By Leigh Coleman
BILOXI, Mississippi (Reuters) - A federal agency's practice of returning weakened dolphins to deeper Gulf of Mexico waters is thwarting efforts to probe dolphin deaths after last year's BP oil spill, scientists said on Saturday.
The National Oceanic and Atmospheric Administration, meanwhile, confirmed that two dolphins stranded in low tide on the Louisiana coastline were returned to water deep enough for them to swim away.
"These animals had no signs of external oil and were deemed healthy and robust," NOAA spokeswoman Kim Amendola said, adding that beach releases are a viable option in some circumstances.
"The animals were pushed to deeper water by our stranding network partner, the Louisiana Department of Wildlife and Fisheries, and swam off on their own," Amendola said.
Researchers said weakened and stranded dolphins should instead be rescued and tested.
Moby Solangi, director for the Institute of Marine Mammal Studies in Mississippi, said returning the dolphins to deeper waters also was undermining efforts to determine who is responsible for the rash of sea animal deaths in Gulf waters.
"We are not able to conduct necropsies on these animals any more either," Solangi said. "This is all because of the BP criminal investigation."
The U.S. Justice Department is using dolphin testing in its investigations into the 2010 explosion aboard the Deepwater Horizon drilling rig, which killed 11 workers and unleashed a torrent of oil that fouled the shorelines of four states.
"I know that everyone thinks they are doing their best but we must have answers and help every marine animal we can," Solangi said.
Fifteen of the 406 dolphins that have washed ashore in the last 14 months had oil on their bodies, the oceanographic administration has said.
Deepwater Horizon oil was confirmed to be on six bottlenose dolphins, one had an unknown oil and two have not been tested.
Meanwhile, wildlife biologists contracted by the National Marine Fisheries Service to document spikes in dolphin mortality and to collect specimens and tissue samples for the NOAA were quietly ordered in late February to keep their findings confidential.
"Scientists try to get to the truth of the matter when the government is worried about political ramifications," said Dr. Mark Peterson of the College of Marine Sciences at the University of Southern Mississippi.
"It would be a good idea to test a stranded dolphin, but I guess someone has to worry about the cost of taking the animal to a rehabilitation facility," Peterson said.
NOAA declared "an unusual mortality event," ongoing since last February, after a spike in the number of dead dolphins washing up in Louisiana, Mississippi, Alabama and Florida.
There have been 153 deaths this year, 65 of them newly born or stillborn calves, NOAA said last week.
Since mid-March, about 120 dead sea turtles also have been found, although the carcasses had no visible traces of oil, said Barbara Schroeder, NOAA Fisheries national sea turtle coordinator.
(Editing by Barbara Goldberg and David Bailey)