Authorities say cases of gunshot wounds, mutilations and other injuries have been detected in recent months.
Scientists from the Institute for Marine Mammal Studies in Gulfport have responded to four dolphin strandings. And on Friday, a team went to Deer Island and found a second dolphin dead with a portion of his jaw missing.
A dolphin found dead earlier near Gautier had a 9mm bullet wound.
"It went through the abdomen, into the kidneys and killed it," said Moby Solangi, IMMS executive director.
He said recovering the dolphins and performing a necropsy to determine the cause of death is heartbreaking for his staff.
"We think there's someone or some group on a rampage," he said. "They not only kill them but also mutilate them."
The public can help by reporting any information they may know about the mutilations, he said. He also asked recreational and commercial fishermen to be on the watch for anyone killing the dolphins.
Attacks on the dolphins carry fines and jail sentences.
Solangi said he doesn't know why anyone would want to kill the dolphins. "They already are under a lot of stress from the oil spill, the dead zone," he said. However, in the past fishermen and charter boat captains have been convicted of harming dolphins they thought were taking bait or fish.