Manatee Death Toll Is At An All Time High, And Nobody Knows Why (VIDEO)

Sea cows, mermaids of the sea, gentle giants, whatever you want to call them -- manatees are in trouble.

The economy of Crystal River, Florida is dependent on a healthy manatee population. It is the only place, outside of Belize, where people can legally swim with the manatees, and tourists come from far-and-wide to get close to these aquatic mammals that can weigh over 1,000 pounds.

Last year, however, something unusual happened to the manatee population -- there were 829 known deaths, the highest annual death toll ever recorded.

Though there are many contributing factors to why manatees are endangered (boat strikes, cold temperatures, disease, infant mortality), scientists are stumped by what killed the record number in 2013.

It is known that 276 manatees were killed in southwestern Florida by a red tide -- toxic algae eaten and breathed in by the animals in fatal quantities.

But according to National Geographic, nobody knows what killed the manatees in the Indian River area of east-central Florida. "It started out pretty baffling, and to this point they still have no clue," Bob Bonde, a research biologist with the U.S. Geological Survey, told NatGeo.

The debate goes on, but the gentle giants are still threatened. To learn more about manatees and what you can do to help, visit Save the Manatee Club.