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At least 54 bottlenose dolphins have died mysteriously in Florida's Indian River Lagoon since January. Today, the federal government is stepping in to help find out what's killing them.

In a normal year, that number would be closer to 22.

On July 24, NOAA declared the mass die-off an "Unusual Mortality Event" – a declaration that will send federal resources and scientists to help teams already on the ground in Florida. It's the lagoon's worst dolphin die-off on record, and the cause is mysterious.

"This has become a national investigation, instead of a local investigation," said Megan Stolen, a marine biologist with Hubbs-SeaWorld Research Institute, the nonprofit organization that has been investigating and keeping track of the dolphin deaths so far. "This will definitely help us."

It's the second time this year that NOAA has declared an Unusual Mortality Event for marine mammals in the lagoon, a 156-mile-long estuary that runs along Florida's Atlantic coast. In April, a mass manatee die-off received the same designation. This is the third time a UME has been declared for dolphins in the lagoon. What caused the others, in 2001 and 2008, is still a mystery.

The lagoon is a treasured but troubled ecosystem, and has been besieged by a combination of nutrient run-off, pollution, and algal blooms – ingredients that have created a lethal situation for 112 manatees, roughly 300 pelicans, and 54 dolphins since last July. Scientists don't yet know if the die-offs are linked, or if there are multiple killers on the loose in the estuary. Multiple investigations are ongoing, with teams trying to find out whether algal toxins, or pollution, or something else is to blame. (See our interactive map below, or click to open it in a new tab.).

Stolen became concerned about the dolphin deaths in January. But it wasn't until late spring that the carcasses really began to pile up; at one point, scientists were retrieving a dolphin a day from the northern and central lagoon. The die-off is affecting dolphins of all age classes and sexes. Some of the bodies are intact, others have been scavenged by sharks. Unlike the dead manatees, which appear normal except for being dead, the dolphins are emaciated – thin and bony. But whether they're starving because of disease, or a toxin, or a lack of food is still unknown. Clues are scarce, and only one sick dolphin has been found alive.

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Now, Stolen says, the die-off has slowed a bit. In July, five dolphins have been pulled from the lagoon's brackish water.

"The last few dolphins have been calves," she said. "Newborn babies."

It's not clear yet whether the calves, three of them, are casualties of the mysterious scourge. But, Stolen says, "We would expect that if moms are getting hit by the UME cause, that we would start seeing dead calves as well."

She and her colleagues will continue to monitor and respond to situation as NOAA's team determines which direction to take the investigation in. "We are starting to look in [the dolphins'] stomachs now," she said. "Normally when we do a necropsy, we kind of scoop everything out of their stomachs and put it in a bag. What we’ll do now is we’ll separate the liquid from the solid."

The liquids are good for toxin analyses, and the solids will tell researchers what, exactly, the dolphins have been eating – and if there are any clues to be found in their last meals.

The interactive map below works much better if you open it in a new tab.

Maps by Greg Miller.