ZANZIBAR, Tanzania —  At least 400 dead dolphins were found on Zanzibar's northern coastline early Friday, alarming villagers, fishermen and tour operators, residents said.

It was not immediately clear what killed the dolphins, whose carcasses were found along a 2½-mile stretch of Nungwi, a popular tourist destination in this semiautonomous Indian Ocean archipelago, said Narriman Jidawi, a marine biologist at the Institute of Marine Science in Zanzibar.

Villagers, fishermen and hotel residents found the carcasses and alerted officials, residents said.

Mussa Aboud Jumbe, Zanzibar's director of fisheries, immediately went on the state radio to warn the public against consuming meat from the dead dolphins, adding that authorities were trying to determine the cause of their death.

The mass deaths is 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 that is perched atop a small cliff overlooking the beach.

Dolphin tourism has replaced hunting in Zanzibar, where they once were used as bait for shark fishing.

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.