Abstract: Apparatus for producing an objective assessment of the size of fish in an aquaculture setting. The apparatus includes a sonic transducer (30) with a transmitter that is driven to generate time-spaced pulses that move along a sonic beam path (32) of small spread angle, and a sonic detector that detects echos of the transmitted pulses which result whenever a fish enters the sonic beam. An analyzing circuit analysis the echos to provide an estimate of the size of the fish that produced the echoes. The analyzing circuit detects the time between the leading and trailing edges of an echo, which indicates the thickness of the fish. A measurement of the peak amplitude of the echo and of the integral of echo energy also indicates the size of the fish.