Common dolphins feed on fish. This can be a problem for those trying to catch the fish on which these dolphins are preying. In fact, the United Nations reported that dolphins along the California coast eat an average of 300,000 tons of anchovies each year, whereas commercial fisherman take in only 110,000 tons. Because of this, many angry fisherman who catch these enemies in their nets often kill them.

(Allen, 1979;   http://whales.ot.com/).