Fisheries yield in Japanese lakes exhibit long-term declining trends. Oligotrophication of lakes and an increase in exotic fish such as largemouth bass and bluegill, which mainly feed on other fish, have been indicated as the causes. In Japan's major lakes, however, levels of organic matter have been fairly stable, and there is no clear evidence of oligotrophication. Moreover, Japanese lakes with particularly high fisheries yield are brackish lakes in which fresh water and seawater are mixed, but freshwater fish such as largemouth bass and bluegill cannot live in brackish lakes. Thus, causes of the decline in fisheries yield that are accordance with the actual situation have not been elucidated.

The researcher conducted a survey of Lake Shinji in Shimane Prefecture, in collaboration with the University of Tokyo, Shimane Prefectural Institute of Public Health and Environmental Science, Nagoya City Environmental Science Research Institute, and Chiba Institute of Technology. The results indicated the possibility that neonicotinoid insecticides used in rice paddies and other watershed areas kill or otherwise harm the creatures on which eels and smelt feed, and thus indirectly caused the rapid decline in eels and smelt.

There are reports that neonicotinoid insecticides may have led to the mass disappearance of honeybees, and the trend is toward stricter regulations in Europe and the United States. However, the impact on fisheries around the world has not yet been clarified. The staple food that occupies the majority of farmland in Europe and the United States is wheat, but in Japan this staple food is rice. Neonicotinoid insecticides are water soluble, and these results indicate the possibility that when used in rice paddies, they flow out and impact the environment of rivers and lakes.