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    The three-frequency method for classifying the species and assessing the size of two euphausiids (Euphausia superba and Euphausia crystallorophias)

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    M. Azzali, G. Lanciani and I. Leonori (Italy)
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    In this paper an acoustic method for identifying two euphausiid species and estimating their length is described. The approach is in fact an outgrowth from both the fluid sphere and Bayes rule methodologies.
    This paper explores applications of the multi-frequency method using data from three expeditions to the Ross Sea (1980-90; 1997-98 and 1999-2000), where the environmental conditions, the sampled areas, the instrumental and the sampling strategies varied.
    First, on the basis of the echo-integrations, made simultaneously either at two or at three frequency, and of the results of net samplings, the thresholds and the decision criteria to recognize the two species are established.
    Next the acoustic estimates of euphausiid lengths, derived from the fluid sphere model, are compared with lengths collected from net samplings.
    Finally, the developed criteria and algorithms are effectively applied to estimate E. superba biomass found in the area of the Ross Sea investigated in the December 1997 and in January/February 2000. The results are compared with those obtained from the standard method.