Since mid-May, 46 bottlenose dolphins have stranded themselves and died on beaches throughout the Indian River Lagoon, alarming scientists who say that's six more dolphins than they usually find in an entire year.

Three of the dead dolphins have been found in Mosquito Lagoon, on the southern end of Volusia County.

The deaths have been declared an "unusual mortality event," and an investigation is under way, said Wendy Noke-Durden, a research biologist with Hubbs-SeaWorld Research Institute.

Marine biologists from across the country are comparing notes and sending samples for testing, with Hubbs serving as the coordinating agency.

So far, scientists don't know why the dolphins are dying, Noke-Durden said.

The die off does give scientists a sense of deja vu. It's the latest in a string of mysterious animal deaths and diseases in the lagoon over the past decade.

In 2001, 34 dolphins stranded. Two years earlier, 100,000 horseshoe crabs died. Then in 2002, puffer fish in the lagoon suddenly became toxic to the people who ate them.

Scientists also have worked to pinpoint the causes of lesions and tumors on sea turtles and dolphins in the lagoon. Found on more than a third of the dolphins, it's too soon to tell if the tumors or lesions have any role in the latest deaths.

Last winter, a spate of dolphin and manatee deaths happened during a toxic algae outbreak, but scientists don't think the latest deaths are related because they haven't found high levels of toxins in the dead dolphins. Researchers have found respiratory problems in some and brain lesions in others.

Most of the dolphins have been emaciated, with no food in their stomachs, said Blair Mase, the southeast regional marine mammal stranding coordinator for the National Marine Fisheries Service. The deaths appear to be happening among the most vulnerable dolphins -- newborns and older males.

The deaths could be the result of harmful algal blooms or even regular diseases or toxins, Mase said. They had suspected a form of toxin released by a kind of algae called pyrodidium. A surge of that was reported in August. It produces a luminescence and causes the paddles of kayakers to glow in the dark.

They've tested for biotoxins and viruses and are "covering all the bases," Mase said. "We've got just enough information to go, 'Hmm, I wonder what that means?'

"Maybe something's going on that's not due to one cause," Mase said. "We may have numerous things going on that are causing this."

Now scientists have a new concern. They'll be watching the lagoon carefully in the coming months to measure how all the releases of floodwater from the St. Johns River affect the lagoon ecosystems. The lagoon spans 156 miles along Florida's east coast and includes three water bodies. It is connected by a canal to the St. Johns.

One challenge the biologists face is the condition of the dolphins when they're found. Many of the dolphins have been too decomposed to get good quality samples.

"The better condition the animals are in when they're found, the better results we can get," Noke-Durden said. Anyone who sees a stranded dolphin in the lagoon is asked to call the state's wildlife alert hot line at 888-404-3922.