Irwin died last week after a stingray barb pierced his chest as he recorded a show off the Great Barrier Reef.
Stingray bodies since have been discovered on two beaches in Queensland state on Australia's eastern coast. Two were discovered Tuesday with their tails lopped off, state fisheries department official Wayne Sumpton said.
Sumpton said fishermen who inadvertently catch the diamond-shaped rays sometimes cut off their tails to avoid being stung, but the practice was uncommon. Stingrays often are caught in fishing nets by mistake and should be returned to the sea, Sumpton said.
Michael Hornby, the executive director of Irwin's Wildlife Warriors conservation group, said he was concerned the rays were being hunted and killed in retaliation for Irwin's death.
"It may be some sort of retribution, or it may be fear from certain individuals, or it just may be yet another callous act toward wildlife," he said.
He said killing stingrays was "not what Steve was about."
"We are disgusted and disappointed that people would take this sort of action to hurt wildlife," he said.
Stingrays are usually shy, unobtrusive fish that rummage the sea bottom for food or burrow into the sand.
They have a serrated spine up to 10 inches long on their tails, which they can lash when stepped on or otherwise frightened.
The spines emit toxins that can kill many small creatures and cause excruciating pain in humans. Few people have died from the poison, but the spines can badly tear flesh and the wounds are prone to infections, including tetanus.
Hornby said people should treat stingrays with caution, but "there is still no need to ... kill or mutilate these important animals."
By Meraiah Foley