Investigations Into the Relation Between Integrated Echo Voltage and Fish Density

An echo integrator, working in combination with an echo sounder, measures the total voltage of the fish echoes received from a given depth interval and sums these voltages over time. Investigations were made on the relation between integrated echo voltage and fish density for juvenile sockeye salmon (Oncorhynchus nerka) in Lake Washington and Pacific hake (Merluccius productus) and Pacific herring (Clupea harengus pallasi) in Puget Sound. When fish were distributed at densities at which they could be acoustically resolved as individual targets, the relation between net catch and integrated voltage was linear. At higher densities when the fish echoes were multiple targets, the net catch was related to the square of the integrated voltage.