Marine mammals live in and around the ocean. They are warm-blooded, breathe air, have hair at some point in their lives, give birth to live young (as opposed to laying eggs), and nourish their young by secreting milk. Dolphins, whales, seals, sea lions, walruses, polar bears, manatees, and dugongs (manatee relatives) fall into this category.

Historically marine mammals have garnered a high level of public support and legal protection. During the 1960s the television show Flipper entertained American audiences with stories about a highly intelligent and love-able dolphin that befriended and helped a family. Such tourist attractions as Marineland in Florida and SeaWorld in California began featuring acrobatic dolphins and whales in very popular shows. The growing environmental movement seized on the public interest in marine mammals and lobbied for measures to protect animals that many people believed to be extremely smart and sociable.

At the time purse-seine fishing was widely practiced by commercial tuna fishers in the eastern tropical Pacific Ocean. This fishing method involved the use of enormous nets, often hundreds of miles long that were circled around schools of tuna. Many dolphins were inadvertently captured because they tend to mingle with fleets of tuna in this part of the ocean. Nontargeted animals captured during commercial fishing activities are called "bycatch." Dolphin bycatch became a major public issue. Hauling in the enormous tuna-filled nets was a lengthy process. As a result, the air-breathing dolphins were trapped for long periods under water, and often drowned. It is estimated that more than 400,000 dolphins and porpoises died per year in this manner during the late 1960s. Public outcry over these killings and general concern for the welfare of marine mammals led the U.S. Congress to pass the Marine Mammal Protection Act of 1972.