(CNN) -- A bottlenose dolphin was found dead with a deep head wound on a southwest Florida beach in May. Marine officials are offering a $38,000 reward for help track down its attackers.
Officials found the impaled dolphin stranded along Captiva Island, near Fort Myers. The fatal injury was probably from a spear-like weapon, the National Oceanic and Atmospheric Administration said Thursday.
The wound was 6 inches deep and ended at the dolphin's skull. It "completely punctured" the tissue in its path, NOAA bottlenose dolphin conservation coordinator Stacey Horstman said.
A necropsy revealed that the dolphin was alive when it was impaled, evident by hemorrhaging at the site of the wound, she said.
The dolphin was found in a "begging position," which isn't natural for wild dolphins, Horstman said. It's a learned behavior enforced when people feed or interact with the animals.
"Folks don't realize that feeding a dolphin in the wild sets off this domino effect," she said.
The dolphin was probably fed by humans throughout its life, she said. The US Marine Mammal Protection Act bans people from feeding, hunting, harassing or capturing dolphins and other mammals.
Feeding dolphins and changing their behavior could inadvertently cause their death, Horstman said. The animals learn to approach boats in open water, increasing their susceptibility to human-related injuries like getting tangled in a fishing net or cut by a boat propeller.
"That's when we start to see injuries like boat strikes or intentional harm, because the animal is getting so close," she said. "And sometimes, those close interactions aren't always welcome."
The dolphin killed in May is the 26th found stranded with evidence of intentional harm by humans in the Gulf of Mexico since 2002.
NOAA is offering $38,000 to anyone who can lead officials to the attacker, but Deputy Special Agent in Charge Manny Antonaras said it hasn't received any leads. Tipsters are encouraged to call 1-800-853-1964.
Dolphins face many dangers
Across the Gulf of Mexico, bottlenose dolphins are battling separate -- and unknown -- dangers.
Since February, at least 290 dolphins have been found dead or injured in states bordering the northern Gulf, a rate three times higher than the historical average. NOAA declared it an unusual mortality event.
Some were covered in lesions consistent with exposure to fresh water, which might have spilled into the Gulf during the winter, officials said.
The previous year, 177 dolphins were found dead along Florida's southwest coast in another unknown mortality event, though NOAA reported that some tested positive for a toxin that causes red tide, a harmful algae bloom.
The common bottlenose dolphin is considered a vulnerable species by the International Union for Conservation of Nature, with about 10,000 adults in the wild.