Bull. Jpn. Soc. Fish. Oceanogr. 77(2), Page 53-58, 2013   Determination of upper boundary of an acoustic blind zone produced by the rugged bottom during a survey using a quantitative echosounder Hideaki Tanoue1,3 †, Teruhisa Komatsu2 and Akira Hamano3 1 Ocean Policy Research Foundation, 3-4-10 Toranomon, Minato-ku, Tokyo 105-0001, Japan 2 Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan 3Present: National Fisheries University, 2-7-1 Nagata-Honmachi, Shimonoseki, Yamaguchi 759-6595, Japan † E-mail: h-tanoue@09.alumni.u-tokyo.ac.jp Natural reefs are generally good fishing grounds. To use fish resources in a sustainable manner, it is important to estimate and monitor fish biomass. However, fish biomass survey using trawl is not applicable to natural reefs because the rugged bottom breaks or tangles trawl nets. In such grounds, it is effective to use a quantitative echosounder for estimating fish biomass. Nevertheless, the rugged bottom produces an acoustic blind zone above the bottom, where volume back-scatter from fish under beam is shaded by echo from the rugged bottom within the beam at a depth shallower than the bottom on the beam axis. It must be taken into consideration to estimate fish biomass more accurately. We conducted fish biomass survey in a natural reef, Hachirigase, southwest of the Sea of Japan by using a quantitative echosounder in June 2000. This study aims to develop a statistical method to identify acoustic bottoms, upper boundary of blind zone, with a discriminant analysis by analyzing integral layers of each ping with the minimum thickness from volume back-scatter of echograms obtained by the survey. We examine an effect of blind zone thicknesses determined with different methods by extrapolating volume back-scatter of fish above the acoustic bottom to the blind zones to estimate fish biomass in the reef expressed as volume back-scatter. Key words: fish biomass, quantitative echosounder, acoustic bottom, acoustic blind zone, natural reef