Cracking Open the Tuna Code

Ecology is only part of the equation when it comes to understanding bluefin tuna.

Published February 9, 2016

From the beach, the Tuna Research and Conservation Center (TRCC) near Monterey, California, looks more like a surf shop than a laboratory: it’s a low-roofed, white-walled shack that squats just a well-cast fishing line away from the Pacific Ocean. Within the center’s dim confines, however, the facility reveals itself as a marine biologist’s paradise. Three dozen young bluefin tuna circle a carousel-sized fiberglass tank, their flanks flashing with tiger stripes, their crescent tails casting frothy wakes. The fish—at around 15 kilograms, a fraction the size of the 300-plus-kilogram giants they will someday become—aren’t scheduled for feeding, but the appearance of human visitors has them revved up with Pavlovian enthusiasm. One tuna leaps from the tank, the crash echoing above the aqueous burble of the room’s pumps.

Atop a wooden staircase overlooking the tank, Barbara Block, a Stanford University professor and one of the TRCC’s founders, watches with a proud half-smile. “This is the only place in North America with live bluefin tuna,” she says. “And we treat them like Olympic athletes.”


The Tuna Research and Conservation Center in the small city of Pacific Grove, near Monterey, California, is a joint initiative of Stanford University and the Monterey Bay Aquarium. Photo by Tyson V. Rininger/Monterey Bay Aquarium

Few of evolution’s wonders are so stunning as the bluefin, a tireless predator that can grow nearly to the length of a whitewater kayak. The fish dives as deep as two Empire State Buildings, swims as fast as a racehorse can run, migrates farther than humpback whales to feed and spawn. It owes many of its extraordinary talents to endothermy, commonly known as warm-bloodedness: a bluefin can boost its internal temperature up to 21 °C above the temperature of its surroundings, allowing it to venture into frigid waters that would kill its cold-blooded counterparts.

For centuries, the bluefin’s rich red musculature protected it from exploitation, as sushi chefs favored fish with whiter, milder flesh. But no longer: changing tastes have made bluefin among the most coveted sushi species, and the fish today faces grave peril, mostly from Japanese diners. Intense fishing pressure has depleted the Pacific bluefin’s population to less than four percent of historic levels, and Atlantic stocks have plummeted below fifteen percent. Bluefin are global travelers: fish that spawn in the Mediterranean often cross the Atlantic to forage along the eastern shores of North America, and tuna that spend their adolescence near California return to breed in the Japanese waters where they were born. That wanderlust makes tuna hard to protect. They cross international boundaries and regulatory jurisdictions without a thought for their own conservation. The ravenous global pursuit of bluefin, author Paul Greenberg has written, represents “the last great wild-fish gold rush the world may ever see.”

Studying the esoterica of tuna physiology, then, might seem the ichthyological equivalent of fiddling while Rome burns. Block, however, believes that bluefins’ warm-blooded bodies provide the key to understanding their global movements—and, therefore, to their conservation.


Barbara Block and her associates are the first scientists to have measured the bioenergetics of wild fish. Photo courtesy of Barbara Block

“Science in the ocean often gets driven by ecology. Most people see animals moving around and say, ‘Oh, they’re just following their prey,’” Block says, climbing down from the tank. “But a physiologist stumbles on the scene and says, ‘Wait a second—their bodies have rules.’” With the publication of a new paper in 2015, Block’s decades-long exploration of those rules reached a new peak: she and her colleagues became the first scientists ever to have measured the bioenergetics of wild fish, thereby unmasking precisely how much energy pelagic predators derive from their prey. Learning those rules might help usher in a new style of marine management—and, in the process, save the bluefin tuna.


Though Block has not always been a bluefin devotee, she has a career-long fascination with warm-blooded fish. As a PhD candidate at Duke University in the 1980s, she cut her teeth studying billfish, so-called “cranial heaters” capable of warming tissues around their eyes and brain, an adaptation that may help them detect flashes of silver in dark waters. She tracked her radio-tagged billfish for days at a time, gauging their swim speeds and diving depths. Some scientists fall in love with species, others with ecosystems. Block grew enamored with physiological phenomena.

“I live and breathe trying to understand why animals do what they do,” says Block in her office adjacent to the TRCC. The space is less workstation than marine science museum. Her eclectic collection includes a leatherback turtle skull, a plastic model of a tuna spine, and five decades of fish-tracking tags, from translucent cylinders packed with circuitry to steel tubes, intended to be returned by fishermen, that are stamped with the phrase “Big Reward $$$.” Block, who has shaggy hair and amused brown eyes, speaks in brisk, declarative statements; she conveys the impression of perpetual motion. In some ways, she’s migratory herself: the day I visit her in California, she has just returned from six weeks in the South Pacific, and she’s soon flying to Alaska to tag salmon sharks, another warm-blooded creature.

Her year-round home, however, is the center, a facility she helped establish in 1994 to investigate yellowfin and bluefin tuna physiology, the latter a species she reverently calls the “pinnacle of bony fish evolution.” Research conducted at the TRCC has helped solve mysteries such as how juvenile bluefin respond to crude oil exposure (hint: not well) and how the bluefin’s heart functions in frigid water (it’s very good at cycling calcium). According to Kathryn Dickson, comparative physiologist at California State University, Fullerton, the TRCC is uniquely capable of tackling big questions. “It’s really difficult to study these fish: they’re so large, and they need huge volumes of water,” Dickson says. “That’s why the TRCC has been so instrumental in understanding some of the physiological questions, because they can keep these fish alive.”


The Tuna Research and Conservation Center is uniquely equipped to keep captive bluefin tuna alive, enabling long-term study. Photo by Tyson V. Rininger/Monterey Bay Aquarium

In the early 2000s, Block began to examine a question that would help define the course of the TRCC’s research: how did water temperatures affect bluefins’ bodies? While endothermic animals are less susceptible to changing water temperatures than their cold-blooded counterparts, they’re far from immune. After all, your own internal furnace has to work harder when it’s too hot or cold. Surely tuna, too, had an ideal thermal range.

To find out, Block and her team hoisted juvenile tuna—captured on barbless hooks off the coast of San Diego and housed at the center—into a swim tunnel that allowed them to swim in place. By measuring dissolved oxygen in the water while the fish “jogged,” Block’s team showed they indeed had an optimal ocean temperature envelope, 15 to 20 °C, at which they gulped the least oxygen and swam most efficiently. In theory, Pacific bluefin would adhere to that same narrow temperature band in the wild.


Tuna Research and Conservation Center staff often use internal sensors to gather data on the bluefin tuna’s movements and feeding habits. Photo by Stanford University/Monterey Bay Aquarium

A few years later, the TRCC’s researchers turned their attention to a related mystery: how tuna’s feeding habits affected their body temperature. When you metabolize a cheeseburger, your body generates both energy and heat, just as a running engine sizzles to the touch. But while it’s easy to watch a human devour a quarter-pounder at a diner, it’s a whole lot harder to observe a tuna chowing down in the middle of the Pacific. In theory, scientists could figure out how much any individual tuna was eating if they could somehow monitor the change in its body warmth: internal temperature could serve as a proxy for eating. Metabolic heat, then, was the master key to comprehending bluefin feeding patterns—knowledge that could help scientists pinpoint oceanic hotspots favored by these top marine predators.

Enter Rebecca Whitlock. In 2010, Whitlock implanted amber-colored tags the size and shape of rubber erasers into the bellies of three captive tuna. Then she crouched in the rafters above the tanks at the TRCC and rained food onto the voracious fish. “You’d drop one sardine and they’d all go for it,” Whitlock recalls. “It could be quite difficult to tell which fish ate what.” Sticking colored markers to their fins made matters easier. By analyzing how food consumption affected internal temperature, Whitlock could determine how much, say, 500 calories of sardines or squid would elevate a tuna’s body heat. She’d cracked the code.

TRCC scientists had figured out the temperature at which bluefins’ bodies ran the best and how eating warmed up the fish. Now it was time to apply those rules to wild fish—with data from the ocean.


Fortunately, Block and her colleagues had plenty of information from which to extrapolate. From 2002 to 2009, the TRCC team had hooked more than 500 young wild Pacific tuna off the coast of southern California and Mexico, and outfitted them with the eraser-sized tags in their bellies. With the help of fishermen, the researchers recovered more than 170 of the data-logging sensors. Having figured out how prey consumption affected body temperature in the laboratory, Block and Whitlock could apply their new models to that existing data set and figure out exactly how wild Pacific bluefin tuna’s movements correlated with their feeding habits.


Physiologist Barbara Block tags a giant bluefin tuna at sea. Photo courtesy of Tag-a-Giant

In her ocean-view office, Block cracks open a laptop to show me what she found. She points to a graph on which a red line, representing an individual bluefin’s body temperature, climbs from 20 to 25 °C—an indication that it’s just feasted on baitfish or squid. “This is one of the first times in the history of marine animals that we can watch prey ingestion at the level of kilocalories,” she says.

She pulls up another slide, a map on which a constellation of animated dots, each representing a tuna, zip from Mexico’s Baja Peninsula to Northern California and back again. Clearly these fish roam widely—but why?

When Block and her team had analyzed a different set of tuna migrations for a 2010 paper, they’d come to a standard ecological conclusion: the fish generally follow plankton blooms that attract other sea creatures. But when they incorporated sea-surface and body-temperature data in the new study, a simple two-step became a nuanced physiological ballet. The energetic demands of tuna’s warm bodies appeared to be at least partially steering their migrations. In winter, storms chill the waters off the California coast to below 15 °C—the bottom end of bluefins’ optimal temperature range, at which point their hearts slow and most fish cruise down to Mexico. As temperatures rise in late summer and early autumn, the increased metabolic strain pushes the fish north again. In October and November, they make predatory forays into nutrient-rich cold water, and then dart south into the “thermal refuge” of more temperate seas.

By determining where the fish had eaten, Block and her colleagues could identify the crucial hunting grounds in the “Blue Serengeti.” But the scientists were in for a surprise: the tuna sometimes spurned the high-productivity areas where you might expect them to feed. Not even the enticement of abundant food could lure them out of their physical comfort zone. Big meals only help if your body can efficiently digest them. Physiology, rather than ecology alone, helps guide the tuna’s movements.


This illustration represents the migration of bluefin tuna through the Pacific Ocean, tracked by researchers at the Tuna Research and Conservation Center. Illustration by Mark Garrison

“Most ecologists haven’t been taking the energetic cost of digestion into account in their models,” says Block, who published her results with Whitlock and others in the journal Science Advances in September 2015. “But tuna can’t sit in hot water and eat the same meal as they could in cooler waters, so they move north. People don’t appreciate how physiological limits are driving what animals do.”


So why, precisely, does this matter? After all, scientists, Block included, have learned an awful lot about tuna behavior by simply tracking their wanderings. If we know where fish move, do we need to know why?

For Elliott Hazen, a University of California, Santa Cruz, ecologist and coauthor of the 2015 tuna migration study, the answer is an unequivocal yes. Marine conditions change constantly—look no further for proof than “the Blob,” the mysterious swath of warm water that recently wreaked havoc on Pacific ecosystems—and hotspots come and go. Fisheries regulations, he believes, need to respond accordingly. “Static protected areas can work well for stationary organisms like abalone,” says Hazen. “The next step is to identify and protect moving oceanographic habitat for pelagic predators.”

Creating mobile protected areas may seem like a daunting task, but it’s already standard procedure in the waters north of the Hawaiian Islands. There, the National Oceanic and Atmospheric Administration (NOAA) operates a service called TurtleWatch, which helps longline fishermen avoid patches of ocean where sea-surface temperatures are favorable for loggerhead turtles.

Hazen envisions applying similar strategies to the Gulf of Mexico, where yellowfin tuna fishermen often kill Atlantic bluefin as by-catch. Although Block’s research led NOAA to close some areas in the Gulf of Mexico to longlining in 2015, dynamic restrictions might someday prove to be an even more precise approach—one that maximizes target catch while preventing by-catch. “This could be a win-win,” Hazen says. “You could better target the closures, so you end up closing less ocean and the fisheries in turn have more area to operate.”


Bluefin tuna can exceed 300 kilograms and are heavily targeted for sushi. Photo by Randy Wilder/Monterey Bay Aquarium

And then, of course, there’s climate change, which has helped raise global sea surface temperatures by as much as a full degree Celsius over the past 140 years. A better understanding of the bluefin tuna’s thermal limits could help scientists and fisheries managers predict how the fish will respond to further warming and identify future protected areas.

Back in Block’s office, the physiologist gestures out her window toward the cerulean waters of Monterey Bay, a vibrant tableau of marine biodiversity. Hundreds of cormorants air out their wings atop guano-stained rocks just offshore, and humpback whales blow curtains of mist into the warm breeze. To Block, the scene conjures more questions. “We struggle to answer even the basic question of why the whales are here today and not four days ago,” she says ruefully. Until scientists can measure when animals are eating, how much energy they’re taking in, and how energy flows through a food web, Block adds, they won’t truly understand ocean ecosystems.

Any fisherman who’s tracked seabirds to a bait ball recognizes one of nature’s most immutable laws: predators follow prey. But a marine animal’s niche is also dictated by the ocean conditions where its heart functions the best, where it most easily metabolizes food, where its blood vessels exchange heat most effortlessly. Time to grant physiologists a seat at the marine management table.