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A simple and precise system for population estimation of epi-megabenthos using an underwater photographic camera is proposed. A standard 35 mm film camera with motor wind mounted in an underwater case is attached on a stainless steel frame to photograph 1 m2 of sea bed. The camera is triggered by a rigid foot switch set off on contact with the sea bed; thus, only vertical lowering is needed in operation. Two-stage samphng was employed to estimate the extensively cultured Japanese scallop Patinopecten yessoensis; estimated densit~es were highly accurate and relative standard errors were within only 0.05 of the means. The estimated amount of harvest approximated closely the actual harvest. Precise density changes in a cohort, from seed release time to harvest, could be followed, providing essential information for fishery management of scallops. The distribution pattern of the population was analyzed. Contagious distribution patterns of the scallops were detected on a large scale, and they showed large areas of patchiness, but random distribution of individual scallops within the patches was observed on a small scale. Contagiousness decreased with age. Factors affecting the distribution pattern on large and small scales are discussed in relation to substrate types, current flow with subsequent downstream dispersal during swimmiig movements, and possible food depletion. K E Y WORDS: Scallop . Distribution . Population estimate Underwater camera . Two-stage sampling

Distribution and abundance of cultured scallop Patinopecten yessoensis in extensive sea beds as assessed by underwater camera