On the relation between echo intensity and fish density

Abstract

On the relation between echo intensity and fish density. FiskDir. Skr.Ser.HavUnders., 16: 301-314. Integrated echo intensities for a wide range of fish densities were measured. The experiments were carried out on live saithe (Pollachius virens) and sprat (Sprattus sprattus) which were kept in a net cage. Echo intensities were measured at 38 kHz and 120 kHz and pulse lengths ranging from 0.1 ms to 0.6 ms. The echo intensity was proportional to fish density below certain density limits. At high fish density a shadowing effect was observed. Factors encountered during survey work on schooling fish which indicate shadowing are also discussed. The exact density values at which shadowing occurs, appear to depend on parameters such as fish species, size, orientation, and probably also the vertical extention of the school.