An Apollo Beach couple rushed to help what looked like a distressed manatee Saturday, only to find the mammal already deceased on the beach.
"As soon as we rolled it over you could see the gashes in its head," said Eddie Hefner. "It's two inches deep, three inches deep."
Pictures show the female manatee on Apollo Beach suffering from deep, parallel wounds.
"It was a boat propeller, as deep as they were and as large as gashes were, this isn't something that could have happened against a rock or from another animal," said Jennifer Hefner.
Florida Fish and Wildlife investigators said Tuesday the cause of death was likely by boat.
ABC Action News found the number of manatee deaths caused by watercraft are on the rise in both Hillsborough County and across Florida. FWC reports there were 7 manatee deaths in Hillsborough County in 2016, that's two more than the year prior. There were 104 reported deaths statewide in 2016, up from 86 in 2015.
FWC officials can not confirm the boat responsible was involved with Gasparilla Saturday.
But the Hefner's say they witnessed several boaters ignoring restricted speed zones Saturday night.
"We saw a lot of speeding," said Jennifer Hefner. "They threat the lives of every manatee that's out here."
And hope to see more water patrols keeping a watchful eye during major bay events.
"The presence of authority, law enforcement, it kind of helps people do the right thing," said Eddie Hefner.
FWC officials suggest always designating a manatee spotter while boating, especially in the area near Apollo Beach, where manatees are known to gather this time of the year.
Intentionally harming a manatee is a felony offense.