Predators of the High Seas

Tyger, tyger, burning bright, in the forests of the night.

—William Blake, The Tyger

When we think of wild animals stalking prey, closing with a burst of speed, jaws snapping to consummate the kill, our typical image is of a lion, a jaguar, or a tiger. But tuna, and other high seas species like speedy mackerel and bonito, colorful mahi-mahi, and titanic marlin and swordfish, are every bit as imperious as predators. They are brawny animals, their large muscles and long-distance journeys imposing substantial metabolic costs that must be met by regular, successful hunts. Prey must be found, not an easy task in the enormous, desert-like empty spaces of the sea. As far back as 60 million years ago, tuna and their ancestors slipped through the waters of the world’s oceans in pursuit of a meal.³ Over millennia they evolved into exceptional swimming machines, specialists in pelagic hunting far from land. Nearly every feature of these large-bodied predators, both external and internal, is tuned to achieve tireless endurance and staggering velocity. The speed of a yellowfin tuna in Hawaiian waters was clocked at 46 miles per hour, on par with a cheetah in mid-hunt, despite the hindrance imposed by pushing through seawater 800 times more dense than air.⁴

A tuna’s primary engines are powerful flank muscles that run down both sides of these enormous aquatic missiles. A full-grown Atlantic bluefin tuna can reach 15 feet in length and exceed half a ton in bulk.⁵ Alternating waves of muscular contractions warp the fish’s robust body into a shallow C-shape, pushing against the water as the oscillation travels down the length of the animal. In large, formidable swimmers like tuna and billfish (marlins, swordfish, and the like), the tail and rearward portion of the body provide the propulsion, while the forward half remains nearly stiff, stabbing through the water like a spear. They cover hundreds of miles in search of seas rich with prey, using an internal magnetic compass to help navigate the vast, empty expanses.⁶ Tuna even accomplish celestial navigation thanks to a semi-transparent and light-sensitive “pineal window” atop their skull.⁷ Such tigers of the sea push relentlessly through mile after mile of seawater, but conserve their energy for the final hunt. Blue marlins (Makaira nigricans), considered among the world’s fastest fish, travel mostly at a leisurely pace of just 1 mile per hour for extended periods of time.⁸ Once they accelerate to strike, however, these fearsome giants can exceed 40 mph.⁹ Many open ocean predators literally travel around the world as they transition from juveniles feeding on local fish larvae to adults motivated by larger but more remote prey, and by the drive to reproduce. A black marlin (Istiompax indica) tagged and released near Baja California was later recaptured in New Zealand, a swim of more than 6000 miles.¹⁰ By comparison, the English Channel, considered by many to be the acme of long-distance swimming, is only 21 miles across; a bather would have to recross the channel 284 times to equal the marlin’s feat.

Most strong swimming fishes, particularly those that cover great distances like billfish and tuna, are torpedo-shaped: their conical nose parts the water like a submarine, and their tails narrow severely. Water closes smoothly behind them, without making eddies that would cause drag like a race car trailing a parachute. To further cut down on drag, their scales may be small and smooth, as in tuna, or wholly absent, as in adult swordfish. Marlins have scales covered by a glossy layer of skin to further ease their passage through the water. The skin can change colors, to blend in with the background or to express emotion. Striped marlin (Kajikia audax) are known to flash silvery blue bars to the intensity of neon lights when they are excited. Many fishes possess multicolor vision, allowing them to perceive subtle communications sent by the color of their skin.¹¹ Mackerels, slender relatives of tuna, owe their pearlescent shine to miniscule but nearly iridescent scales loaded with guanine and sheathed by a thin layer of skin. The very word mackerel means “pimp” in medieval French, possibly because their shiny appearance lends them an aura of a slippery fellow sporting excessive jewelry. Even today, Tokyo slang slingers use the term “mackerel gals” to refer to women in glittery clothes.¹² Hip-hop artists in the United States bent the term into their own lexicon, the hippest glossing themselves as “mack daddy.”

Anything that juts out will brake an animal as it passes through the water. Ironically, in a drag race, drag is your worst enemy. Fins pose a conundrum. They are necessary for steerage, aiming the fish up or down, and serve as brakes when a hard turn is required. Pectoral fins, located foremost on the body, provide most of this control. But in tuna, their scimitar-shaped fins can nestle into shallow divots in each flank, so while the fish is cruising the pectorals are plastered to the body: stroke your hand over them and you will feel not the slightest bump or bulge. The dorsal fin is able to disappear completely, folding exquisitely into a deep slot like a Swiss Army knife. And in both tuna and mackerel, just forward of the forked tail, the body is decorated with two ranks of short serrations, delightfully called finlets. These finlets also reduce drag, in a way that today is being exploited by competitive swimmers. When water flows over a completely smooth surface, it slips imperfectly, tumbles, and tiny eddies develop; their backward motion sucks at the skin and slows progress. The finlets, however, trap a thin layer of water between them and the body. Ocean water now slides alongside that trapped layer, provoking less turbulence and reducing drag. Today, swimsuit manufacturers are making competition suits out of material with tiny teeth that mimic this effect in the never-ending quest for more speed in Olympic waters. But all these adaptations for high speed, in fish anyway, are for naught if those swimming predators cannot find prey to chase.

On the high seas, most of the life is found near the surface. Solar-powered phytoplankton are fed on by a diverse ecosystem of zooplankton: miniature crustaceans, diminutive jellyfish, precocious fish larvae, and more. The rich broth of the sunlit waters, a soup bubbling with life, is where small schoolers like anchovies and herring come to feed if they dare to run the risk. Meandering in well-illuminated waters, engrossed in plucking tiny organisms for a morning meal, small fishes join a mortal game of roulette with their lives on the line. Predators like tuna and mackerel and marlins will find them, sooner or later, and launch violent attacks on the schooling prey. Most of these predators use a keen sense of smell to track groups of prey from a distance: yellowfin tuna can detect a single scent molecule in five trillion molecules of water.¹³ Once the predators follow the odor to the quarry, they rely on sharp vision to make the final kill. In response, many smaller prey species pursue a simple strategy: feed by dark of night, hide by light of day. But where to hide? The big blue sea is featureless, lacking the cracks and crevices exploited for safety by their reef-dwelling cousins. The answer is to drop below the sunlit zone, descending as deep as 1000 feet, where they shroud themselves in the dimmest of waters, awaiting nightfall when they can rise again to feed in safety.

A wink is as good as a nod to a blind horse, but for a predatory fish in the dark, an empty mouthful of cold water is nowhere near as good as a meal. Evolution has provoked an arms race between how deep predators can hunt and how far down prey have to drop to find safety. First, like most animals that hunt in dim light—nocturnal mammals, frogs, owls—deepwater predators have enormous eyes. Their huge retinas have more power to detect scarce glimmers in coal-black waters, allowing them to spy the dim outlines of prey. In 2012 an eye the size of a softball washed up on the shores of Florida that once belonged to a swordfish (Xiphias gladius), a species that makes repeated deep dives to hunt lanternfish and other dark water species. Even more impressive is the 11-inch eye of a giant squid, who also stalks fish that inhabit the aquatic twilight, but is itself a favorite meal of sperm whales. Their colossal eye may serve as an early warning system, detecting incoming whales just beyond their sonar range of about 400 feet.¹⁴

Bigeye tuna, who dive deeper and more routinely than any other tuna species, have eyes half again larger than their closest relatives. Furthermore, those oversized eyes have high levels of guanine in the tissue lying just behind the retina. Guanine acts like a mirror: photons that slip past the light-sensing retina without detection are bounced back for a second, reverse pass, effectively doubling the ability to see in dim light. This reflective layer, dubbed a tapetum lucidum, is possessed by many nocturnal animals and is responsible for eye-shine, the green or orange iridescence that bounces back to the viewer when a light or camera flash strikes the eyes. Sweet-natured house cats look satanic in snapshots because of the reflection from their night-adapted eyes. Bigeyes combine this doubly sensitive retina with oversized peepers, allowing them to stalk prey in deeper and dimmer waters than any other tuna. Managing the frigid cold in those inky depths, however, is another story.

Plunging into the Frigid Deep

They say the sea is cold, but the sea contains the hottest blood of all.

—D. H. Lawrence, Whales Weep Not!

For centuries our understanding of tuna, marlins, swordfish, and other ocean giants was limited to the experiences of fishermen. Virile anglers like Zane Grey waxed poetic about the speed and power of these fishes and reported herculean battles on the high seas once the hook was set. But in the late twentieth century, a surprisingly different picture emerged. Stanford University marine biologist Barbara Block has spent years attaching telemetry tags to these giants, recording their depth, position, and the water temperature for weeks or months at a time. “We try to use the same chips that are in your computer, the same devices that allow you to talk to satellites and cell phones,” Dr. Block explains, “and put them on big animals like white sharks and tunas, and we follow them across the globe. What we’re trying to do is figure out how do big animals live in the ocean ecosystem.”¹⁵ The picture that emerged has shattered old stereotypes. Block’s tags revealed that large ocean predators lead a fairly sedentary existence, swimming languidly near the surface, lolling and basking in the sunshine, rarely exceeding even a couple of miles per hour. “The fact that the cruising speeds of all these fish are exceedingly low suggests that slow swimming is a way to minimize the cost of locomotion over long distances.”¹⁶

With improved battery life and dozens of collaborators, an ambitious program tracked these animals swimming across half the globe, and startling patterns emerged. “We got a glimpse for ten years of how the Pacific Ocean worked,” Block marvels. “What we discovered was there was a pulsatile movement of the animals according to seasons. Animals you thought would wander everywhere were basically going away and coming home, going away and coming home … It was a finely-tuned periodicity much as you’d expect on the plains of Africa in which animals were going through large migrations on a seasonal scale.”¹⁷ Block’s team also made a serendipitous discovery, that tuna undertake repeated staccato dives into deep waters, staying but briefly before returning to the shallows.¹⁸ Why, they asked? The answer can be given in one drawn-out syllable: BRRR!

Three problems plague fishes descending into frigid waters to reach delicious and lethargic prey without chilling themselves to the bone in the process. Swimming muscles are debilitated, their contractions just half as quick with the 18 °F temperature drop common in dives to 3000 feet.¹⁹ The chill robs swimming musculature of the burst energy needed to capture prey. Cold water also affects the efficiency of the most important muscle, the heart, making it pump less vigorously. Reduced blood flow slows oxygen supply to the body, further diminishing swimming vigor. Third, sensory systems respond less well in a cold environment, and in particular the retina of a fish’s eye becomes less sensitive overall and less capable of detecting rapid movements like prey gliding across the field of vision. Deepwater hunters must adopt behaviors that limit their exposure to frigid water, and they have evolved key modifications to better tolerate repeated forays into the icy deep.

While fish generally are cold-blooded, large predators like swordfish, marlin, tuna, and mahi-mahi are homeotherms, capable of keeping themselves warm in all but the coldest waters. Alternating contractions of their flank and tail muscles burn fats and sugars in the bloodstream, a cellular blaze that warms the body and revs up swimming performance, giving them an advantage over the cold, sluggish fishes they are hunting. To conserve that valuable heat, their bodies have evolved over millions of years to slow its loss to the ocean waters. People in the cold feel it first on their extremities, with fingers, toes, and noses growing numb in an icy wind. In warm-blooded fishes, much of the heat generated by muscles is lost from the fins and tail, rather than toes. As we have seen, dorsal fins vanish into narrow slots, an anatomical disappearing act that also insulates those fins against the cold. To abate the loss of heat from their tails, tuna and their compatriot hunters rely on a plumbing trick called counter-current exchange. Warm blood traveling aft from the body flows in arteries lying directly alongside veins returning from the tail. The former warm the latter, so blood reaching the tail is pre-cooled and little overall heat is lost to the sea. This heat retention network can be dialed up or down by the fish. Wild swimming bigeye tuna diving into frigid waters use counter-current exchange to slow heat loss, but when ascending to the shallows they disengage the heat retention network, allowing balmy waters to warm the body more rapidly.²⁰

Beyond physiological mechanisms, fishes can modify the amount of heat they generate through behavioral adaptations, in effect setting the furnace thermostat higher. When large ocean predators enter cold waters during a hunting dive, they shift from slow, wide swings of the tail to faster, tighter wiggles that produce more heat.²¹ The more truncated stroke is akin to shivering, and it generates warmth at the expense of forward propulsion. That thermostat adjustment trick works only within a narrow range: albacore tuna can maintain their body temperature in waters above about 52 °F, but in colder seas, muscle heat alone cannot defend them, and their bodies began to chill. Once that happens, the only alternative is to return to shallower, sun-warmed waters, like Minnesota kids coming in from ice skating for a hot chocolate in the warming house. Tuna in frigid waters hunt by trampolining, explaining the bounce dives discovered by Barbara Block. They make short forays to depths where the temperature is too chilly for their internal furnace, staving off the cold as long as they are able with shiver-like swimming, and capturing as many slow-moving prey as they can manage; then they rise to the surface to warm up before another dive. Off the cost of Baja California, Kurt Schaefer and his team recorded yellowfin tuna diving repeatedly to depths below 800 feet, presumably hunting, and then returning quickly to the surface to warm up.²² One yellowfin made as many as thirty dives in a day, plunging and rising about every twenty minutes. That’s a lot of hot chocolate for one fish.

The bounce diving strategy only works when the proverbial warming house is open, that is, when the shallows are heated by the sun. Tuna make these repeated dives only during the day, but stick to the surface at night. Albacore tuna, who call both tropical and temperate waters home, bounce dive only in equatorial latitudes where the hot tropical sun provides welcome warmth after hunting at below 1000 feet.²³ In colder, temperate latitudes, albacore do not make regular dives, and as a consequence they subsist on a less diverse diet of mostly shallow-water prey. Bigeye tuna, who are larger bodied and more capable of thermoregulation, can routinely venture to depths beyond 1500 feet, but again only during daylight hours.²⁴ Blue marlin follow much the same pattern: in the Gulf of Mexico they repeatedly make daytime dives to just below the cooling zone found 200 feet down, then rise and loiter in balmy surface waters to recover.²⁵ This behavior, widespread among the high seas predators, confirms the enormous value of the prey schools lurking below the thermocline. Cold, however, is not the only challenge predators must surmount to hunt effectively in the deep; they must also contend with the risk of asphyxiation.

Muscles have simple requirements: like a campfire they need fuel and oxygen to perform. Fuel comes in the form of hunted prey, but without oxygen campfires do not burn and muscles cannot contract. Cramping sets in, fuel cannot be hunted, and death can follow. As in humans, oxygen is delivered to fish muscles by hemoglobin picked up by gills and shipped through the bloodstream. Tuna have some of the largest gill surface areas²⁶ and highest hemoglobin levels of any fish, both facilitating rapid and ample uptake of oxygen to power their carnivorous assaults.²⁷ They are challenged, however, because the ocean’s oxygen minimum zone lies around 1000 feet, precisely where diving tuna go to hunt. As blood passes across a tuna’s gills it cools rapidly, and that chilled blood goes straight to the heart. In land animals, hemoglobin’s affinity for oxygen climbs as temperatures drop: cold blood is more “sticky” to oxygen. Normally beneficial—flapping wings and jumping legs are warm and need more oxygen—this relationship would starve a tuna’s heart of the life-giving gas, as too much would remain lodged in the slushy bloodstream. Amazingly, bluefin and other deep divers show a reverse temperature dependence: their blood unloads oxygen more when it is chilled than when it is warmed.²⁸ Remarkable among animals, this mechanism ensures plenty of oxygen is supplied to the heart so that it can beat strongly despite the cold and pump oxygenated blood swiftly to the rest of the tuna’s body. In the flanks where muscles are hard at work, carbon dioxide builds up as a by-product, much like a campfire belches smoke. That carbon dioxide causes blood to release oxygen, an acidity-mediated response named the Bohr effect a century ago. In tuna, the Bohr effect, rather than warmth, signals the hemoglobin to release oxygen to the swimming muscles. Diving tuna truly fit a poignant description penned by the University of Manchester’s Holly Shiels: “a warm fish with a cold heart.”²⁹

The final key response by diving predators to the demands of hunting in cool waters is to protect their sensory systems. When icy water chills a fish venturing into the depths, it cools not only swimming muscles but also, critically, the brain and the eyes. A colder brain is less responsive and more slowly processes signals, like an older computer that sluggishly opens large, high-resolution photos. The retina of an eye at low temperature is less sensitive to light than in a warmer eye, and crucially it is less able to resolve fast-moving objects. Marlin and swordfish have excellent vision, and like tuna they frequently hunt in deep and dimly lit waters. They commonly stalk prey at depths beyond 2000 feet, but swordfish have been detected as far down as 9400 feet, where the waters are inky black and temperatures hover near freezing.³⁰ To counter the cold, these formidable hunters possess a unique heating organ that wraps their enormous eyes and rather small brains like an electric blanket.³¹ Its brown-colored tissue is richly supplied with blood and packed with energy-producing cellular machinery that raises the eye temperature by 25 °F. As a result, the warmed retina enjoys a sevenfold improvement in what researchers call the flicker fusion rate, the speed at which the eye can take discrete pictures of the world whizzing past.³² A delicious squid sprinting across the field of vision of a cold eye with a slow flicker rate would appear as an indistinct blur, but a warmed eye resolves the prey into crisp, stop-motion images. A fiery-eyed marlin can instantly measure the speed and direction of the harried squid and intercept its path for a welcome cold-water lunch.

Swordfish, Marlins, and Sailfish

A sword never kills anybody; it is a tool in the killer’s hand.

—Seneca, Letters to Lucilis on Morals

On a cool August morning in 1886, captain Franklin Langsford guided the fishing schooner Venus out beyond Halibut Point, a rocky headland some 30 miles northeast of Boston, on a quest for swordfish. It would be his last day at sea. Just before midday, the crew sighted a 300-pound swordfish swimming near the surface and hastily launched the ship’s dory. With a powerful harpoon thrust, Langsford speared the fish, attaching a length of line and a buoy. The captain returned to the boat for lunch, leaving the swordfish to wear itself out. But the fish did not tire. Having dined, Langsford boarded the dory, grasped the buoy, and began to reel in the line. The swordfish pulled mightily, the taut line twanged, the captain hauled, poised with a lance to land a mortal blow. But in an instant that would change the course of Langsford’s life, the swordfish turned and charged the boat. The line went slack, and the captain, leaning back in a small boat pitching on the waves, fell over backward. In that moment the swordfish, perhaps in a fit of vengeful rage, perhaps merely driven by blind instinct, hurled itself at the bottom of the dory and thrust its prodigious spear through the hull, piercing the wood by some two feet. Poseidon was not kind to Langsford that August day, because the captain had fallen in the dory on precisely the spot where the swordfish struck, and he was stabbed by the tip of the spear deep in his pelvis. “I think I am hurt, and quite badly!” he exclaimed, accurately diagnosing his predicament.³³ Although he survived the initial wound and made it back to land, Langsford perished three days later. The duel, fought on a watery field of honor, ended fatally for both combatants.

The sword of a swordfish, and of its close relatives the marlin and sailfish, is a bony extension of the skull. It is sheathed in skin and projects forward to constitute as much as one-third the total body length. This unique form evolved at least 100 million years ago and has persisted ever since. All three use their swords to slash through schools of prey, as we have seen, stunning or wounding a few unlucky individuals they seize and devour. The slash and dash hunting technique was first inferred from the contents of billfish stomachs: copious squid and fishes bearing distinctive gashes from having been literally put to the sword. Later, underwater cameras recorded the balletic but violent performances. The penchant by small fishes to shoal under floating objects, where they can be hunted by billfish, provided videographers an ideal site to film these behaviors. The aggregating popularity of such objects, for hunted and hunters, may explain bizarre stories of boats having been speared by marlins and swordfish who, unlike the victim of Frank Langford’s harpoon, had no reason to attack a vessel. They may have simply been slashing and lunging their way through a thick, swirling school of fish that gathered beneath the boat and accidentally stabbed the hull. Unintentional or not, such impalements can be disastrous. In 2018 a Filipino fishing vessel was struck by a marlin and promptly sank, stranding five fishermen on a makeshift raft for days. The skipper Jimmy Batiller told rescuers, “It hit the bottom of our boat leaving two big holes. We suspect it was chasing a smaller fish. It swam around the sinking boat for a while, apparently disorientated.”³⁴ Fortunately, men and fish survived their ordeal.

Billfish, like marlins, are nearly the acme of the food chain in the ocean, but not quite. They themselves are preyed upon by toothed whales and may use their bills not only for offense but also for defense against such assaults. This phenomenon was personally observed by one unfortunate diver, the oceanographic filmmaker Mark Ferrari. He was in the water when a wounded swordfish fleeing an attacking pod of false killer whales charged toward him. The injured goliath, maybe in a desperate attempt to escape, or possibly mistaking man for whale, sped at Mark and stabbed him powerfully in the chest.³⁵ Unlike Captain Lansford, Ferrari survived the terrifying impalement, despite losing a prodigious amount of blood. Billfish blades have been found buried in the blubber of captured whales around the world, albeit infrequently. Swords also have been recovered from even more unusual locations, from sea turtles to floating bales of rubber to undersea cables.³⁶ One swordfish observed below 2000 feet by the Alvin scientific submarine in the 1960s actually assailed the vessel, stabbing it with its bill; sadly, the sword became stuck, and the unfortunate swordfish perished as a result.³⁷ There are enough of these odd stories, tinged with fear, retaliation, or curiosity, to suggest we do not yet fully comprehend the ways that billfish wield their peculiar implements.

Between the three types of billfish the tools differ in form and size. A swordfish is appropriately named, as its weapon is a flattened blade; these are also the largest armaments among all billfish, reaching up to 5 feet in length. Marlin and sailfish brandish rapier-like implements, however, round in cross section. The former is named after a marlinspike, the round and pointed metal tool used by sailors to splice ropes. Holes in boats, like the one skewered off Halibut Point, can sometimes be attributed to a specific animal, depending on the shape; the same is true for broken bills left behind in whales or sea turtles.

Swordfish and their relatives also differ in their flamboyant dorsal fins. Swordfish sport a relatively narrow fin that rises steeply from their back, from just behind the gill slit, and ends just as abruptly. In contrast, the aptly named sailfish boasts a fin stretching down the length of its back nearly to the tail, forming a long and broad sail when fully unfurled. Marlins are intermediate, with the topmost fin emerging fore of the gill opening and tapering swiftly to a shallow sail that stands only a few inches above the back. While the narrow fin of a swordfish is permanently upright, both marlins and sailfish can erect or furl their sails. When moving through the water, their sails are folded into narrow slits, like a tuna, to reduce drag and improve swimming efficiency. During hunting, there is evidence that an extended sail stabilizes the fish by steadying the side-to-side motion of its bill while swimming and permitting a stealthier approach to schooling prey.³⁸ That stabilizer may also give the sailfish a firm buttress to brace against when vigorously slashing its sword, generating bill accelerations that outstrip the escape speed of most prey. When not hunting, the sail’s expansive surface area renders it an excellent solar collector. Billfish have been observed loitering at the surface, using the extended sail to warm themselves after a hunting foray into the frigid depths, capturing the sun’s heat and funneling it to the stout body.

All billfish are enormous compared with most fishes in the sea, their bulk helping them stay warm during hunting dives. More cylindrical in shape than tuna, they similarly bear sickle-shaped tails. Females are considerably more hefty than males: full-grown adult swordfish can reach 15 feet in length and exceed 1400 pounds.³⁹ Blue marlins attain similar bulk but stretch to more than 16 feet.⁴⁰ Sailfish are more diminutive, reaching 8 or rarely 11 feet, but they still represent a large and fearsome hunter if you are a sprat on the run. All three are powered by immense muscles, making up the majority of their weight and requiring prodigious consumption of prey for fuel; small fishes and squid are favored as main courses. Unlike tuna, billfish tend to hunt singly or in small groups rather than in large schools, which dramatically affects the way they, in turn, are hunted by humans.

Fishing nets are designed to catch schooling fish, whether tiny anchovies or beefy and gregarious tuna, but they cannot reliably be employed for solitary targets like marlins and swordfish. Historically, these giants were caught on hook and line, in one-on-one battles with sports fishers. Commercial fisheries, in an effort to boost catch rates, have multiplied that approach, trailing staggeringly long lines behind fishing boats with thousands of baited hooks. One long line may stretch for 100 miles in the ocean, the distance from Philadelphia to New York City. They are dragged through productive waters for a few hours, then hauled in to harvest the fishes that took the bait. Predictably, this indiscriminate technique also ensnares a distressing amount of bycatch. Sharks, rays, sea turtles, and even seabirds like albatrosses are attracted to the bait, snagged by the hooks, and drowned. Deployment of tinsel-like streamers above the surface, and C-shaped hooks below, has eased the mortality of seabirds and marine animals, but the practice is inherently unselective and destructive. Whether line-caught or netted, billfish unfortunately share with tuna the twin traits of being economically valuable and slow to reproduce, a combination Bruce Collette at the National Marine Fisheries Service calls “double jeopardy.”⁴¹ Such species are pursued intensely by fisheries because the meat fetches a high price, but their late maturity means they are unable to replace themselves quickly enough to sustain overenthusiastic harvests. Consumer demand, unfortunately, has yet to submit to biological realities. And the hazards of swordfish and marlin harvesting affect not only the fishes themselves. Once those expensive steaks reach a diner’s table, an unseen danger to the consumer lurks inside.

At the End of the Chain

Even in the vast and mysterious reaches of the sea we are brought back to the fundamental truth that nothing lives to itself.

—Rachel Carson, Silent Spring

In 2009, the actor Jeremy Piven took an unexpected leave of absence from his leading role in the hit Broadway show Speed the Plow, citing muscle fatigue and neurological dysfunction. The cause? His blood mercury level tested at more than six times the safe limit, likely caused by his obsessive consumption of sushi. Playwright David Mamet, who penned the script quipped, “So my understanding is that he is leaving show business to pursue a career as a thermometer.”⁴² But mercury poisoning is no laughing matter. In the 1860s when London scientists synthesized compounds of methyl mercury, the element’s organic form, two lab technicians died from accidental poisoning. Their deaths must have been ghastly. Nearly a century later, the Chisso chemical company in a coastal Japanese town called Minamata discharged methyl mercury for decades into the waters of the bay.⁴³ There it was absorbed by fish and shellfish that were eaten by residents, particularly the families of fishermen. The consequences of this diet of pollution were disastrous. Adults were stricken by numbness, dizziness, and lack of coordination. Children suffered from severe physical deformities, acute neurological disability, and agonizing deaths. Over a thousand people succumbed, and many more were tormented for the rest of their lives by debilitating symptoms. The term “Minamata Disease” became synonymous with methyl mercury poisoning.

Epidemiologists were slow to diagnose this so-called disease, in part because they could not imagine how the miniscule concentrations of methyl mercury measured in the bay’s waters could provoke such profound effects. After an extensive investigation they discovered that mercury concentrations were being magnified in the seafood eaten by local families. Trace but persistent levels in the water soared to around 20 parts per million (ppm) in the bay’s mollusks and fish, then skyrocketed to 700 ppm in the hair of residents.⁴⁴ For comparison, the Environmental Protection Agency standard for “safe” methyl mercury levels in seafood is just three-tenths of a ppm; Minamata families were eating fish nearly a hundred times more poisonous.⁴⁵ Researchers eventually concluded that the marine food chain, a noble succession of organisms that progressively concentrate sunlight into dinner table fillets, had been corrupted as an amplifier of toxicity.

When a flying fish is swallowed by a mahi-mahi, the larger fish eats not just its victim, but also all the prey that flying fish has ever consumed, and the prey of that prey, and so on. Like nesting dolls, every mouthful contains a multitude. But in each step, only a small fraction of the dinner is permanently incorporated into the diner. The mahi-mahi’s metabolism burns much of the flying fish calories, exhaling carbon dioxide across its gills. Mercury, however, cannot be used to power muscles, and thus is never exhaled. It also has a nasty habit of lodging in fatty tissues, rather than being purged as waste. As months stretch into years, each tiny morsel of mercury eaten by the mahi-mahi accumulates in its tissues. If that mahi-mahi is eaten by a larger predator, like a great white shark, its entire storehouse of mercury is passed on undiminished, and the indigestible toxins become biomagnified. These open ocean hunters, graceful and powerful predators like tuna and mahi-mahi and swordfish, can harbor a chemical curse in their streamlined bodies.

Older and larger fishes tend to have higher mercury levels, because time yields greater amplification, and big bodies are built from many meals. Anchovies, on which Peru built a seafood empire, manifested mercury levels of just two-tenths of a part per million in tests by the US Environmental Protection Agency, while big-bodied and long-lived marlins bore twice that amount.⁴⁶ Similarly, New Jersey researchers studying mercury in canned tuna found levels in “white” tuna were nearly four times higher than in cans of “light” tuna, because the white albacore tuna are larger, about twice the size of light skipjack tuna, and are harvested several years later in their lives.⁴⁷ Faster-growing animals like swordfish also accumulate toxins more swiftly, since their high metabolisms require more frequent meals. Toxicologists in Italy showed that mercury levels in Mediterranean swordfishes more than doubled with a twofold increase in fish length and urged buyers to stay away from the largest specimens.⁴⁸

In the oceans, it is not only mercury that should cause us worry. Many pollutants that reach the sea will biomagnify in marine animals if they cannot be broken down. These include metals like cadmium and lead, complex industrial compounds like persistent organic pollutants (such as PCB and dioxin), and powerful insecticides like DDT. This last chemical (its full name would traverse two Scrabble boards: dichlorodiphenyltrichloroethane) was made infamous thanks to Rachel Carson’s landmark book Silent Spring, her heartbreaking requiem for birds silenced by accumulation of the poison.⁴⁹ Fish, too, are debilitated by the toxins they ingest. Luís Vieira and his colleagues in Portugal have shown that common gobies (Pomatoschistus microps, a denizen of estuaries) are unable to swim as vigorously when exposed to mercury or excess copper in their water, leaving them more susceptible to hungry predators and more likely to pass their toxins.⁵⁰ Our pollution puts fishes, as well as our own bodies, at risk.

Mahi-Mahi, Jacks, and Mackerel

Mahi-mahi (Coryphaena hippurus) are among the most beautiful, and most odd-looking, of the large predators who stalk the big blue ocean. Their common name derives from the doubled Hawaiian word for “strong,” reflecting their robust build and vigorous swimming ability. A glittering gold coloration, however, is the origin of their Spanish name, dorado, or golden one. A deep but narrow body tapers gracefully to the scissor-shaped tail characteristic of speedy aquatic hunters; however, it is the astonishing head, with its high, bluntly domed forehead that stands out. Adult males develop this prominent bony crest as they age, giving them a unique, battering ram-like appearance. Parading a pronounced hump, while unappealing in the ogre of Notre Dame, is apparently quite alluring in large male fishes: salmon, parrotfishes, wrasses, and others develop conspicuous, bulbous head shapes as signals of their virility to would-be challengers contemplating a duel, and to females contemplating a family.

Females and youngsters lack this macho crest, having more normal, rounded heads, but all grown mahi-mahi are equally splashed with some of the most dazzling colors in the sea. Draping the body, atop a wash of aqua green, is a brilliant, lustrous golden color, an unmistakable combination, like a broad beam of sunlight drenching fresh grass. Glittering turquoise spots speckle the flanks, while long dorsal and anal fins of rich cobalt blue frame the sleek body, the former tracing almost the entire length of the back. The brilliant colors may provide a degree of camouflage, as these predators hunt primarily in the sunlit, turquoise waters of tropical and subtropical seas. Sadly, the gleaming colors fade rapidly after they are caught, but underwater they are a stunning sight to behold.

From Hawaii and myriad Pacific islands to tropical countries across the globe, mahi-mahi is widely sought as a food fish. The large bodies yield ample steaks of firm, pale flesh that is delicious: lighter in flavor than tuna or swordfish, but with more savor than bland whitefish. Add to this mild, sweet flavor the fact that mahi are excellent targets for fisheries, and it is easy to see why they are fast becoming a top seafood choice. They reach reproductive maturity in as little as four months and thus accumulate fewer toxins like mercury before capture. Mahi are well known to congregate under floating objects, like tuna, a trait exploited by fishers who dot the seas with all manner of clustered jetsam to draw them to their nets and hooks. They breed abundantly, in the wild and in captivity, rapidly replacing individuals reeled in and sold to restaurants. For these reasons mahi-mahi stocks are routinely ranked by seafood assessment organizations as safe and sustainable.

Despite these advantages, mahi-mahi was not an immediate hit in all places. Perhaps the wildly misleading common name “dolphinfish” was responsible; more likely, people were reticent to eat something unfamiliar. Fisherman Ed Ries relates a tale from the 1930s, when he had to resort to a bit of creative flim-flam to flog a dozen tasty mahi-mahi he had hooked. “We stowed the fish in the rumble seat of a Model A Ford,” he begins, “and took off for L.A. to peddle our catch. To our dismay the small markets and restaurants on our route refused to buy. None had ever seen a dolphin or heard of the Hawai’ian taste treat, mahi-mahi.” But necessity, as they say, is the mother of invention. Inspiration struck Ed when he realized the fish tails closely resembled those of more familiar, and delicious, yellowtail jacks; Ed, in turn, struck the mahi-mahi. “Chop, chop and back into the ice went the dolphins, minus their funny-looking heads. They were quickly disposed of at yellowtail prices.”⁵¹

Yellowtail jacks (Seriola lalandi), the objects of Ed’s deft impersonation-by-beheading, are members of a large group of fishes collectively called jacks that includes amberjack, trevally, pompano, and the confusingly named jack-mackerel, which is not at all a mackerel. All share a deeply forked tail, streamlined shape, and ravenous appetite for predation with their cousins, the tuna. Conversely, humans have a ravenous appetite for jacks: nearly all species are much sought-after for the table. They differ from tuna by being smaller, usually one to three feet in length, and more compressed: flattened from side to side, like a book standing on a shelf. Most are silvery, occasionally with bars or a yellowish wash; in a few species, like the giant trevally (Caranx ignobilis), males turn a deep brown, almost black, when they are of advanced age. African pompanos (Alectis ciliaris), on the other hand, are splendidly decorated as youngsters, with long, tinsel-like threads trailing from the tips of their dorsal and anal fins. The threads disappear by adulthood, suggesting they serve to make the juveniles less appetizing to predators with no wish to get floss stuck in their teeth.

Jacks are predatory and have developed some unique strategies to flush and capture their prey. Giant trevally, well-named for their hefty, almost brutish appearance, have been known to ambush seabirds loitering on the ocean’s surface. Mesmerizing videos from the Seychelles, widely seen in David Attenborough–narrated documentaries, show the formidable fish lurking just below the waves, then bull-rushing an oblivious tern and seizing it in powerful jaws. A few nimble birds take panicky flight at the last instant, spurring the trevally to burst upward into the air itself, leaping as much as a foot out of the water, salt spray glistening in the sun, to snatch a fowl on the wing and drag it down to a watery grave. But in the enormous, mostly empty expanse of the oceans, finding prey can require some creative strategies, and some fish, it would seem, do know jack about hunting.

Frank Parrish, from the Pacific Islands Fisheries Science Center, made a surprising discovery about jacks when he fitted Hawaiian monk seals with underwater video cameras to observe the mammals while feeding. He recorded numerous instances of predatory fish accompanying the seals in what he termed “escort” behavior. The most frequent escorts were giant trevallies, presumably taking a break from plucking birds out of the sky, followed by several other species of jacks. “They were clearly attracted to the [seal’s] intense bottom-searching activities,” which flushed prey buried in the sand or hidden inaccessibly among rocks.⁵² “Jacks routinely positioned their mouths within inches of the seal’s nose to maximize their chances of snatching prey items flushed by the bottom probing of the seal … [and] to capture prey before the seal could catch it.”

While many predatory fishes are solitary hunters like swordfish and giant trevallies, some species hunt in packs, relying on their numbers. Complex routines are choreographed by mackerel as they gather in large schools with mouths open wide and pass through the ocean like a giant spider web, capturing copepod and other morsels as the ensemble glides forward. Mackerels resemble tuna, but are more slender, a javelin to the tuna’s torpedo, and they shine a glittering silver. Many of the thirty or so species have dark patterns on their backs, ranging from loose speckling to wavy ripples to zebra-like barring. Both the shiny scales and the barcode-like markings are thought to help these animals traverse the upper ocean waters in tight schools without so much as a scrape or fender-bender. A fine example is the horse mackerel (Trachurus trachurus), who sports bright mirror-like patches on the tail and cheek, like reflectors on a boat trailer. The reflectivity of the patches shifts dramatically with the tiniest of movements of the head or tail, further signaling to other school members that this fish is adjusting course mid-gallop.⁵³ Taken together, the mix of visual signals from stripes and reflectors helps the school instantly detect, and match, changes in direction and speed, producing the coordinated movement we see as a polarized school.

Like tuna, mackerel are steady swimmers, relying on slow-twitch fibers to produce sustained travel. Their muscles are reddish and keep their bodies warmer than the sea, but they rely to a greater degree on fat as fuel. Mackerel undertake arduous migrations from the high seas to spawn in nearshore shallows, sometimes in staggering numbers. These fish are astonishingly fecund: a single female may release as many as 2 million eggs in a season.⁵⁴ During the pilgrimage and industrious spawning sessions, mackerel power their muscles with stored oils, rather than stopping to feed. To a diner, these fat stores are noticeable as fish oil, a slippery, energy-rich substance that make mackerel the quintessential “fishy” fish. They smell strongly, even unpleasantly to some, and the meat has a similarly potent flavor. Mackerel sequester oil in their muscles, and in cavities around the gut: a quarter or more of a fish’s weight can be composed of oil.⁵⁵ The oils are beneficial to us, as they are offer important sources of vitamins A, B₁₂, and D, as well as the current poster child for dietary health, omega-3 fatty acids⁵⁶.

Mackerel, however, have been unappetizingly dubbed by some Japanese chefs “the fish that spoils while still alive.”⁵⁷ Fishes high in oil tend to rot more rapidly because the fat-rich tissues break down quickly; in turn they yield by-products called lipid peroxides responsible for the distinctive rancid odor of rotten fish. The expression “stinks like a dead fish” should more technically be “stinks like a lipid peroxide,” but it hardly rolls off the tongue as poetically. Better would be to reference the noxiously fetid names given to some of the foul by-products, like “putrescine” and the particularly stomach-churning “cadaverine.” Decay of oils into putrid volatiles may be further accelerated by the abundance of myoglobin in these dark meat fishes, as the oxygen they supply supercharges the breakdown of fats and the production of malodorous stenches. Because of such rapid spoilage, mackerel have traditionally been salted, smoked, or dried as soon after capture as possible. But thanks to their abundance, particularly near coastlines where people with rods and reels and hungry bellies reside, they have always been a popular food fish.

Even an abundant fish can be mismanaged, however, and Atlantic mackerel (Scomber scombrus) provide a worrying illustration of the consequences of multiple countries managing a single, mobile stock. Once sustainably harvested in the North Sea, their schools have been shrinking dramatically, unable to withstand intense fishing pressure levied by overlapping fleets. Geoffroy Lean of the Daily Telegraph describes how climate change, migration, and optimistic fishing quotas all combined to drive northeastern stocks of Atlantic mackerel down to a fraction of their pre-industrial levels. “It began more than four years ago, when Iceland started increasing its landings of the fish, unilaterally upping its quota … The Faroe Islands followed suit, raising their own quota sixfold … Added to what other countries are legally catching, some 900,000 tonnes of mackerel are being landed a year from a fishery that can only stand to yield 500,000 tonnes. That cannot go on for long without disaster.”⁵⁸

There are reasons beyond sound resource management to be cautious about overeating mackerel. These fishes are common sources of scombroid toxicity, one of the most widespread types of seafood poisoning. The word scombroid refers to the taxonomic family of mackerel (and tuna), but the illness can be provoked by nearly any fish that is improperly chilled or preserved. It is instigated by a chain reaction in which bacteria on the fish’s skin convert histidine—a harmless amino acid present in the flesh—into histamine, a toxin that triggers allergic reactions.⁵⁹ Symptoms are variable, with victims typically suffering moderate stomach upset, dizziness, or a bright red rash, usually appearing an hour after the meal, but disappearing within one day; in extreme cases it can cause anaphylactic shock, or even more rarely, death.⁶⁰ Fortunately, treatment with antihistamines is highly effective since scombroid toxicity is an allergic response, not a true poisoning.

Mackerel are known to harbor surprises even more off-putting than just allergens and pungent odors. Many is the time an angler has boated a mackerel and set about cleaning the fish, only to find the insides literally crawling with parasites. These range from tiny crustaceans to worms and flukes that do not kill the fish, using it instead as a stopover on a circuitous life cycle that normally includes several hosts. Anisakis, a roundworm that infests many fish species as well as marine mammals, can cause nausea or vomiting in people if the fish is eaten raw. Even more nauseating is the description by the Centers for Disease Control and Prevention of how unfortunate diners may learn they have just eaten the roundworm: “Some people experience a tingling sensation after or while eating raw or undercooked fish or squid. This is actually the worm moving in the mouth or throat.”⁶¹ Fortunately, freezing or cooking fish eliminates the risk of nearly all parasites; this is why most sushi, although eaten raw, is deep-frozen first.

Mackerel are so heavily parasitized that one study of nearly 2000 horse mackerels in the eastern Atlantic registered a total of forty-five different kinds of squirming colonizers.⁶² But much can be learned from such diverse infestations. Parasites are often specific to restricted geographic regions, and the assemblage in a single fish can serve as a kind of bar code revealing the fish’s past: its birthplace, the waters where it has swum, and the stock to which it belongs. By parsing the map of parasites in numerous fish, researchers discovered that Spanish mackerel (Scomberomorus commerson) in northern Australia was divided into five distinct populations, with about 5 percent of fish moving in their lifetime between the stocks.⁶³ So, parasites are not just disgusting and dangerous, they can be quite helpful … so long as you do not swallow them.

Some fish have the power to provoke revulsion even without the help of parasites. Escolar (Lepidocybium flavobrunneum), a deepwater cousin of mackerel, has gained fame as “the most dangerous fish in the world,” not because it is a menace to bathers, but for the peril it poses to diners.⁶⁴ That peril has a name: keriorrhea, a sanitized clinical term for a revolting condition of uncontrollable, malodorous, waxy diarrhea. Escolar shares with mackerel the tendency to store energy in fats, but mostly in the form of indigestible wax-esters. Also found in carnauba wax—principal ingredient in shoe polish, car waxes, and furniture polish—wax-esters cannot be processed by our digestive systems. Food writer Harold McGee primly describes the greasy results: “The wax esters therefore pass intact, their lubricating properties undiminished, from the small intestine into the colon, where a sufficient quantity will defeat our normal control over the ultimate disposition of food residues.”⁶⁵ While eating a few ounces may provoke no ill effects, larger portions may send the unfortunate diner rushing for the restroom. So foul are the results that escolar has been banned in Japan since 1977, where it is listed as toxic.⁶⁶ Elsewhere, the fish is sometimes distressingly marketed as “white tuna” by sushi restaurants, a deliberate mislabeling that can catch customers off guard, so ask before you eat.

The Tyger’s Future

The solution often turns out to be more beautiful than the puzzle.

—Richard Dawkins, Unweaving the Rainbow

In tuna, marlins, and the other tigers of the sea, every detail has been fine-tuned over millennia for a singular purpose: cruising the vast expanses of the great blue oceans in search of prey to hunt. The streamlined body shape, the acute senses, their tolerance for deepwater cold, and their unrivaled swimming endurance all contribute to a transfixing and majestic work of predatory art. People will fly across the world to see a cheetah hurtling over the Serengeti, or a pride of lions tear into an antelope, but they are barely aware of swordfish and tuna as equally extraordinary hunters. Granted, it is difficult to arrange an underwater safari to witness marlin and tuna in action. But like John Cage’s symphony of pure silence, an unwatched tuna is nevertheless magnificent. These open ocean hunters deserve our appreciation, our awe, and our protection. What they habitually inspire, however, are our tableside appetites.

Barbara Block extends the comparison to terrestrial carnivores: “We wouldn’t go into Africa and eat the lions, zebras and elephants, in most cases. We are basically doing that in the ocean. We are not looking at wildlife in the ocean as anything but food, and we could leave to our children an ocean without these animals.”⁶⁷ One of the strongest drivers, according to Block, is the astronomical price that some tuna can command, and the lengths to which people will go to catch them. “It’s a cocaine-of-the-sea type of problem where many people want it and no one’s paying attention to the rules. Pirated tuna is a really big problem.” Having used advanced technology in her own studies, she advocates its application to the problem of piracy. “What if we could barcode every tuna that’s landed and keep track of them,” she asks. “My dream is really to make a tag … that allows us to keep track of the fishery in a more accurate manner from point of landing to market, so we don’t have any pirating.” Soon, that dream may be realized: pilot programs in Mauritius attach a digital tag when a fish is caught, then store data in a blockchain so consumers can see the fish’s entire history—from hook to plate—just by scanning a barcode.⁶⁸

It may take more than just tags to save the ocean’s large predators, but it has been done before. Wolves and mountain lions had been driven to the brink of extinction in North America before conservationists launched campaigns to restore these magnificent carnivores to the landscape. Despite boisterous but partly misinformed objections by hunters and ranchers, wolves have returned to many US states and robust populations thrive in Yellowstone and elsewhere. Quarrels with interest groups occasionally boil over, but earlier predictions of widespread disaster have failed to materialize. Instead, these wild predators inspire wonder in photo-snapping crowds who increasingly frequent national parks and hope merely to catch a glimpse. Tours in Alaska and Canada are dedicated solely to showing brown bears and polar bears to tourists, who bask in their august (but safely distant) presence. If a similar future can be created for tuna and billfish, by setting aside key protected areas and reducing fishery harvests, then these spectacular marine hunters will endure, roaming the seas, majestically slicing through the waves to seeking mates and snap up prey.

Like more familiar terrestrial predators, swordfish and tuna actually make their prey stronger, by picking off the weak, confused, and sick among them, thinning the herd and improving the gene pool. They are essential for the health of the ocean. And despite their power and speed, tuna can themselves be prey. Sea lions have been known to cleverly corner and capture them among rock pools in the Galapagos.⁶⁹ Pilot whales and killer whales feast on them in open waters.⁷⁰ But even more feared by all tuna are a group of silent ocean predators, formidable and relentless, the only other fishes swift enough to outrace them and massive enough to overpower them. These creatures of ancient origin are misunderstood, much maligned, and unjustly dreaded by humans. They are sharks, and they are the most awe-inspiring and formidable predators that rove the seas.