It is shown in this study that for the fixed box trap, the size at first capture (ICWc) was less than the size at maturity (ICWm50%) for female broodstock. If ICWm50% was considered, the fishermen should change the fixed box trap mesh size. By applying the concept of optimum mesh size (mo), the mesh size of the fixed box trap should be enlarged to 4.50 cm to reduce the impact on immature stock. Optimum mesh size can reduce the destruction of young populations without reducing the catch efficiency for crabs of marketable size [8]. Selection performance is a critical point in the construction of many gear types. For example, mesh size modification in Japan and increase in the number of trap funnel entrances have improved trap fisheries according to studies on selectivity and catch efficiency