For reasons not completely understood by scientists, dolphins and yellowfin tuna are often found swimming together, with dolphins closer to the surface and the tuna beneath them. It is thought that the tuna may be trying to take advantage of the dolphin's superior food-finding talents. Fishermen have long known of the association between the two species, and traditionally have used the presence of dolphins as a lead to finding schools of tuna. In the days when tuna were caught by the pole and line, this presented no problems as dolphins were rarely involved or injured. This changed dramatically in the 1950's when a fishing technique known as purse-seining was introduced, where many dolphins are killed in the process, as they become entangled in the nets and drown, or are crushed as the nets are pursed and hauled in.
Thousands of dolphins and small whales throughout the world continue to be hunted for food, oil, fertilizer and other products.
What may turn out to pose the greatest threat to dolphins (and other marine animals for that matter) is contamination of the marine environment. Pesticides, PCBs, heavy metals, plastic particles, radioisotopes and other industrial wastes are released into the oceans, bays and rivers in unknown amounts daily. Many of these pollutants do not break down in the environment, or take a very long time to do so. For species with a wide geographical range, environmental degradation may not be as critical, but those species that have a limited or localized distribution may become quite vulnerable. The populations of several of the river dolphins have been drastically reduced by a combination of man-made factors including pollution, damming, increases in boat traffic and shoreline development. In addition, many dolphin deaths have been attributed to swallowing man-made objects including balls, nets and pieces of plastic.
Dolphins belong to the family Delphinidae. Dolphins possess a distinct beak. Their teeth are conical in shape. Most species of dolphins are larger than porpoises, with the males usually being larger than the females. The family Delphinidae is the largest and most diverse family of the cetacean order and includes 26 living species. Dolphins of some kind occupy virtually all oceans and major seas as well as some large river systems. Their distribution, however, is not random. Each species has become specialized to fit into a particular niche. In terms of their feeding habits, all dolphins are carnivores. Some feed exclusively on either fish or cephalopods the class of marine invertebrates including squid, octopus and cuttlefish), while others have a more varied diet including fish, squid, crabs, shrimps and lobsters
In Chile, some of the world's most endangered dolphins, the black dolphins, are hunted to provide bait for crab pots for the king crab industry. Catches of several thousand are reported each year in the late 1990's. Turkey has greatly reduced dolphin populations in the Black Sea to produce oil and chicken feed. In the Faroe Islands and parts of Asia, a type of fishing known as "drive fishing" has been practiced for centuries by fishermen who view the fish-eating marine mammals as competition for their catch. The fishermen use boats and large nets to "drive" schools of dolphins into shallow bays and harbors where they are killed, often brutally. The meat is often sold for human or animal consumption, or made into fertilizer. Japan is believed to be the largest consumer of dolphin.