Scientists probing the deaths of hundreds of dolphins that beached themselves on Tanzania's Zanzibar archipelago last week said Tuesday they had no idea yet what caused the phenomenon.

Scientists probing the deaths of hundreds of dolphins that beached themselves on Tanzania's Zanzibar archipelago last week said Tuesday they had no idea yet what caused the phenomenon. Seismic or sonar (activity) can cause vomiting," it said, noting that dead dolphins "had basically empty stomachs".

At the same time, the report said heavy cold rains and high seas, like those reported by fishermen in the area at the time, that lead to panic among tightly knit dolphin pods are suspected to have caused similar events.

"Affected animals will relentlessly follow one another ashore, as if crippled by widespread panic, even when there is clear access to open water," it said.

Bottlenose and humpback dolphins are popular attractions for tourists visiting Zanzibar's famed spice islands where snorkelers can swim with the creatures as they frolic in the ocean. However, mass strandings such as last week's are unusual with the last recorded event -- when about 60 pygmy whales beached themselves -- occurring in Zanzibar in the 1940s, the scientists said.