Use of acoustic descriptors to identify sardine and anchovy schools on the Sicilian Channel continental shelf

Sardine pilchardus and anchovy stocks are a commercially important resource for the purse seine fisheries in the South Sicilian continental shelf (Patti et al. 2004). These pelagic species form mono or plurispecific schools during the day for feeding, defense and reproductive advantages and tend to disperse during the night (Pitcher,1993). Sardine and anchovy biomass in Southern Sicily is managed mainly on basis of acoustic techniques that record the backscattering energy came from the schools fish present in a sampled area. The acoustic techniques are generally not able to discriminate between the echoes of co-occurring species. The inability to acoustically identify the species composition could be an important source of error in the estimate of stock biomass. At the moment, echo traces are assigned to fish species following a procedure that is based on the proportion of fish found in the pelagic hauls. Because of the difficulty of splitting the biomass into species, the classification of fish schools is a recurrent worry among scientists (Swartzman et al., 1994). If we were able to broaden our knowledge on the school data from the echograms it would be possible to improve the discrimination between the species. Consequently, more accurate biomass estimates could be obtained. The aim of this study is analyze bathymetric, morphometric and energetic characteristics of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) schools detected during echosurveys carried out in the Sicilian Channel (Fig. 1) to evaluate which descriptors could be used to better discriminate between the two species.