SACRAMENTO — High levels of selenium are suspected of being a cause in the deaths of 150,000 eared grebes earlier this year at the Salton Sea in one of the worst bird die-offs ever, U.S. Fish and Wildlife scientists say.
Selenium, a trace mineral more toxic than arsenic, drew national attention a decade ago when it was blamed for thousands of bird deaths and deformities at Kesterson National Wildlife Refuge in the San Joaquin Valley.
The levels of selenium in the Salton Sea eared grebes rivaled the levels found in 1983 at Kesterson, where biologists discovered that selenium was killing adult birds and causing grotesque deformities in bird embryos.
Concerned that elevated selenium levels in the surviving Salton Sea grebes will result in deformities in embryos this summer, the Fish and Wildlife Service has asked refuges in the Western United States and Canada where eared grebes nest to watch for unusual numbers of unhatched eggs.
Salton Sea, an oasis for migratory birds and a prime wintering site for eared grebes, long has had relatively high amounts of selenium. But tests this year showed that eared grebes had up to four times more selenium in their systems than in 1989, said Dan Audet, a Fish and Wildlife contaminant specialist in Carlsbad who is leading the investigation.
Analysis of dead grebes found selenium levels in their livers, where the toxin collects, averaging three times the amount considered normal. Some grebes had selenium levels five times normal. At such levels, deformities in embryos are likely, scientists have found.
Audet and other scientists have concluded that no grebes died of acute selenium poisoning, but they suspect that selenium was one of the causes of the deaths. It could have weakened the birds' immune systems, making them more susceptible to disease.
Audet said federal laboratories have ruled out several common avian diseases. To determine if the grebes ingested the selenium from the Salton Sea, the birds' favored food sources at the desert lake are being analyzed.
"It would be safe to say that selenium probably played a part in the die-off," said William Radke, the resident U.S. Fish and Wildlife biologist at the Salton Sea refuge. "The degree of that (contribution) is still under debate."
Scientists plan to gather in July in San Diego to reach consensus on the causes of death and forge a plan for preventing further die-off at the Salton Sea, a crucial habitat for 380 species and 2 million birds annually.
Eared grebes, diving birds known for their gold-colored crest, ruby eyes and elaborate mating rituals, are not considered an endangered species, though they are protected under the Migratory Bird Treaty Act.
As many as 2 million populate the Pacific Flyway, the migratory path that extends from Canada to South America. Whatever the population, the 150,000 grebes that died represented a significant percentage.
The species, which was also among the hardest hit at Kesterson in the early 1980s, is thought to accumulate selenium more readily than other birds and thus suffer from the poison sooner.
"Many researchers believe that grebes are an indicator species," Radke said. "It is a real possibility that grebes are just the first ones to show ill effects."
The Salton Sea is fed primarily by agriculture drainage in the Imperial Valley. Much of the water originates in rivers, including the Colorado, that slice through country that has soil high in selenium. Selenium is vital in trace amounts, and has several beneficial uses, but is poisonous in higher doses.
As the drought persists, the water level in the Salton Sea is falling, and various toxins are becoming more concentrated. The dead Salton Sea birds had higher than normal levels of other metals and minerals, among them mercury, chromium and molybdenum, Audet said. A byproduct of the long-banned pesticide DDT also was found in elevated levels in the Salton Sea grebes.
Kesterson Reservoir, like the Salton Sea, was filled with irrigation drainage water that contained large amounts of selenium. Federal authorities, trying to solve the selenium problem, closed the Kesterson ponds in 1986.
"I don't think we're close to Kesterson levels (of selenium)," said Dan Dinkler, the Fish and Wildlife Service manager at the Salton Sea wildlife refuge. If the Salton Sea's selenium problem were as bad as at Kesterson, Dinkler noted, other bird species would have succumbed this winter.
"Selenium might be involved--selenium is present," Dinkler said. "But it is not the causative factor that it is in some other areas, like in the San Joaquin Valley. There are some similarities but it is not a carbon copy, not yet anyway."