80 percent of fish sample collected from California’s San Joaquin River Delta have a permanent spinal deformity, which researchers believe was caused by farm wastewater
- Researchers in California’s central valley studied a collection of 1,000 minnows
- They found more than 80 percent of the small fish had a spinal deformity
- The deformity left the fish’s spines in a zig-zag shape resembling a lightning bolt
- The team believe it was caused by exposure to selenium from farm runoff
A team of researchers in California’s San Joaquin Valley have linked widespread spinal deformities in a regional fish to toxic selenium exposure, believed to come from agricultural runoff.
The project, a joint effort from the California Water Science Center, the National Marine Fisheries Service, and UC Davis, began when researchers collected a sample of 1,000 splittail minnows from a pumping station in the San Joaquin River Delta.
The team noticed a severe spinal deformity in the minnows, which left the fish permanently shifted into a kind of zig-zag shape resembling a small lightning bolt.
More than 80 percent of splittail minnows collected from a location in the San Joaquin River Delta had a spinal deformity researchers believe was caused by exposure to selenium through farm runoff
The condition was widespread among the minnows, affecting more than 80 percent.
‘This was not just a few fish, it was the majority of them,’ co-lead Fred Feyrer told EurekAlert!.
To learn more about what had caused the deformities the team looked to the fishes’ ear bones, called otolith, which frequently retain the chemical markers of past exposures.
Using high intensity X-rays borrowed from Cornell University, the team found clear traces of selenium, which they believed was absorbed both from the fishes’ mothers and from their own independent feeding as juveniles.
‘They got it from both directions,’ lead researcher Rachel C. Johnson said.

The researchers believed the selenium exposure was constant for the minnows, with absorption first occurring through their mothers and later continuing as they fed themselves in the contaminated waters

Selenium was first discovered in the San Joaquin Valley in high concentrations during the 1980s, which researchers believe was caused by aggressive farm irrigation practices leeching the chemical out of the ground

Discovering fish with deformities from selenium in the wild is a rare occurrence as they usually either die from natural causes or are eaten by predators
The soil in the western part of the Central Valley is naturally high in selenium, and aggressive irrigation practices to feed the region’s booming $45billion agriculture industry has helped leech the chemical out of the soil and into runoff water.
Selenium was first detected in the San Joaquin Valley in 1983 when polluted waters containing the chemical were found in the Kesterson National Wildlife Refuge.
In the 1980s, researchers also discovered deformities in birds that had been exposed to agricultural drainage water with high levels of selenium.
Selenium exposure in fish has since been traced as far as the San Francisco Estuary, where adult splittail are believed to absorb the chemical by eating Asian clams.
It’s rare to see animals exhibiting the toxic effects of selenium, according to the researchers, because affected fish typically either die and decompose before being discovered, or else are eaten by predators.