp.) were investigated. The objectives of this study were to analyse the echo traces for estimation of species-specific preferences and to examine the influence of species assemblages on the vertical position and shape of the schools. Using MOVIES-B software, 4789 fish schools were detected and isolated. Characteristics investigated were size (height, vertical cross-sectional area), elongation (length/height), back-scattered energy (density), and vertical distribution (bottom depth, school altitude). Comparative analysis of the vertical distribution of echo traces and catch compositions revealed some species-specific preferences. Horse mackerel were found close to the bottom and anchovy at a mean of 16 m above the bottom. No variation in vertical distribution was detected where sprats were mixed with anchovies. In the presence of horse mackerel in the bottom layer, anchovy appeared to be displaced upwards. A modification of the shape of schools was also detected when sprats were mixed with anchovies. In anchovy–horse mackerel mixtures, a suggested modification of behaviour could explain the observed decrease in the density of anchovy schools.