Massive counts of beached dolphins is still a mystery!

Preliminary investigations have failed to yield an explanation of why hundreds of dolphins left their deep offshore habitat, got stranded in shallow waters and later washed up dead on Zanzibar's northern coast, a scientist said Tuesday.

"It is a mystery," Narriman Jiddawi, a marine biologist at the Institute of Marine Science of the University of Dar es Salaam, said after studying tissue samples and the remains of some of the 400 common bottleneck dolphins.

Dolphin carcasses washed up Friday along a 2.5-mile stretch between Kendwa and Nungwi beaches. The dolphins had no bruises to indicate they had been entangled in fishing nets, Jiddawi said.

A U.S. Navy task force patrols the coast of East Africa in counterterrorism operations.

A Navy spokesman ruled out the possibility Navy sonar might have disoriented the dolphins and led to their deaths. He said there were no U.S. Navy vessels within 580 miles of the location in the 48 hours before it happened.

"In the U.S. alone, a person is 10 times more likely to be struck by lightning than for sonar to cause a marine mammal stranding," Lt. William Marks said.

Scientists said they were mystified by the mass deaths.

"A day earlier, fishermen reported seeing them at sea at high tide, but the next morning they appeared dead," Jiddawi said. "We don't know why they left offshore waters in such a large number and got stranded."

Preliminary examination of their stomachs indicated the dolphins had either not eaten for a long time or had vomited severely. Their general condition, however, showed that they had not starved, she said.

Experts planned to further examine the dolphins' stomachs for traces of poison, including from the toxic "red tides" of algae.

Zanzibar's resorts attract many visitors who come to watch and swim with wild dolphins.

The Indo-Pacific bottlenose, humpback and spinner porpoises, commonly known as dolphins, are the most common species in Zanzibar's coastal waters, with bottlenose and humpback dolphins often found in mixed-species groups.