Inhumane act against a bottlenose dolphin

Wildlife authorities appalled by the stabbing of a bottlenose dolphin believe the mammal's normally curious and trusting nature may have led to its killing.

One of a population of about 80 in Victoria's Port Phillip Bay, the female dolphin was found dead with seven stab wounds on Sunday at Eastern Beach in Geelong, west of Melbourne.

The Dolphin Research Institute said a post-mortem examination revealed the dolphin was a healthy female which was about ready to start breeding and had been stabbed with possibly two different knives.

"There was nothing else to indicate that she was sick ... she died from a stab wound to the heart," research director, Anika Warren-Smith, told reporters in Melbourne.

Ms Warren-Smith described the act as "disgusting" and the loss was one the small dolphin population of Port Phillip Bay could not afford.

"I suppose it's similar to people torturing cats and dogs and other wildlife - you just wonder why you would actually consider doing something like this to an animal that is generally very trusting of humans," she said.

"They are very curious and trusting and to betray that is really quite distressing."

Ms Warren-Smith said there was a possibility the dolphin might have swum up alongside a boat, enabling someone to stab her - but she conceded this would be difficult.

"It's a bit of a mystery and that's why we're hoping the public can help us figure out what happened," she said.

The Department of Sustainability and Environment (DSE) said dolphins were legally protected and any person found guilty of killing one could be fined more than $100,000 or face 12 months' imprisonment.

DSE spokesman Ron Waters said he was investigating claims by fishermen that they had seen a dolphin floating dead and had stabbed it to make it sink.

"We've got a vet saying: here's what we found - an animal with a full stomach, in good condition and no other signs of death, and one puncture wound through the heart.

"And we've got this other report ... saying: it was a dead one, mate, and we just plugged it. It was bloated and we sunk a knife into it.

"That explanation is at odds with what the vets have told us."

He said he initially feared the dolphin might have been caught in a net while being stabbed but there was no evidence to suggest this.

"It is very difficult to understand how someone could get that close to an animal that's wild and healthy, to be able to do that amount of damage," Mr Waters said.