Abstract
Using the results from the coastal survey off Fukuoka Prefecture, the importance of using sonar for a clear understanding of the distribution of schools was shown by comparing the figure drawn based on the echo-sounder information with that based on the sector scanning sonar information. The stepwise transmission of a beam of a different tilt from a different position on the transect by sector scanning sonar caused the non-scanned volume. A geometric consideration revealed that the non-scanned volume was, under the present survey conditions, an upside-down triangle with a 32m base and 65m depth every 36m on the section 2 m from the transect. Its apex first deepened then became shallower, and its base was shortened with an increase in the distance from the transect.
