A series of savage attacks on dolphins with knives, rifles and spear guns has angered and mystified wildlife experts in Australia. The attacks have all taken place within 80km of the coastal city of Adelaide, in South Australia, in waters that support a healthy population of whales, dolphins and sharks. At least six dolphins are known to have been speared, shot or stabbed in the past three years. Several of the attacks have occurred along Adelaide's Port River, one of the few places in the world where wild dolphins live close to a major city. The latest incident was last month, when a female bottle-nosed dolphin with suspicious wounds was found in the river, which lies about 15km from the city centre. A week earlier, another dolphin was found floating off the coast after being stabbed through the brain with a spear. Many of the dead and disfigured animals have been found by marine biologist Mike Bossley, 58, who has been studying the resident population of about 300 dolphins for 14 years. 'Adelaide is so lucky to have these dolphins here and yet we really abuse them,' Mr Bossley said. 'Nowhere else in the world can you live in a city so close to such a large dolphin population, and yet we've stabbed them, speared them, shot them and polluted their home. It's just getting ridiculous the number of killings we've had.' Killing a dolphin is punishable with an A$50,000 (HK$198,000) fine or two years in prison and local wildlife officers say they have no idea who is carrying out the attacks. They cannot even be sure if it is one person or several. One theory is that fishermen sometimes fire shotguns over the heads of pods of dolphins to scare them away from shoals of fish, and may on occasion have aimed too low. The dolphins are an easy target because although wild, they have become used to humans and are naturally curious. 'They're extremely inquisitive and they seem to find people fascinating. Often they'll ride on the bow wave of a boat and they will sometimes roll over just for fun,' Mr Bossley said. Such behaviour may explain one particularly sickening attack, when a young dolphin was found fatally speared through its exposed underbelly. For years, Mr Bossley has been lobbying for the establishment of a sanctuary for dolphins in the Port River. In addition to giving the dolphins greater protection from deliberate cruelty, he wants to see the heavily polluted river cleaned up. Past autopsies have revealed that dolphins inhabiting the river have extremely high traces of heavy metals in their bodies. Pollutants such as polychlorinated biphenyls compromise the immune system of the animals, and several have been found to have developed deformities. Meanwhile, conservationists wait for the next attack. Dolphins may be among the smartest creatures on the planet, but their intelligence offers little defence against someone determined to do them harm.