1Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, 2T/S Ushio-maru of Faculty of Fisheries, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
The crangonid shrimp Crangon uritai is one of the predators for marbled sole Pseudopleuronectes yokohamae juveniles in the coastal area of Hakodate Bay, Japan. The catch efficiencies of Hirota's sledge net (mouth size: 60 cm width and 40 cm height) with a distance wheel for C. uritai and P. yokohamae juveniles were obtained to estimate their accurate population densities. Assuning the catch efficiency of the drop trap (a kind of box type quadrat and 0.5 m2 covered) to be 100%, the catch efficiency of the sledge net was 36-68% for the shrimp and increased with the shift to small body size cohort and so large female individuals disappeared. In contrast, the catch efficiency for P. yokohamae juveniles was lower than that for the shrimp and fluctuated (8-38%), which would be caused by the high escape ability of large juveniles.