The recent collapse of two fisheries in Japan was found to be linked to the use of pesticides by nearby rice farmers, according to research, which warned similar impacts are likely to be found around the world.
The study, published in the journal Science, showed an immediate plunge in insect and plankton numbers in a large lake after the introduction of neonicotinoid pesticides to rice paddies. This was followed by the collapse of smelt and eel populations, which had been stable for decades but rely on the tiny creatures for food.
It was the first time a study revealed the effects of pesticides on fish. Previous studies in Europe have linked neonicotinoids to die-offs in other freshwater species including mayflies, dragonflies and snails and also to falling populations of farmland birds that feed on insects.
Prof Olaf Jensen, at Rutgers University in the US and not part of the research team, told The Guardian: “This study, although observational, presents compelling evidence. A fishery that was sustainable for decades collapsed within a year after farmers began using neonicotinoids. This is a large and astoundingly fast response.”
Researchers looked at data from Lake Shinji spanning the decade before and the period after the introduction of neonicotinoids in 1993, from which point the pesticides started running off into the lake. They found neonicotinoid concentrations in the water frequently exceeded levels that are toxic to aquatic invertebrates.
Amongst the worst affected, the midge Chironomus plumosus, an important food source for smelt, ranked the highest. It vanished completely from all 39 locations sampled in 2016, despite being abundant in 1982. Another important food source, an abundant zooplankton species, Sinocalanus tenellus, fell by 83%.
The study found annual catches of smelt fell 90% in the decade after neonicotinoids were introduced, compared with the decade before. Catches of eels dropped by 74% over the same time period.
“Several alternative explanations for the collapse were evaluated and rejected: invasive species, hypoxia, or changes in fish stocking cannot plausibly explain the observations,” said Jensen. Furthermore, catches of icefish, which do not rely on the affected invertebrates for food, remained unchanged.
The researchers noted that they also studied other factors that might have led to fishery collapses, such as nutrient depletion or changes in oxygen or salt concentrations. They report that they were not able to find any evidence showing that there might have been something other than pesticides killing the food fish ate leaving them to starve.