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Histograms for the different sets of data associated to haul no. 4 explained in Figure 1. The left panel histograms correspond to (a) the raw Sv data for the whole echogram (excluding sea bottom); (b) the first 100 m from the surface; and (c) the rectangular area enclosing the schools. The histograms of the averaged Sv data for the same sets are represented in the right panels (d, e and f). The red and green lines in the bottom histograms are the Gaussian PDFs fitted by the EM algorithm to the weak and strong (likely the schools) echoes. The black vertical lines in (c) and (d) represent the optimal thresholds found for both sets, −54.2 and −60.4 dB, respectively.
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