The natural injustice of the death of Steve Irwin, the daredevil conservationist who wrestled with crocodiles and dangled cobras but was killed by a relatively docile fish, has apparently motivated some of his most ardent supporters to wreak revenge.

Eight days after the documentary maker was killed in a freak encounter with a stingray while snorkelling off the north Queensland coast, and while his native Australians continue to mourn him, the authorities are investigating the possibility that the species that took Irwin's life is being targeted in acts of retribution.

Up to 10 apparently mutilated stingrays have washed up in coastal waters since his death, prompting Queensland state officials to call on fans of the self-styled "wildlife warrior Australian bloke" not to retaliate against the species that killed their hero.

Two stingrays - typically placid creatures - were found dead yesterday with their tails sliced off at Deception Bay, north of Brisbane, adding to the toll of eight already discovered since Irwin's death in waters further north.

The actual stingray which delivered the fatal blow to Irwin as he swam in shallow waters with a cameraman to film "stuff on the reef and little animals" for a programme featuring his eight-year-old daughter could, of course, still be alive.

"We do find dead stingrays with their tails cut off from time to time. People usually do it if they are worried about getting stung by a stingray, or they just do it maliciously, but it is pretty rare," said Wayne Sumpton, a senior biologist in Queensland's fisheries department.

"We do not know if these incidents are motivated by Steve Irwin's death. At the moment that is just speculation."

Conservationists who worked with the documentary maker have been swift to condemn any form of retaliation against the animal kingdom for his death, pointing out that cutting the tails off normally placid fish conflicts with Irwin's conservationist beliefs.

Killing stingrays, said Michael Hornby, executive director of Irwin's conservation group Wildlife Warriors, is "not what Steve was about".

"We are disgusted and disappointed that people would take this sort of action to hurt wildlife," he said. "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."

Irwin died after the serrated barb of a stringray penetrated his heart. Although the venom embedded in the tail spines of stingrays can kill small creatures and cause acute pain to humans, it is extremely rare for the shy fish to kill humans.

It is thought the stingray that killed Irwin struck its tail with a reflex powerful enough to puncture a hole in the 44-year-old's chest in what is said to have been only the third recorded fatal stingray attack in Australia.

Irwin's family and friends held a private funeral around a campfire at his Australia Zoo at the weekend. A public memorial service will be held today.