Kirk here. This survey has been promised for a very long time:

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The survey on residents of the Shiranui Sea coast in Kumamoto and Kagoshima prefectures will be conducted for the first time under a special law that took effect in 2009 to provide relief for victims of the disease, which was officially recognized in the Kumamoto city of Minamata on May 1, 1956.
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The "under a special law" part means that the government committed to doing the survey when they made the law. So far, the only data about health effects is from those who have come forward to request certification, etc. -- despite fears of social stigma and discrimination. The government has been dragging its feet because revealing the extent of the damage reveals the extent of its own negligence (failure to protect the health of people living in the area). Now after 17 years, the government is finally starting the survey but their methods are not what patient groups are asking for.