Toxic pesticides dumped off the coast of Southern California decades ago are lingering — deep in adjacent ocean sediments and in the fish that inhabit those habitats, a new study has found.

Current levels of dichlorodiphenyltrichloroethane (DDT), an agricultural insecticide banned in 1972, remain highest in sediments and fish closest to old spill sites, according to the study, published Monday in the Proceedings of the National Academy of Sciences.

DDT contamination remains persistent in the Palos Verdes region, about 15 miles off the coast of Catalina Island, according to previous research.

From about 1948 to 1961, barges contracted by DDT manufacturer Montrose Chemical Corporation released waste containing up to 2% pure DDT directly into the Pacific Ocean in this area, previous studies they showed

An estimated 100 tons of the chemical ended up in Palos Verdes offshore sediments. The Environmental Protection Agency eventually declared the area a hazardous waste site in 1996, and four years later, a judge ordered Montrose to pay $140 million in restitution.

Since then, research has confirmed contamination and health problems in local sea lions, dolphins, bottom-feeding fish and California coastal condors. As such, the ways in which compounds move through the food chain have come under increasing scrutiny.

Monday’s study was able to reveal strong relationships between DDT concentrations in fish and their precise habitat, diet and location — information that the authors said they hope will lead to more accurate fish consumption advisories.

“I was surprised by how strong the relationship was,” said lead author Lillian McGill, a postdoctoral researcher at the Scripps Institution of Oceanography at the University of California, San Diego, in a statement.

This relationship, McGill continued, was “strong enough to reasonably predict DDT concentrations in a fish based on where it was caught and its diet and habitat.”

To investigate these connections, the scientists compiled information from nine data sets that included 60 fish species in the area from 1998 to 2021.

After analyzing chemical concentrations in both ocean sediments and fish, the researchers used statistical models to disentangle relationships between DDT’s distribution and the species’ location, diet, and habitat.

In encouraging news, the researchers said they found that DDT contamination of fish has declined over time and that most fish caught in the region are safe to eat.

“The jury is still out, but I think one of the main reasons for the decline is that contaminated sediment is slowly being buried by new sediment,” co-author Brice Semmens, a Scripps marine biologist, said in a statement.

“This could be why we see less of it in the food web over time,” Semmens added.

However, scientists have identified some notable exceptions, including bottom dwellers such as halibut that have been caught near highly contaminated areas such as Palos Verdes.

In general, the researchers noted that the highest concentrations of DDT were found near these landfills, even though they had been decommissioned for half a century.

These findings, the authors pointed out, attest to the ability of certain chemical pollutants to persist in the ocean over time. However, their relatively localized presence and predictability could help both in reducing exposure and in future assessments of other affected species, according to the researchers.

“If DDT contamination moves through the ocean in predictable ways, then we can mitigate our exposure as we use the ocean,” Semmens said.

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