The Nature Conservation Society of Japan, a Tokyo-based non-governmental organisation, has released a research report on the effects of a dam at the mouth of the Nagara River in Mie Prefecture. The report mentions an accumulation of sludge on the riverbed and a declining population of corbicula japonica shellfish. It further questions the necessity for the dam. The research began in 1994, one year before the construction of the dam was completed. Local residents, university researchers and students studied changes in water quality, wildlife habitat and other factors. Sonar equipment was used to study the condition of the riverbed. It was discovered that up to two metres of sludge had accumulated downstream from the dam between 1994 and 1998, compared to 16 cm before its completion. Research by local fishery cooperatives unveiled that the presence of corbicula japonica had declined after peaking in June 1994. None of the shellfish were found after the dam became fully operational. Gifu University researchers studied the condition of ditch reed fields above the dam and found that they had been reduced by half.