The most recent case was of a dolphin found dead off the coast of Mississippi, its lower jaw missing.
Officials from the National Oceanic and Atmospheric Administration said Monday they're asking everyone from beachgoers to fishermen to wildlife agents to be on the lookout for injured or dead dolphins - and any unusual interaction between the mammals and people.
"It's very sad to think that anyone could do that to any animal," said Erin Fougeres, a marine mammal scientist for NOAA's southeast office in St. Petersburg, Fla. "There have been some obviously intentional cases."
Fougeres said five dolphins have been found shot. In Louisiana, two were shot in 2011 and one in 2012. And in Mississippi, three were found shot this year, the most recent one last week, which was first reported by the Sun-Herald newspaper.
Besides the shootings, a dolphin in Alabama was found with a screwdriver stuck in its head over the summer. Another in Alabama had its tail cut off, and that animal survived. Still others were missing fins or had cuts to their bodies.
"I think it is outrageous," said Moby Solangi, the executive director of Institute for Marine Mammal Studies in Gulfport, Miss. "These animals are very docile, very friendly and they're very curious. They come close to the boats, so if you're out there, you'll see them riding the bows. And their curiosity and friendship brings them so close that they become targets and that's the unfortunate thing."
Dolphins are among the species protected by the 1972 Marine Mammal Protection Act. Violators can be fined up to $10,000 per violation and sent to prison for a year.
The California-based Animal Legal Defense Fund said it is offering a $5,000 reward for information leading to the arrest and conviction of whomever harmed the dolphins.
The gruesome discoveries are heartbreaking for Gulf Coast scientists, who follow the population. Fougeres said that two months before the 2010 oil spill disaster off the coast of Louisiana, dolphins began stranding themselves and that there were unusually high mortality rates - possibly due to a cold winter that year.
Since then, the spill and another cold winter in 2011 have contributed to several deaths within the Gulf's dolphin population, experts say. Investigators have also found discolored teeth and lung infections within some of the dead dolphins.
Since Feb. 2010, experts have tallied more than 700 recorded dolphin deaths.
Experts have also found increased "human interaction" cases, which include dolphins tangled in fishing lines - and the more violent incidents.
Fougeres cautions that some of the dolphin mutilations might have happened after the animal died from natural causes and washed ashore. She said that in the case of the dolphin with the lower jaw missing, someone could have cut off the jaw for a souvenir after the animal died.
"We have to do a necropsy on the animal and collect tissue samples to try to determine whether or not the injury was pre-or post-mortem" she said.
She also said that the increase in cases might be due to NOAA's dolphin stranding network becoming better trained to notice cruelty cases or unusual deaths.
Some have suggested that the deaths are the work of a few angry fishermen who are upset about bait-stealing dolphins. Yet the majority of fishermen say that while dolphins can be annoying, they wouldn't harm the creatures.
"I don't know who to suspect ... I was really sickened when I read about it," said Tom Becker, of T&D Charters out of Biloxi, Miss., and head of the Mississippi Charter Boat Captains Association, said he's never had a problem with dolphins.
The mammals tend to swim behind his boat until a fish too small to keep is tossed over the side.
"You'll see him under your boat," Becker said, about the dolphin. "He'll get it before it can reach the bottom. I usually leave the area if they're doing that."
Fougeres said she doesn't think the dolphins are being targeted by a gang of people or even by a lone, sick individual.
"The cases are fairly spread apart," she said. "I don't think there is one dolphin murderer out there."
She added that anyone who sees a dead or stranded dolphin, or spots people harassing a marine animal can call the NOAA Enforcement hotline at 800-853-1964.