底拖網漁業的網目選擇性研究－I-50 mm網目選擇性試驗的初步報告

Abstract

In order to find out the mesh selectivity of trawl net, the 50mm of cod end and 16mmcover net in mesh size were used, and a total of 15 experimental hauls in four cruises were made in the water along the coast off Fan-Liao, Fan-San, and Hai-Ko in May and August 1978, respectively (Fig. 1, Tab. 2) Numbers and body length of fishes retained in cod end and cover net were recorded. The experimental results were summerized as follows: 1. The selection ranges in body length for Saurida sp., Nemipterus ovenii, Nemipterus bathybius and Terichirus japonicus were found wider than the other species due to higher sensitive mesh selection. (Tab. 3) 2. The selection curves for Saurida sp., N. ovenii and N. bathybius were ”S” shaped, while that for T. japonicus was a simple smooth curve, (Fig. 3) 3. The mean body length for each species estimated at the 50% selection point was158mm for Saurida sp., 148mm for Nemipterus ovenii, 118mm for Nemipterus bathybius, and 120mm for Trichiurus japonicus. 4. Trash fishes less than 50mm in body length were all escaped from the cod end, but only about 50% of other species were escaped. The amount of cathes will be greatly reduced due to the enlargement of mesh size. 5. The resistance of trawl net with covering net on cod end was increased 11% of total drag force under 3.4 konts of towing speed.