The present research adopts with the application of the sequential mesh regulations on bottom trawl to study how to protect economic immature fishes. The experimental design includes first the determination of the relationships of fish size and the price per weight (PPW) of the economic demersal fishes sold as the one-hundred Dollars-a-plate manner in Tunkang and Zihguan fish markets in the southwestern coast of Taiwan. The obtained results were used to yield the catch of smaller size fishes and the lower PPW for regulating the economic immature fishes. The three kinds of regulated fish size were defined as the smallest surveyed size, smallest economic size, and smallest matured size. These regulated fish size were used as the body lengths of 50% selection (L50) to calculate the regulating mesh size. This study presents that four steps are needed for the sequential mesh regulations of bottom trawl in the southwestern Taiwan. The first step is to regulate the mesh size between 35 and 45 mm for banning bottom trawler to catch fishes smaller than the smallest surveyed fish size. The objective of first step is to reduce the bycatch without affecting the landings of economic immature fishes sold in the fish market. The second step is to regulate the mesh size between 50 and 60 mm for banning bottom trawler catch fishes smaller than the smallest economic fish size, and to reduce the catch of smaller size fishes and the lower PPW of economic immature fishes. The third step is to regulate the mesh size between 65 and 70 mm for banning bottom trawler to catch fishes smaller than the smallest matured fish size. This is also to reduce the catch of economic immature fishes, and to increase the recruitment. The fourth step is to regulate the mesh size between 70 and 82 mm for banning bottom trawler catching the lower PPW of economic fishes that is not large enough to reach their optimal PPW. The present research provides important data results for regulating the mesh size of bottom trawler operation for the achievement of sustainable coastal fishery in Taiwan.