Sharks are afraid of dolphins when outnumbered because they like to eat animals that are much smaller than them, including baby dolphins, but adult dolphins will protect their young by trying to kill a shark any time they see one. Dolphins are usually successful at killing the sharks that they find because they attack in groups, ramming their bodies into the shark until it dies.
There is mounting evidence that sharks are not so much afraid of dolphins as they are intelligent enough to stay away from the dolphins when they are in large groups. Sharks will, from time to time, try to attack adult dolphins from above, behind or below to avoid being noticed by the dolphin's echolocation process.
Sharks are most likely to attack, and will sometimes even kill, dusky dolphins, bottlenose dolphins, humpback dolphins and risso's dolphins when the dolphins are in small numbers. It is difficult to make a definitive statement because the predator-prey relationship depends on many differences, including the number of sharks, the number of dolphins, the species of shark and the species of dolphin.
In Western Australia, the Shark Bay Ecosystem Research Project is working to further examine the natural relationship between both sharks and dolphins.