Photo/IllutrationShinjiko in Shimane Prefecture, connected to the Sea of Japan, is a brackish lake and home to a variety of fish and shellfish. (Asahi Shimbun file photo)

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A team of researchers has found that neonicotinoid pesticides could wreak havoc on the ecosystem in rivers and lakes, the first discovery of its kind in the world.

According to the findings, catches of eel and Japanese smelt decreased dramatically in Lake Shinjiko in Shimane Prefecture in the 1990s, highly likely because those agrochemicals started to be used in nearby rice paddies around that time.

“The possibility of neonicotinoids exerting effects on not only rice fields but also the ecosystem in rivers and lakes was suggested,” said Masumi Yamamuro, an aquatic environment professor at the University of Tokyo, who is also a special fellow at the National Institute of Advanced Industrial Science and Technology. “Similar phenomena may have occurred in other rivers and lakes.”

The findings were published in the U.S. journal Science on Oct. 31.

In their study, Yamamuro and her colleagues discovered that Lake Shinjiko saw a sharp decrease in the zooplankton population after the first neonicotinoid was approved in Japan in 1992 and its use began during the rice planting season in May 1993.

The average population of zooplankton dropped 83 percent over the period from 1993 through 2004, compared with that for between 1981 and 1992, according to the scientists.

As a result, almost no Japanese smelt, which consume zooplankton, have been caught in the lake since 1994, although an average of 240 tons of the fish were previously hauled ashore annually. The average annual catch of eel also dropped from 42 tons to 10.8 tons.

An increasingly smaller amount of fish has been caught in lakes and marshes in Japan, and the reduced phytoplankton population and the increased number of alien species that prey on indigenous fish have been blamed for the decrease.

But because no phytoplankton decrease has been reported in Lake Shinjiko and non-native freshwater species cannot inhabit the brackish water, characterized by both seawater and fresh water, the devastated fishery yields can solely be attributed to the zooplankton decrease caused by neonicotinoids, according to the team.

Neonicotinoids have been widely replacing organophosphorus pesticides for controlling stinkbugs and other pests in rice paddies, killing cockroaches, removing fleas from pets’ bodies and other purposes.

However, as neonicotinoids are suspected of causing cancer and connected with the widespread deaths of honeybees, tighter restrictions have started to be imposed on them outside Japan.