Environmental groups have petitioned the federal government to list the declining population of great white sharks off the coast of California as an endangered species.

The northeastern Pacific Ocean population of great whites is genetically distinct and in danger of extinction, according to the petition. Researchers have estimated that there are about 340 individuals in the group that are mature or nearly so.

“There could be fewer than 100 breeding females left,” said Geoff Shester, the California program director of Oceana, an international group focused on protecting the world’s oceans. “Numbers in this range are lower than most species currently listed as endangered.”

Environmentalists said they filed their petition late Friday, but the federal National Oceanic and Atmospheric Administration could not yet confirm the submission when contacted Saturday.

If granted, the designation could lead to changes in fishing practices and could spur research aimed at restoring the population, said Jim Milbury, a spokesman for the federal agency.

Oceana prepared the petition, along with the Center for Biological Diversity and the group Shark Stewards.

“The main threat we are concerned about is bycatch of white shark pups off Southern California and across the border into Mexico, primarily in entangling gill net fisheries targeting halibut, yellowtail, swordfish, thresher sharks and white sea bass,” Shester said in an email.

In addition, young great whites off the Southern California coast have “the second-highest mercury level on record for any sharks worldwide, six times higher than levels shown to cause physiological harm to other ocean fish,” said Ashley Blacow, a policy and communications coordinator for Oceana. The sharks also had liver tissue with high levels of the contaminants PCB and DDT.

Great white sharks can live about 30 years and reach a size of 6,600 pounds and a length of 20 feet.

There are about 2,015 species listed under the federal Endangered Species Act. Of these, roughly 1,400 are found in part or entirely in the U.S. and its waters. Fish on the list include certain salmon and sturgeon populations, and the largetooth and smalltooth sawfish.

howard.blume@latimes.com

Times staff writer Tony Barboza contributed to this report.