Excessive exposure to selenium is linked to the severe deformities in the spines of an imperiled California migratory minnow.

The Sacramento splittail (Pogonichthys macrolepidotus) is only found in the waters between the Sacramento-San Joaquin Delta and the San Francisco Estuary and is considered a species of “special concern” by the State of California. During a 2011 fisheries study, researchers found that more than 80 percent of 1,000 juvenile splittail collected near a pumping station showed signs of spinal deformities. Their findings are published in Environmental Science and Technology.

"This was not just a few fish, it was the majority of them," said Fred Feyrer, a research fish biologist at the US Geological Survey's California Water Science Center and co-lead of the research, in a statement

Selenium is a naturally occurring trace element and is essential to the development of humans and animals, but high levels can be toxic and have been shown to cause deformities in fish, birds, and mammals through both parental transfer and accumulation through diets. In some cases, the element can be more toxic than arsenic, as was the case during a mass mortality event that killed some 15,000 seabirds near the Salton Sea in 1992, reported in the Los Angeles Times.

Rice-sized ear bones known as otoliths record chemical traces that fish are exposed to much like tree rings leave records of the plant’s environment. Researchers examined the fishes’ otoliths using high-intensity X-rays to measure selenium concentration in order to determine where and at what point the minnows were being exposed.

"We found that the otoliths record a diary of selenium exposure from birth to death, and were the key to unraveling this mystery," said lead author Rachel C. Johnson, a research biologist at NOAA Fisheries' Southwest Fisheries Science Center and the University of California Davis.

Though individuals varied widely in the number of days they were exposed to the element, researchers found that the fish had absorbed selenium from “both directions” – both from their parents during embryo development and while feeding as juveniles. In an earlier study, the researchers observed high concentrations of selenium in some adult splittails feeding in the San Francisco Estuary that exceeded protective criteria set forth by the US Environmental Protection Agency.

Oil refineries within the estuary and agricultural practices upstream are the two leading causes of human-influenced selenium in the region between the river and estuary where splittail feed, migrate, and reproduce. Splittails are known to reproduce during wet years like 2011, which also see engorged rivers spread into floodplains to open new habitats. What is unclear is whether splittails are encountering higher levels of selenium during these wetter years or consistently over time – a difficult theory to test as deformed fish are less abundant in the wild because of their heightened mortality rate.

The researchers add that understanding how fish encounter selenium could inform management agencies’ policies on migratory species and paves the way for diagnosing sources, pathways, and potential avenues for excessive selenium exposure.