DETERMINATION OF COMPOSITION AND VERTICAL STRUCTURE OF FISH COMMUNITIES USING IN-SITU MEASUREMENTS OF ACOUSTIC TARGET STRENGTH

A split-beam echo-sounder was used to estimate in situ acoustic target strengths (TS) of fish from a number of different fish communities on the South African continental shelf. The TS and size distributions (o btained by aimed trawling) were used to describe the size structure an d vertical distribution of these communities. TS distributions obtaine d from several monospecific populations of juvenile Cape hake (Merlucc ius capensis), the dominant species present, were self-consistent, and there was good correspondence between modes in the TS and length dist ributions of juvenile hake, adult round herring (Etrumeus whiteheadi), anchovy (Engraulis capensis) recruits, pelagic goby (Sufflogobius bib arbatus), horse mackerel (Trachurus trachurus capensis), and ribbon fi sh (Lepidopus caudatus). Average TS values for all these species, both per individual and normalized by weight, are presented and compared w ith published values. The use of TS information in studies of the smal l-scale community structure and dynamics of fish populations is discus sed. it is concluded that the method can be effective on low-density, multispecific assemblages such as those in our study, avoiding many of the pitfalls of conventional net sampling.