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Controversial Pesticides Are Suspected Of Starving Fish

Nov 2, 2019

There's new evidence that a widely used family of pesticides called neonicotinoids, already controversial because they can be harmful to pollinators, could be risky for insects and fish that live in water, too.

The evidence comes from Lake Shinji, which lies near Japan's coast, next to the Sea of Japan.

Masumi Yamamuro, a scientist with the Geological Survey of Japan, says the lake is famous for its views of the setting sun. "It's amazingly beautiful," she says.

Lake Shinji was also the site of thriving fisheries. People harvested clams, and eels, and small fish called smelts. But, Yamamuro says, about a decade ago, people noticed that fish populations had declined drastically. "I was asked to investigate the cause of this decrease," she says.

It was a puzzle. Yamamuro says the decline in fish populations did not seem to coincide with anything that people were keeping track of, like the lake's salinity, or levels of pollution.

But she noticed something curious. One kind of fish in the lake was doing fine. This one had a more diverse diet; it could eat algae, as well as tiny insects in the water. The eels and the smelts that were dying off relied on insects and crustaceans for food. And that food source was vanishing.

"So we concluded [that] something killed the food of the eels and the smelt," Yamamuro says.

She and her colleagues now believe that they've identified the culprit: pesticides called neonicotinoids.

The evidence is circumstantial. Right around the time the fish started having problems, early in the 1990s, farmers near the lake started using these pesticides on their rice paddies to control insect pests. Yamamuro also found traces of these chemicals in some parts of the lake. Those levels, she thinks, are high enough to cause problems for tiny aquatic animals. Also, neonicotinoids kill insects, but not the algae that the thriving fish were eating.

She and her colleagues just published their findings in the journal Science.

Jason Hoverman, an ecologist at Purdue University, in Indiana, says this study doesn't really prove that neonicotinoids are guilty. There's no historical data showing levels of neonicotinoids in the lake back when the fish started to die off.

But he says that it is logical to suspect them, and the new report is a good reminder that chemicals can have really complicated effects on an ecosystem.

"When we think about chemicals, we often just go right to direct toxicity, not thinking about the food web implications; the food of the fish, and the impact of the chemicals on that food," he says.

Neonicotinoids have become really controversial in recent years. That's partly because of how widely they're used. Corn and soybean and other seeds that are coated with these pesticides are planted on close to 200 million acres of land every year. Also, neonicotinoids are extremely toxic to bees and other pollinators.

Scientists like Hoverman now are starting to pay much closer attention to the effects on insects that live in streams and rivers. "These chemicals can definitely end up in water. We apply them on land, but they don't stay on land. The question becomes, are they at levels that are high enough to cause a problem?" he says.

Hoverman says that in some cases, it looks like they may actually be causing problems. But much of the time, scientists still are searching for the answer.

Copyright 2019 NPR. To see more, visit https://www.npr.org.

SCOTT SIMON, HOST:

There's a widely used family of pesticides that's controversial because it can harm pollinators like bees. Now there's evidence it could also be risky for organisms in water, like insects and fish. That evidence comes from a lake in Japan and NPR's Dan Charles has the story.

DAN CHARLES, BYLINE: Lake Shinji lies near the coast, next to the Sea of Japan. Masumi Yamamuro, a scientist with a geological survey of Japan, says the lake is famous for its views of the setting sun.

MASUMI YAMAMURO: It's amazingly beautiful.

CHARLES: Do people come, and to look at the sunsets to see the lake?

YAMAMURO: Yes, yes. We have a special spot for that.

CHARLES: Historically, there were thriving fisheries here. People harvested clams and eels and small fish called smelts. But Yamamuro says about a decade ago people noticed the fish were declining rapidly.

YAMAMURO: And I was asked to investigate.

CHARLES: When she started investigating she realized the decline in fish population did not seem to coincide with anything that people were keeping track of, like the lake's salinity, or levels of pollution. But she noticed something else, one kind of fish in the lake was doing fine. This one had a more diverse diet; it could eat algae as well as tiny insects in the water. The eels and the smelts that were in trouble, they mostly ate insects and crustaceans and that fish food was disappearing.

YAMAMURO: So we concluded something killed the food of the eels and smelts.

CHARLES: She now thinks she knows the culprit, pesticides called neonicotinoids. The evidence is circumstantial. Right around the time the fish started having problems, farmers near the lake started using these pesticides on their rice paddies to control insect pests and Yamamuro found traces of these chemicals in some parts of the lake. Enough, she thinks, to cause problems for tiny aquatic animals. Also, these chemicals kill insects, not the algae that the thriving fish were eating. She and her colleagues just published these findings in the journal Science. Jason Hoverman, an ecologist at Purdue University in Indiana says the study does not really prove that neonicotinoids are guilty. There is no historical data showing levels of pesticides in the lake back when the fish started to die. But he says it is a reasonable suspicion and it's good to remember that chemicals can have really complicated effects on an ecosystem.

JASON HOVERMAN: When we think about chemicals, we often just go right to direct toxicity of a chemical, we're not thinking about the food web implications. That link between the food of the fish - right? - and the impacts of the chemicals on that food.

CHARLES: Neonicotinoids have become really controversial in recent years, partly because they're used so widely. Corn and soybean and other seeds that are coated with these pesticides are planted on close to 200 million acres of land every year in the United States, and they're extremely toxic to bees. Now scientists like Hoverman are starting to look more closely at their effects on insects that live in streams and rivers.

HOVERMAN: These chemicals can definitely end up in water, right. So we apply them on land and they don't stay on land. The question becomes, are they at levels high enough to cause a problem?

CHARLES: He says in some cases it looks like they may be. But many times scientists are still searching for the answer.

Dan Charles, NPR News.

(SOUNDBITE OF THOR AND FRIENDS' "WHOSE FINGERS") Transcript provided by NPR, Copyright NPR.