What Eats a Tuna? Predators and Human Impact

Tuna sit in the middle of a surprisingly busy stretch of the ocean food web. Despite their speed, size, and endurance, they are hunted by orcas, large sharks, billfish, jumbo squid, and even other tuna. But the predator that dwarfs all others in its impact on tuna populations is us. Humans have been catching tuna for at least three thousand years, and modern industrial fishing has pushed several stocks from abundance to severe depletion within decades.

Orcas and Marine Mammals

Killer whales are among the few ocean predators capable of taking down a full-grown adult tuna. A 2024 study documented an orca attacking and consuming an adult Atlantic bluefin tuna that weighed an estimated 81 kilograms and measured 175 centimeters in length. The biologging tag on the tuna recorded the entire predation event: the fish was chased from a depth of about 126 meters, accelerated to a peak of over 3 g during escape attempts, and was brought to the surface in a pursuit lasting more than 19 minutes. Separate video tags on nearby orcas showed them working cooperatively to capture and handle tuna, confirming that this is a coordinated hunting strategy rather than a one-off encounter.¹

Other toothed whales also interact with tuna, though often in a less dramatic way. False killer whales and pilot whales have been observed taking tuna directly off commercial longlines, a behavior known as depredation. In the tropical western Atlantic, this kind of feeding by cetaceans on hooked fish has become a recognized problem for the fishing industry, and the whales themselves risk injury or death when they tangle with the gear.² In French Polynesia, sharks actually steal from longlines more often than toothed whales do, though overall less than two percent of the total longline catch showed signs of depredation by either group.³

Sharks, Billfish, and Other Pelagic Hunters

The open ocean is full of large, fast predators that overlap with tuna in both habitat and diet. Blue marlins, wahoos, and even other tuna species all eat scombrids, the family that includes tunas and mackerels. A study of North Atlantic feeding patterns found that scombrids were important prey for blue marlins, yellowfin tuna, and wahoos, and that these three predators showed significant diet overlap precisely because they all rely on the same kind of prey fish.⁴ A large blue marlin can weigh several hundred kilograms and has the speed and bill to stun or kill a tuna before swallowing it. Mako sharks and great white sharks are also documented tuna predators, with makos in particular being fast enough to chase down skipjack and small yellowfin in the open water.

What makes this interesting is that yellowfin tuna are simultaneously predator and prey within the same ecological niche. They eat smaller scombrids while also being eaten by larger marlins and sharks. This dual role places tuna at a critical junction in pelagic food webs, linking the small schooling fish they consume with the apex predators above them.

Jumbo Squid

One of the more surprising tuna predators is the jumbo squid, also known as the Humboldt squid. These cephalopods can reach over a meter in mantle length and are aggressive, fast-moving hunters. In the eastern Pacific, purse-seine fisheries have reported catches of jumbo squid in the same sets as tuna, sometimes exceeding a hundred tonnes of squid in a single haul. Accounts from inside the purse seine describe squid actively attacking skipjack and yellowfin tuna, leaving visible bite damage on the fish.⁵ While jumbo squid mainly eat lanternfish and other small prey from the deep scattering layer, they are opportunistic enough to go after tuna when aggregations overlap near the surface during the daytime.

When Tuna Eat Their Own

Tuna do not spare their own species. Cannibalism has been documented in Atlantic bluefin tuna starting at the larval stage. In Mediterranean spawning grounds near the Balearic Sea, researchers found that post-flexion bluefin larvae shifted their diet from tiny crustaceans to eating smaller bluefin larvae from other cohorts. This cannibalism was density-dependent, meaning it appeared when enough larvae of different sizes were packed into the same patch of water at the same time.⁶

This cannibalism does not end in the nursery. Adult tuna regularly eat juvenile tuna of the same or related species. Research in the western Pacific found that different age classes of tuna, from small juveniles to large adults, were strongly correlated in space. The predation of adult skipjack tuna on smaller, juvenile-sized tuna creates a dynamic that actually benefits other species: when large tuna chase small ones to the surface, seabirds like shearwaters take advantage of the commotion to feed on scattered prey.⁷ So tuna eating tuna has ripple effects that reach well beyond the water’s surface.

Three Thousand Years of Humans Hunting Tuna

Humans have been tuna predators since at least the Phoenician era. Atlantic bluefin tuna were fished and traded across the entire Mediterranean basin, and the city of Cádiz in southern Spain has a documented connection with this fishery stretching back roughly three thousand years, referenced by Greek, Roman, and Arab authors. The almadraba, a labyrinthine net system designed to intercept tuna on their annual migration, dates to the late fourteenth century and is considered the oldest known industrial fishing technique.⁸ A single bluefin tuna weighing 150 to 200 kilograms can yield up to 24 different cuts, each with distinct fat content and culinary applications. For centuries, this artisanal method followed the fish’s natural migration cycles, and the scale of harvest was limited by the technology.

That changed dramatically in the twentieth century. Industrialized fishing fleets, powered by diesel engines and guided by sonar and satellite data, multiplied the human take of tuna to a level no natural predator could match. Today, commercial tunas and billfishes provide enormous catches globally, but stocks vary widely in condition, from lightly exploited to severely depleted. Research has shown that tuna stocks are most likely to be depleted when the species has high commercial value, a long lifespan, a relatively small initial population, and has been fished intensively for a long time.⁹ Atlantic bluefin tuna, the most valuable tuna species, checks every one of those boxes.

Fish Aggregating Devices and Industrial Pressure

One of the most consequential developments in modern tuna fishing is the fish aggregating device, or FAD. These are floating structures, sometimes as simple as a raft with dangling ropes, deployed to attract fish in the open ocean. Since the 1990s, purse-seine fisheries have increasingly used FADs in the western Pacific, which has boosted catches but also raised the harvest of juvenile bigeye and yellowfin tuna and vulnerable bycatch species like sharks.¹⁰ Ecosystem modeling suggests that cutting FAD fishing effort by at least half could allow tuna and shark biomass to recover and return ecosystem structure to something resembling a pre-industrial-fishing state within about a decade.

The problem is not limited to the Pacific. In the northeast Atlantic, purse-seine fishing under FADs has been linked to overfishing of little tunny, a smaller tuna relative. Length-based assessments indicated that the stock was grossly overfished, with low spawning potential per recruit, suggesting the fishery may be unsustainable.¹¹ FADs are effective precisely because they exploit tuna’s natural tendency to aggregate around floating objects, a behavioral trait that evolved long before industrial fishing existed.

Longline fisheries, which set thousands of baited hooks across miles of ocean, present a different set of problems. The average bycatch rate for tuna longline fisheries has been estimated at over 20 percent of total catch, meaning that for every five animals pulled from the water, at least one was not the intended target. The unintended victims include sea turtles, seabirds, marine mammals, and sharks, species that reproduce slowly and are poorly equipped to absorb heavy incidental mortality.¹²

How Tuna Shape the Food Web Around Them

Because tuna are large, abundant, and highly mobile, their feeding has measurable effects on the communities they pass through. When Atlantic bluefin tuna migrate into the North Sea, their consumption of prey fish above roughly one kilogram can substantially reduce local abundance in that size class. At the same time, smaller fish that fall below the tuna’s preferred prey range actually benefit: with fewer large competitors and predators eating them, these smaller fish experience less pressure and their numbers increase. The effect cascades downward through the size spectrum, weakening with each step and becoming negligible by the time it reaches zooplankton.¹³

Remove tuna from this picture, and the cascade runs in reverse. In the central Pacific, researchers used ecosystem models to reconstruct food web dynamics over nearly five decades and found that the depletion of large predators, including large tuna, reduced predation mortality on smaller fish. For small yellowfin tuna, this decrease in predation pressure from above was large enough that it may have contributed to apparent increases in their biomass, essentially a predator-release effect. For other species like skipjack, however, the decrease in natural predation was overwhelmed by the increase in fishing mortality.¹⁴ The implication is that heavy fishing does not simply remove tuna from the ocean; it rewires the relationships between everything that tuna eat and everything that eats tuna.

Tuna also create foraging opportunities for species that follow them. As noted earlier, seabirds consistently forage in areas where tuna are most abundant, particularly where dynamic ocean features like eddies and fronts concentrate both adult and juvenile tuna in the same patch of water.⁷ When tuna drive small fish to the surface during feeding, they make those prey accessible to birds that cannot dive deep. Dolphins, too, often associate with tuna schools for similar reasons. A decline in tuna abundance could leave these surface-dependent foragers with fewer opportunities.

Climate Change and Shifting Habitats

Rising ocean temperatures are beginning to redraw the map of where tuna can thrive. Modeling studies project that tuna distributions in the Atlantic will start shifting poleward as early as 2030, with significant declines in habitat suitability at lower latitudes. Under higher-emission scenarios, equatorial regions could face range restriction or even local extinction of some tuna species.¹⁵ This means that tropical nations currently dependent on tuna fisheries could see their stocks migrate away, while higher-latitude waters may gain new tuna populations they have not historically managed.

In the Pacific, the picture is more nuanced. Projections suggest that bigeye tuna habitat will contract and shift eastward by the end of the century, while yellowfin tuna habitat is expected to expand across a wider area.¹⁶ These two species respond differently to changing temperature, salinity, and dissolved oxygen levels, so climate change will not simply push all tuna in the same direction. For fisheries managers, this means that today’s quota boundaries and jurisdictional arrangements may no longer match where the fish actually live within a few decades. And for tuna predators, both natural and human, the rearrangement of tuna distributions will ripple through entire food webs that have been organized around current migration patterns for thousands of years.

The Aquaculture Equation

Tuna ranching, the practice of capturing wild juvenile or adult tuna and fattening them in sea pens, has grown into a significant industry, particularly for Atlantic bluefin tuna. But ranching does not reduce pressure on wild populations the way land-based livestock farming replaced hunting for meat. The fish are still taken from the wild; they are just fed and fattened before sale. And the feeding itself creates its own ecological cost. Producing one kilogram of ranched bluefin requires roughly 9.3 kilograms of forage fish as feed, a conversion rate of about 11 percent.¹⁷ That forage fish, typically sardines, anchovies, and mackerel, forms the base of the ocean food web. If those stocks are not carefully managed, tuna ranching adds pressure to the same marine ecosystems it nominally aims to supplement.

The appeal of ranching is obvious: a fattened bluefin tuna is worth far more per kilogram than a lean wild-caught one, particularly in the Japanese sashimi market. But the math of feeding nine kilograms of wild fish to produce one kilogram of tuna means the net effect on ocean biomass is a loss, not a gain. True closed-cycle aquaculture, breeding tuna from eggs in captivity, has been achieved experimentally but remains far from commercial scale. Until it matures, tuna farming will continue to lean heavily on wild fish populations.

Parasites and Disease

Not all threats to tuna come from teeth and hooks. Blood flukes of the genus Cardicola are parasites that infect the gills and cardiovascular system of bluefin tuna, and they are a growing concern in ranching operations. In southern bluefin tuna ranched in Australia, researchers found that as the intensity of Cardicola DNA in the gills increased, the fish’s condition index decreased, meaning the parasites appeared to suppress growth.¹⁸ In the wild, tuna encounter these parasites but maintain large enough populations that individual losses do not threaten the species. In the confined, high-density environment of a sea pen, however, parasite loads can build up and weaken fish more systematically.

Other parasites found in tuna include various species of tapeworm, roundworm, and copepod parasites that attach to the skin or gills. Anisakis, a nematode that can cause illness in humans who eat raw or undercooked fish, is commonly found in tuna muscle tissue. Freezing fish before serving it raw, which is required by food-safety regulations in many countries, kills the larvae. For the tuna themselves, most of these parasites are a chronic drag rather than a lethal threat, reducing growth and condition without typically killing the host outright. In heavily parasitized individuals, though, reduced fitness could make the fish more vulnerable to predation by the sharks, orcas, and billfish that are always watching for weakness in the school.