also began, including requested research by the Kumamoto Prefectural Government to the University of Kumamoto and the setting up of the Health Science Research Team by the Ministry of Health and Welfare (MHW; presently the Ministry of Health, Labor and Welfare (MHLW)).
At the early stage, an infectious disease or similar was thought to be the possible cause. In March 1957, the Team of the MHW reported: "Poisoning by eating fish or shellfish caught in Minamata Bay is suspected as the most likely cause at present. It is not yet known which toxic substance has caused the pollution of fish or shellfish, but a type of chemical substance or metal is believed to be the most likely cause".
Because of this suspicion that the eating of fish or shellfish caught in Minamata Bay was the cause of the disease, the Fishermen's Cooperative in Minamata City voluntarily restrained from fishing in Minamata Bay in August 1957 on the administrative guidance of the Kumamoto Prefectural Government. By this time, the Kumamoto Prefectural Government had decided to opt for a policy of the total prohibition of fishing in Minamata Bay with the application of the Food Sanitation Act and requested the MHW to decide on the legality of this policy in August 1957. The response by the MHW was that the said policy could not be legally enforced as there was no clear evidence of the toxic contamination of all fish and shellfish in a specified area of Minamata Bay.
The background for this response was uncertainty regarding the cause while the Team of the MHW was focusing on selenium, manganese and thallium as the casual substance.
Half Measure Investigation of the Cause
In September 1958, Chisso changed the discharge system for plant effluent from the acetaldehyde manufacturing process. Before this change, effluent was discharged directly to the Port of Hyakken in Minamata Bay. Under the new system, effluent was stored in the Hachiman Pool prior to discharge of the supernatant to the mouth of Minamata River. However, this new system led to the emergence of new patients near and north of the river mouth from March of the following year. In October 1959, the Ministry of International Trade and Industry (MITI; presently the Ministry of Economy, Trade and Industry (METI)) instructed Chisso to remove the new drainage channel, and discharge from the Hachiman Pool to the mouth of Minamata Rive was stopped in the following November.
In July 1959, the Minamata Disease Study Team of the Faculty of Medicine, University of Kumamoto reported that it had reached the conclusion that the substance causing Minamata Disease was a mercury component, most likely an organomercury compound. Some scientists, however, did not support this organic mercury theory.
At the Ministerial Liaison Conference on Countermeasures for the Minamata Food Poisoning held on November 11, 1959, a researcher from the University of Kumamoto reported the suspicion of organomercury poisoning caused by the plant effluent. Some of the participants pointed out that no similar disease had been reported in connection with effluent from other similar chemical plants and that the process of inorganic mercury changing to organic mercury was not clearly established. The Food Sanitation Investigation Council which met on the following day simply returned its verdict to the Minister of Health and Welfare that the main cause of Minamata Disease was likely to be some kind of organomercury compound without mentioning the original source of pollution.
The Special Committee on the Minamata Food Poisoning which had been set up under the Food Sanitation Investigation Council in January 1959 to investigate the cause of Minamata Disease was dissolved on November 13 of the same year.
Subsiding of the Problem
Following the presentation of the organomercury theory by the University of Kumamoto, local fishermen