MIAMI — Like a macabre marine mystery, the carcasses — many badly deteriorated and tossing about in the surf — first turned up along the coast of New Jersey in June. Soon, droves of them washed up in Virginia, the Carolinas, Georgia and most recently Florida, their winter home.

So far this year, nearly 1,000 bottlenose dolphins — eight times the historical average — have washed up dead along the Eastern Seaboard from New York to Florida, a vast majority of them victims of morbillivirus. Many more are expected to die from the disease in the coming months.

The high death toll from the resurgence of the virus, which killed 700 dolphins in an outbreak 25 years ago, has alarmed marine scientists, who say it remains unclear why the dolphins have succumbed to the disease. The deaths, along with a spate of other unrelated dolphin die-offs along Florida’s east and west coasts, raise new questions about the health of the ocean in this part of the country and what role environmental factors may be playing, scientists said.

“Marine mammals are very good sentinels for ocean and human health, and they really act like the proverbial canaries in a coal mine,” said Dr. Greg Bossart, a veterinary pathologist and senior vice president in charge of animal health at the Georgia Aquarium. “They give us an idea of what’s occurring in the environment.”