The Minamata Fishing Cooperative had managed to win small payments of "sympathy money" from the company in 1926 and again in 1943, but after the outbreak of Minamata disease, the fishing situation was becoming critical. Fishing catches had declined by 91% between 1953 and 1957. The Kumamoto prefectural government issued a partial ban on the sale of fish caught in the heavily polluted Minamata Bay – but not an all-out ban, which would have legally obliged it to compensate the fishermen. The fishing cooperative protested against Chisso and angrily forced their way into the factory on 6 August and 12 August, demanding compensation. A committee was set up by Minamata Mayor Todomu Nakamura to mediate between the two sides, but this committee was stacked heavily in the companys favour. On 29 August, the fishing cooperative agreed to the mediation committees proposal, stating: "In order to end the anxiety of the citizens, we swallow our tears and accept". Chisso paid the cooperative ¥20 million (US$183,477 — about US$1.7 million in 2021 value) and set up a ¥15 million ($137,608 — about 1.25 million today) fund to promote the recovery of fishing. Since the change of route of wastewater output in 1958, pollution had spread up and down the Shiranui Sea, damaging fisheries there as well. Emboldened by the success of the small Minamata cooperative, the Kumamoto Prefectural Alliance of Fishing Cooperatives also decided to seek compensation from Chisso. On 17 October, 1,500 fishermen from the alliance descended on the factory to demand negotiations.
The disease developed without any prior warning, with patients complaining of a loss of sensation and numbness in their hands and feet. They became unable to grasp small objects or fasten buttons. They could not run or walk without stumbling, their voices changed in pitch, and many patients complained of difficulties seeing, hearing, and swallowing. In general, these symptoms worsened and were followed by severe convulsions, coma, and eventually death. By October 1956, forty patients had been discovered, fourteen of whom had died: an alarming case fatality rate of 35%. Finding the cause
Researchers from Kumadai also began to focus on the cause of the strange disease. They found that the victims, often members of the same family, were clustered in fishing hamlets along the shore of Minamata Bay. The staple food of victims was invariably fish and shellfish from Minamata Bay. The cats in the local area, which tended to eat scraps from the family table, had died with symptoms similar to those now discovered in humans. This led the researchers to believe that the outbreak was caused by some kind of food poisoning, with contaminated fish and shellfish being the prime suspects. On 4 November, the research group announced its initial findings: "Minamata disease is rather considered to be poisoning by a heavy metal, presumably it enters the human body mainly through fish and shellfish." Identification of mercury
As soon as the investigation identified a heavy metal as the causal substance, the wastewater from the Chisso factory was immediately suspected as the origin.