The appearance of marine animals during the improvement of water quality in Dokai bay, northern Kyushu, Japan

The recent appearance of marine animals in Dokai Bay, northern Kyushu, was revealed with the first biological investigation in about twenty years following the improvement of water quality in the bay. Fishermen had not been able to catch any fish in the bay between 1943 and 1980, due to excessive water pollution caused by the discharge of untreated liquid waste from Kitakyushu's heavy industry and chemical production factories. Qualitative investigations were carried out at three stations, once each season from May 1989 to January 1990, using a small trawling net and two gill nets. Seventy-one species of marine animal were confirmed to have appeared in the bay: 5 species of molluscs, 30 species of arthropods, and 36 species of fishes. The total number of species and total catch weight increased gradually from the inner part to the outer part of the bay. The lowest occurrence of aquatic animals was observed in summer at the inner part of the bay where no biota zone had been found, however a large quantity of Acanthogobius flavimanus and a few young stages of Penaeus japonicus and Charybdis japonica recovered there in winter. It is suggested that the bay is used as a growing ground for the entire life cycle of Oratosquilla oratoria.