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A B S T R A C TDifferences between the results of pot fishing and hand-net collecting of the Japanese mitten crab, Eriocheir japonica (De Haan, 1835), were examined to increase the accuracy of sampling natural population in freshwater and tidal areas of a small river. Large adult crabs were caught in pots; small juveniles mostly collected with the hand nets. The difference in size composition in the freshwater river is probably caused by changes in the dietary preferences and behaviors that occur as crabs grow larger. The sex ratio was strongly male-biased in pot fishing in the tidal river in contrast to hand-net collecting (no bias), and males captured in the pots were slightly larger than those caught by hand-nets. The sex ratio bias observed in pot fishing in the tidal river can be explained by mating behavior, whereas the differences in the size of males between the two methods can be explained by the dimorphism and alternative mating strategy of male E. japonica. The proportions of ovigerous females in the tidal river in early winter were larger in pot than in hand-net samples. Differences in the proportion of ovigerous females between the two gears in the tidal river may have resulted from the presence of two groups differing in reproductive season and the wandering activities of ovigerous and non-ovigerous females. Although pot fishing is useful for studying reproductive ecology, the resulting data is sometimes strongly biased. In contrast, hand-net collecting is essential for gathering a complete range of crabs from all size classes and the data generated by this method is more representative of their distribution pattern. It is necessary to make up for the shortcoming of each sampling method.

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This study measured the flow speed distribution using actual small traps to investigate the effects of their materials and shapes by analyzing the characteristics of the water flow patterns around them. Five types of trap (netted semi-cylinder shaped, wire semi-cylinder shaped, heart shaped, box shaped, and a cylinder shaped traps) constructed from different materials were used in the experiment. The flow speed distribution around each trap was measured in a flume tank. Flow speeds in the flume tank were 0.1, 0.2, 0.3, and 0.4 m/s. The webbing of the traps was different in shape and had various diameters of twine. The diameters of twine ranged from 0.55 to 1.00 mm. The relationship between the solidity ratio of the twine and flow distribution of the traps was also analyzed. For the netted semi-cylinder shaped and cylinder shaped traps, the area of reduced flow speed extended along the downstream area and its width slightly increased with distance from the trap's entrance. The heart shaped trap is shown to have the largest solidity ratio and this resulted in a high reduction of the flow speed downstream from the trap.KEY WORDS: flow ratio, flow speed distribution, solidity ratio, trap.