Separation efficiency of a two-level trawl with a separation device

For better utilization of the bottom fish resources, it is desirable to introduce a new trawl net with a separation device, which is functional in sorting and releasing small and immature fish. For this purpose, we designed a two-level trawl with square-meshed separation panel transversely attached to the mouth of the lower level. The main objectives of this paper are to present the operating characteristics and separation efficiency of the test gear. The experimental trawlings were carried out in the East China Sea aboard the training vessel Koyo-maru in 1996. At a towing speed of 3.5 knots, the height of the net-mouth at the lower and upper parts of the gear were about 3.0m and 7.0m, respectively. Of 8 hauls tested on July 21-23, a total number of nearly 15,000 individuals were caught in the upper codend, while about 3,000 were caught in the lower codend. Separation efficiency for 9 species were discussed by taking the length compositions of fishes collected in the upper and lower codends into consideration. As a result of a statistical comparison between the body size distributions of the fish caught in the lower codend and that in the upper codend, the gear was effective for these species.