Dolphins are close to the top of their food chain with few natural predators other than sharks. When faced by a predator, dolphins often circle, head butt or use their tails to hit the other animal in self defense. According to the Sarasota Dolphin Research Program, sharks often attack dolphins from behind or below as shown by bite scars.
Sometimes, dolphins and sharks swim in nearby water without interacting. They often feed on the same school of fish. The Sarasota Dolphin Research Program indicates that there are questions as to whether sharks attack dolphins for food or to protect their own territory.
Humans are one of the greatest threats to dolphins. Pollution, including pesticides, plastics, heavy metals and materials that disintegrate slowly in the ocean, concentrate in dolphins. These animals are commonly injured or killed in collisions with boats, with a propeller collision causing a great deal of damage. Humans unintentionally kill dolphins when fishing, especially when using purse seine fishing for tuna. In some locations, humans hunt and kill dolphins with harpoons for food, even though the meat is high in mercury and poses a health threat to humans who consume it.
Because of consumer outcry against dolphins killed when netting tuna, several companies label the canned tuna they produce as dolphin-free. While efforts have reduced the number of dolphins killed when fishing for tuna by 50 percent, dolphins still die in this manner, according to Wikipedia.