A team from the Zanzibar-based University of Dar es Salaam's Institute of Marine Sciences said a preliminary investigation into the mass stranding of nearly 600 bottlenose dolphins had failed to yield an explanation.

"We have net yet been able to determine what has exactly caused the strandings," it said in a report. "We are still doing more investigation and ... in a few weeks' time we may come out with some answers."

As of Tuesday, researchers said about 540 of the dead mammals, which washed ashore on four beaches on semi-autonomous Zanzibar's main island of Unguja on Thursday and Friday, had been buried, but that other carcasses could be found.

Analyses of samples taken from the dolphins revealed that their stomachs were all unusally empty, suggesting they vomited in a confused or frenzied state although residue showed they were not poisoned.

Some samples are to be sent abroad but dissections done here found nothing to explain why they may have become agitated but researchers said they may have suffered from "confused echo-location," often caused by human interference.

As the preliminary report showed the carcasses had "no signs of net marks or bruises or cuts" and no evidence of pollution, scientists said that maritime sonar devices, seismic activity or poor weather might be to blame.

"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.

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