The tuna-dolphin association is strong, persistent and geographically restricted: it is a feature of the eastern tropical Pacific and is not a general property of yellowfin worldwide. Large yellowfin swim beneath herds of pantropical spotted dolphins and spinner dolphins, sometimes for days.
Because dolphins surface to breathe and tuna do not, the dolphins made the tuna visible from a boat. From the late 1950s purse-seine fleets chased the herds and set nets around them to capture the tuna underneath. Dolphins drowned in the nets in enormous numbers, and estimated cumulative mortality over the following decades runs into the millions.
United States law, international agreement and dolphin-safe labelling later forced backdown procedures, observers and mortality limits, and reported dolphin deaths in the fishery fell by orders of magnitude. What still is not settled is why the association exists at all. Shared prey, shared oceanographic fronts, and predator detection have all been proposed, and no single explanation is agreed.