Near range errors in sound scattering measurements of fish

Abstract

In the theory of the fish abundance estimation by quantitative echo sounders, we assume that object fish are at far range compared with the pulse length or the beam spreading. This assumption, however, deteriorates when the target fish are at near range. The time delay error which is caused by the finite pulse width, becomes serious, as Fernandes and Simmonds revealed, especially when we calibrate the quantitative echo sounder by the echo integrator outputs. This paper reviews our theory of fish abundance measurement focusing on the near range error to show necessity to incorporate a time delay in the range compensation function of the sounder. Further, we enumerate several near range errors and discuss them to give practical insight or countermeasures.