Separation efficiency of a two-level trawl using a separation grid-panel with different mesh sizes

For sustainable utilization of the demersal resources in the East China Sea, we designed a two-level trawl, in which the net-mouth for the lower level was covered with a grid-panel (square-mesh net panel) to separate marketable fish from the smaller or noncommercial fish. The main objectives of this study were to investigate the separation efficiency and size-selectivity of the gear. The experiments were conducted using three different mesh-bar sizes, 60, 90 and 120mm, on the grid-panel. Fishing experiments with the gear were carried out in the East China Sea aboard the training vessel Koyo-maru in 1996 (60mm mesh-bar), 1997 (120mm) and 1998 (90mm). At the towing speed of 2.5-4.0 knots, the heights of the net-mouth at the lower and upper parts of the gear were about 3-4m and 7m, respectively. Separation efficiencies, defined by the ratio of the catch in the upper cod-end to the total catch of the three mesh bar sizes were compared. The separation efficiencies for the 60, 90 and 120mm mesh-bar size grid-panels were about 80, 65 and 50%, respectively. As a result of the comparison of the size-selectivity of the different mesh-bar sizes for 5 species, the 90 and 120mm mesh-bar gridpanel have the appropriate size-selectivity for the separation of the smaller fish. In order to maintain the quantity of fish entering the upper cod-end, the 120mm mesh-bar grid-panel may be too large. Accordingly, the 90mm mesh-bar size is the recommended size for the grid-panel of the bottom trawl in the East China Sea.