Banned DDT discovered in Canadian trout 70 years after use, research finds

Residues of the insecticide DDT have been found to persist at “alarming rates” in trout even after 70 years, potentially posing a significant danger to humans and wildlife that eat the fish, research has found.

Dichlorodiphenyltrichloroethane, known as DDT, was used on forested land in New Brunswick, Canada, from 1952 to 1968. The researchers found traces of it remained in brook trout in some lakes, often at levels 10 times higher than the recommended safety threshold for wildlife.

“DDT is a probable carcinogen that we haven’t used in 70 years here [Canada], yet it’s abundant in fish and lake mud throughout much of the province at shockingly high levels,” said Josh Kurek, an associate professor in environmental change and aquatic biomonitoring at Mount Allison University in Canada and lead author of the research.

The research, published in the journal Plos One, discovered that DDT pollution covers about 50% of New Brunswick province. Brook trout is the most common wild fish caught in the region, and the research found DDT was present in its muscle tissue, in some cases 10 times above the recommended Canadian wildlife guidelines.

Researchers said DDT, which is classified by health authorities as a“probable carcinogen”, can persist in lake mud for decades after treatment and that many lakes in New Brunswick retain such high levels of legacy DDT that the sediments are a key source of pollution in the food web.

“The public, especially vulnerable populations to contaminants such as women of reproductive age and children, need to be aware of exposure risk to legacy DDT through consumption of wild fish,” said Kurek.

Throughout the 1950s and 60s, half the province’s conifer forests were sprayed with DDT, a synthetic insecticide used to control insects carrying diseases such as malaria and typhus. Canada banned the use of the substance in the 1980s.

The 2001 Stockholm convention on persistent organic pollutants banned DDT worldwide for mass agricultural use, although it is still permitted in small quantities for malaria control.

“This mess can’t be cleaned up,” said Kurek. “DDTs can persist in lake mud for decades to centuries and then cycle in the food web. The best approach is to manage the public’s exposure of legacy DDTs by encouraging everyone to follow fish consumption guidelines and consider reducing exposure.

“Our findings are a clear wake-up call to abandon our overreliance on synthetic chemicals. Lessons need to be learned so we don’t repeat past mistakes. Our study hopefully informs on other contaminants that we apply broadly today, such as road salt and herbicides like glyphosate. We absolutely need to do things differently or our ecosystems will continue to face a lifetime of pollution.”