(a) Raw S_v echogram associated to haul no. 4 for which the EM algorithm was applied to three sets of data: the whole echogram excluding echoes below the sea bottom; the first 100 m from the surface, delimited by the horizontal line; and the rectangular area enclosing the schools. The locations of the random samples taken from the schools (+) and their surroundings (×) are also represented. (b) Averaged echogram using a 5 × 5 kernel for which the EM algorithm was also applied to the same sets of data as in (a). The colour scale is the same for both echograms.