KEY WEST – A nuclear-powered submarine used two different types of active sonar to navigate over several days as it trained off the Florida Keys last week, including the day of a massive dolphin stranding in Marathon, the U.S. Navy said late Monday.
At the time, the submarine was approximately 39 nautical miles southwest of Marathon, where about 80 rough-toothed dolphins — nearly 30 of which have since died — beached suddenly late Wednesday.
The submarine, the Connecticut-based USS Philadelphia, was in the Keys for about 10 days, the Navy said.
A Navy spokeswoman said it was premature to speculate on the cause of the strandings and whether the incident had anything to do with sonar use. Necropsies and tests are underway on the dead animals by fisheries biologists from the National Oceanic and Atmospheric Administration. That process could take months to complete.
“The cause is not known and I cannot speculate, but every effort will be taken between federal agencies to determine what might have caused the stranding,” said Lt. Cmdr. Jensin Sommer, spokeswoman for Naval Submarine Forces, based in Virginia.
Some scientific reports say there is evidence that marine mammals may have a particular sensitivity to active sonar. The technology allows subs and ships to spot targets and other vessels by emitting sound waves that bounce off objects, revealing distance and location.
Marine mammals rely on sound for just about everything, from feeding to finding a mate to communicating, which different animals do at different frequencies.
Dolphin and whale strandings, however, are not unusual in Florida and can stem from a variety of circumstances, from a sick dolphin leading a pod onto shore to harmful algae blooms.
SUB’S SONAR USE
After it surfaced last Monday, the Philadelphia used mid-frequency active sonar on its bow in reduced visibility to “provide for the submarine’s ability to avoid potential contact with other vessels at sea” for a period of 21 minutes, Sommer said.
On three other days, Feb. 27, March 1 and March 2 — the day the normally deep-water dolphins mysteriously beached on offshore flats — the sub used high-frequency active sonar mounted on its sail while it was submerged to help it “avoid other ships” before it came to the surface, Sommer said. She did not know how long the high-frequency sonar was used, but said it was “short duration” and of low intensity.
High-frequency sonar is considered to have a shorter range than medium frequency or low frequency. Factors like water temperature and salinity can also affect how far the sound travels.
After a whale stranding in 2000 in the Bahamas, the Navy acknowledged in a report the existence of some marine mammal sensitivity to sonars, but has also argued at times that the extent of any cause-and-effect is scientifically vague.
The Natural Resources Defense Council, an environmental group, successfully sued the military in 2002 to limit use of new, low-frequency sonar believed by some to be particularly damaging because of its ability to travel extremely long distances. Some types of sonar can be extremely loud — as much as 235 decibels at close quarters, equivalent to the noise made by a Saturn V rocket on takeoff — according to the council.
“It’s too early to draw a conclusion, but the Navy’s use of active sonar near the stranding site heightens our concern that sonar played a role in harming these animals. We already know that exposure to high-intensity sonar can kill marine mammals. A full transparent investigation is needed to get to the bottom of it,” said Joel Reynolds, director of the council’s Marine Mammal Protection Project.
STRANDED WHALES
Within the past two months, at least 35 whales of three species stranded off the Outer Banks of North Carolina. The stranding coincided with Navy sonar exercises in the area, though a final cause of death for the animals hasn’t been determined.
In recent years, military sonar use has also been alleged as a factor in the strandings of porpoises off the coast of Washington state, and of melon-headed whales off the coast of Hawaii, while other potentially sonar-related strandings have occurred in Greece and the Canary Islands.
The International Whaling Commission last year issued a report supporting a link between active sonar use and whale deaths.
Some researchers believe that sonars may disorient or scare marine mammals, causing them to surface too quickly and creating the equivalent of what divers know as the bends — when nitrogen is formed in tissue by sudden decompression, leading to hemorrhaging.
A POSSIBLE CLUE
In South Florida, biologists are hoping that necropsies of the dolphins who died after the stranding may shed some light on what prompted the incident. One key clue may be the condition of the animals’ acoustic tissue, a potential indicator of sonar damage.
“As with every marine mammal stranding, we are conducting a thorough investigation,” said Laura Engleby, a biologist for NOAA Fisheries, which coordinated a massive response to the stranding last week and is now spearheading efforts to determine its cause.
“It’s way too early for us to know what caused this, and our scientists are collecting as much data as possible.”
Eleven dead animals were examined and their tissue sampled over the weekend, with more scheduled for necropsy later this week.
At least 20 of the dolphins managed to make it out to sea within a day of the incident.
But at least 28 in poor condition or pain were subsequently euthanized or perished on their own.
Researchers and volunteers are still trying to nurse back to health 26 rough-toothed dolphins that survived after being moved to several South Florida dolphin rehab centers.
By Jennifer Babson, The Miami Herald
