Frolicking on the surface, short-beaked common dolphins (Delphinus delphis) are accompanied by yellowfin tuna (Thunnus albacares) in the eastern tropical Pacific. The feeding frenzy of the large tuna drives schools of smaller fish toward the surface, and all the commotion attracts red-footed boobies (Sula sula) that plunge into the sea to nab fish.

The nutrient-rich waters off the Pacific coasts of Mexico and Central America support a wide array of fish. In this scene, the yellowfins chase after frigate tuna (Auxis thazard) and flying fish (Cypseurus angusticeps).

Flying fish (Cypseurus angusticeps)

CONSERVATION: Safe for Dolphins?
When the tuna industry began using purse-seine nets to fish for yellowfin tuna in the 1950s, they also started catching dolphins. Knowing that tuna tend to school near dolphins in the eastern tropical Pacific, fishing boats would set their nets around dolphin pods and cinch the bottom to bag the tuna. Sadly, many dolphins drowned in the process—over a million by 1970.

The United States first protected dolphins through the Marine Mammal Protection Act in 1972. Twenty years later, the US embargoed any tuna that wasn't labeled "dolphin-safe." While these measures help, regulators must keep an eye on how the fishing industry continues to affect the future of both tuna and dolphin populations.

Fish Facts Named for the yellow finlets along the back end of the body, yellowfin tuna reach weights of 200 kilograms (440 pounds). Like most other rayfin fishes, they have gills instead of lungs, and they lay eggs. Open-ocean predators, these tuna feed on squids, crustaceans and smaller fishes, and they often school beneath floating objects-including dolphins. © James Watt/Mo Yung Productions / www.norbertwu.com

Fish or Finned Mammal?
Sleek and swift dolphins may look like fish, but they are actually air-breathing mammals.

Dolphins may have fins and flippers, but that doesn't mean that they are fish. A closer look reveals characteristics inherited from land-based mammalian ancestors. Like other mammals, dolphins have hair as newborns—if only sparse stubble. They give birth to live young, which nurse from the mother's mammary glands. And dolphins must surface regularly to breathe through nostrils—the blowhole—located on the top of the head.

Surface Sentinels One of three dolphin species to occur with tuna, the common dolphin spends the day in schools numbering in the hundreds. By late afternoon they break into smaller groups for nightly foraging for small fish and squid. Peter Howorth / Mo Yung Productions / www.norbertwu.com

Pelagic Potluck
Like oases in a desert, patches of life are scattered throughout the open ocean. It may be no coincidence that tuna, dolphins and seabirds band together where food is available.

Survival at sea means locating food. Miles from land, life occurs in patches that depend on the ever-changing supply of nutrients in the water. Ocean currents, upwellings and storms all affect what can grow, starting with fleeting blooms of tiny floating plankton that form the basis of the food web.

In the eastern tropical Pacific, dolphins, yellowfin tuna and seabirds gather where food flourishes, but it isn't clear who follows whom. Some scientists think that deep-swimming tuna herd schools of small fish, driving them up to the surface where dolphins and birds wait. Others say that tuna and seabirds follow the dolphins, which arrive first. Fishing crews know that a pod of dolphins and a flock of seabirds at the surface probably mean there are tuna to catch below.