By Marty Graham
SAN DIEGO (Reuters) - Federal authorities said on Thursday they were investigating the deaths of two California common bottlenose dolphins that stranded and died on beaches just a few miles apart in San Diego a day after the U.S. Navy conducted a brief exercise with sonar.
Jim Milbury, the West Coast spokesman for the National Oceanographic and Atmospheric Administration (NOAA), said that necropsies were being performed and that an investigation was underway.
The dolphins, apparently part of a population of 323 remaining California common bottle nose dolphins, were found on Oct. 21 at Imperial Beach and the Silver Strand, less than 10 miles apart.
Navy officials confirmed that ships were using mid-frequency sonar in the area "for a little over an hour" as part of a training exercise on Oct. 19 and Oct. 20. The ships are part of the Silver Strand Training Complex, according to Lieutenant Julie Holland.
The Navy won federal permission to train with mid-frequency sonar, which sends out a loud ping underwater, in December 2013 despite environmentalists' concerns over the harmful effects of such sonar on underwater mammals that rely on their hearing, including whales and dolphins.
"A single ping can permanently deafen a marine mammal 100 meters (yards) away," said David Henkin, an environmental attorney.
The investigation comes as part of a September settlement with the environmental group, Earth Justice. The group sued the agency for giving the Navy a permit to conduct training exercises using mid frequency sonar and explosives in areas where ocean mammals could be injured or killed.
The settlement includes the requirement that the National Marine Fisheries arm of the agency investigate deaths and notify the public of the deaths and investigations.
Henkin, an Earth Justice attorney, applauded the investigation.
"The fisheries calculate the maximum acceptable death rate caused by humans in this population at 2.4 per year before the population is threatened, so these two deaths are significant," Henkin said. "Before the settlement, there was no obligation to investigate or report the deaths."
NOAA had six months to evaluate whether or not the permit for the Navy should be modified after the investigation, he said.
(Reporting by Marty Graham; Editing by Dan Whitcomb and Sandra Maler)