1Graduate School of Science and Technology, 2Faculty of Fisheries, Nagasaki University, Nagasaki 852-8521, Japan
Fourteen experiments were carried out to estimate quantitatively the actual conditions of discards of by-catch for marketable species from a small-scale trawl fishery in the Ariake Sea. The length and weight of catch were measured and the number and weight of discards were calculated. The results showed that the percentage of the catch of marketable species in weight was an average of 67.4 percent of the total amount of catch and the individuals of discard size in marketable species occupies about 30 percent of the catch weight in that. The discard percentage of marketable species to the total discards in weight was 39.3 percent on the average. Among the discarded fish, frog flounder Pleuronichthys cornutus, the target species was yielded significantly with the average of 38.5 percent of total discard weight in marketable species. The percentage of the discard number to the catch number of frog flounder was much higher in the summer than in the winter with an average of 76.1 percent.