Near range errors in sound scattering measurements of fish

In the theory of the fish abundance estimation by quantitative echo sounder s, we assume that object fish are at far range compared with the pulse leng th or the beam spreading. This assumption, however, deteriorates when the t arget fish are at near range. The time delay error which is caused by the f inite pulse width, becomes serious, as Fernandes and Simmonds revealed, esp ecially when we calibrate the quantitative echo sounder by the echo integra tor outputs. This paper reviews our theory of fish abundance measurement fo cusing on the near range error to show necessity to incorporate a time dela y in the range compensation function of the sounder. Further, we enumerate several near range errors and discuss them to give practical insight or cou ntermeasures.