Environmental groups expressed concern about the potential negative effects oil spilled from a supertanker in Tokyo Bay would have on animal life and tidal flats in the area.

Fish usually breed and mature in the shallows, so the spill could significantly effect them and the animals that feed on the fish, said Tatsuo Nakai, director of education and communication for the Nature Conservation Society of Japan. In addition, unlike an ocean oil spill, Tokyo Bay is almost enclosed, limiting the flow of water between the Pacific Ocean and the bay so that the oil will remain there for a long time, Nakai said. There are fears the tide flats in the bay, which are home to a diverse and very fragile ecosystem, may be at risk, he said.

Tidelands are very sensitive to oil spills, said Yukihiro Kominami, of the Wild Bird Society of Japan. "If the oil is carried in by the tide, it will cover a greater area and seep into the ground in three or four hours, damaging the tide flat ecosystem," Kominami said. Unlike the oil spill off of Fukui Prefecture in January from the wrecked Russian tanker Nakhodka, the number and variety of birds in the Tokyo Bay area is comparatively small this time of year, Kominami said.