An enclosed coastal sea such as the Seto Inland Sea functions not only as nursery ground for juveniles of many species but also as fishing ground for trawl fisheries. A small shrimp trawl net has small-mesh codends. Consequently, in the small shrimp trawl fishery, lots of juvenile flounders smaller than a commercial size are caught and discarded at sea. It was found that the annual amount of juvenile finespotted flounder discarded at sea was about 200,000 in number (1 ton in weight) per a boat in the Seto Inland Sea. Most of these discarded juveniles are dead. The authors believe that the catching and discarding of these juveniles have a detrimental effect on fisheries of flounders in the future. The small trawl fishery also catches and discards at sea shrimp smaller than a commercial size, since the mesh size now used is too small. For the purpose of reducing the amount of discards at sea, we determined the mesh selectivity curves of the small shrimp trawl by Tokai and Kitahara's method. The authors propose, as an objective in managing the present small trawl fishery, that the amount of a few important target species discarded should be made as small as possible without a remarkable decrease in the landing of the commercial size fishes. Moreover, according to the objective the authors attempt to estimate appropriate mesh sizes for the shrimp fishery in the Seto Inland Sea. The result seems to be useful for predicting the amount of the target species discarded at sea, using a fishery model. This mesh size regulation is probably acceptable for the small trawl fishermen.