SOURCES OF ACOUSTIC SCATTERING NEAR A HALOCLINE IN AN ESTUARINE FRONTAL SYSTEM

Salt wedge is a quasi permanent feature at the Rio de la Plata estuary , generating strong vertical density gradients. Zooplankton and ichthy oplankton organisms aggregated below the halocline were insonified uti lizing a scientific echosounder operating with a 120 kHz single beam t ransducer. From acoustic data and CTD measurements, a strong coinciden ce between the depths of the scattering layer and the halocline was pr oven. The organisms from the zooplankton and the ichthyoplankton were caught with a Nackthai sampler and classified into four functional gro ups according to their scattering properties. A multiple regression an alysis shows fish larvae as the most significant source of sound scatt ering at the halocline. Finally, the salt wedge across the estuarine r egime is described employing an echochart. The halocline here showed a quasi constant depth at 7-8 m with the salt water intruding 100 km up stream.