(AP) Hundreds of dead dolphins washed up Friday along the shore of a popular tourist destination on Zanzibar's northern coast, and scientists ruled out poisoning.

It was not immediately clear what killed the 400 dolphins, whose carcasses were strewn along a 2 1/2-mile stretch of Nungwi, said Narriman Jidawi, a marine biologist at the Institute of Marine Science in Zanzibar.

But the bottleneck dolphins, which live in deep offshore waters, had empty stomachs, meaning that they could have been disoriented and were swimming for some time to reorient themselves. They did not starve to death and were not poisoned, Jidawi said.

In the United States, experts were investigating the possibility that sonar from U.S. submarines could have been responsible for a similar incident in Marathon, Fla., where 68 deep-water dolphins stranded themselves in March 2005.

A U.S. Navy task force patrols the East Africa coast as part of counterterrorism operations. A Navy official was not immediately available for comment, but the service rarely comments on the location of submarines at sea.

The deaths are a blow to the tourism industry in Zanzibar, where thousands of visitors go to watch and swim with wild dolphins, said Abdulsamad Melhi, owner of Sunset Bungalows, perched atop a small cliff overlooking the beach.

Villagers, fishermen and hotel residents found the carcasses and alerted officials. Mussa Aboud Jumbe, Zanzibar's director of fisheries, went on state radio to warn the public against eating the dolphin meat, saying the cause of death had not been determined.

But residents who did eat the meat were all doing fine, Jidawi said.

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.