Three dolphins died this month during a Navy training exercise using underwater explosives near the San Diego County coast, authorities said Friday.
Scientists have yet to officially determine what caused the deaths at the Silver Strand Training Complex near Coronado, but examinations of the animals showed injuries consistent with blast trauma.
The unit conducting the underwater training exercises March 4 had scanned the area and spotted no marine mammals before starting a countdown to detonate the explosives about 10:45 a.m., said Cmdr. Greg Hicks, spokesman for the U.S. Navy's Third Fleet.
"They saw the dolphins before the explosives went off, but it came so late it would have put humans at risk to stop the process," he said. "After the detonation, despite all required protective actions taken to avoid marine mammal impacts, three dolphins were found dead in the area."
After the explosion, government biologists retrieved the carcasses and took them to a veterinary lab at SeaWorld to conduct necropsies.
Genetic testing showed that the animals were long-beaked common dolphins, said Sarah Wilkin, a marine mammal biologist for the National Marine Fisheries Service, which is responsible for investigating sick, injured and dead marine mammals.
Samples from the carcasses are being analyzed to rule out other factors that could have contributed to the deaths, such as disease or poisoning.
Wilkin said the deaths should not have a significant impact on the species' population. There are an estimated 15,000 long-beaked common dolphins along the California coast. While protected under the federal Marine Mammal Protection Act, the species is not considered threatened or endangered.
Conservationists have wrangled with the Navy in the past about military operations, but experts said they knew of no previous incidents in the region of dolphin fatalities involving explosives.
Most of the controversy over the effects of military training on marine life in recent years has centered on sonar.
Environmentalists have argued that the Navy's sonar exercises can deafen and even kill whales and other marine life. The Supreme Court ruled in favor of the military in 2008.
The Navy has been working with the National Marine Fisheries Service on permits and protocols for exercises at the Silver Strand facility, Wilkin said.
Environmental groups said the dolphin deaths show that the military needs to take further precautions to protect marine life from explosives.
"It underscores that the Navy trains with a lot of technology that is harmful to the marine environment, said Michael Jasny, senior policy analyst for the Natural Resources Defense Council. "It is therefore imperative that it take every available step to prevent harm."
After learning of the deaths this week, Jasny wrote a letter to the Navy asking for a public investigation into the incident and for the suspension of similar explosives exercises until the chain of events is understood.
The Navy said the Explosive Ordnance Disposal unit involved in the incident "conducted the underwater training in accordance with all operational training and safety guidelines as well as observed all protective measures and assessment protocols and monitoring of the area."
"Obviously, this was a very unfortunate incident," Hicks said.
A Navy investigation, he said, is underway to determine what went wrong and whether further measures may be required to protect marine mammals in future training exercises.
The National Marine Fisheries Service is also looking into whether two additional long-beaked common dolphins that washed ashore dead in La Jolla and Oceanside the following week are connected to explosives training exercises.