Science shows a path to recover Pacific bluefin tuna

The ruby-red slice of maguro presented on a piece of nigiri sushi does nothing to convey the sheer power of Pacific bluefin tuna. These top ocean predators can grow to be twice the size of lions; at top swimming speed, they’re faster than gazelles. But it’s been a huge challenge to halt the decline of these incredible fish.

Pacific bluefin tuna at the Monterey Bay Aquarium. Photo by Monterey Bay Aquarium/Randy Wilder

The Pacific bluefin population is down to just 2.6 percent of its unfished level—yet it continues to face intense fishing pressure. The fish are prized commercially, command staggering market prices, and are difficult to manage because they cross through national and international waters on trans-Pacific migrations.

Monterey Bay Aquarium has long advocated for use of the best available science to inform management decisions that can bring the Pacific bluefin population back to a healthy level. Now researchers at the Aquarium, together with colleagues from Harvard University and the National Museum of National History, have identified new evidence of migration trends that underscore the need for comprehensive fishing restrictions and enforcement across the Pacific—especially in the Western Pacific, where all Pacific bluefin spawn, and where most of the fish are caught.

The source of spawning-age fish

The analysis, published in Science magazine, concludes that—in many years—the majority of spawning-age bluefin tuna in the Western Pacific are migrants who left the waters off Japan when they were just one to two years old, and spent the next four to six years on rich feeding grounds off the coasts of California and Mexico, before returning to the Western Pacific.

New research supports the need to limit fishing for Pacific bluefin tuna — and to enforce the limits — in order to recover the species. Photo courtesy NOAA

If too many of the young fish are caught in the Western Pacific before they can make the migration east, there won’t be enough returning fish years later to maintain or recover the already-depleted population.

And if fishing pressure is too great in the Eastern Pacific, the fish won’t survive to make the migration back to their spawning grounds near Japan.

“These fish were passing through two gauntlets, in the west and in the east, before they had a chance to spawn,” said Dr. Andre Boustany, the Nereus Principal Fisheries Investigator for the Aquarium. “Many fish have to pass through both the Western and Eastern Pacific Ocean. So by taking too many of them out in both locations, we end up with a severely depleted population.

“We need much better management of the fishery in the west, and to continue to at least maintain current management in the east,” he added.
Continue reading Science shows a path to recover Pacific bluefin tuna

Dispatch from the Sea of Japan: Tagging takes teamwork

The Conservation & Science team at the Monterey Bay Aquarium has worked for more than two decades to understand and recover bluefin tuna populations – particularly Pacific bluefin tuna, whose population has declined historically due to overfishing. A key piece of our efforts is tagging bluefin tuna in the wild so we can document their migrations across ocean basins. Much of our work takes place in the Eastern Pacific, but this month we’re partnering with Japanese colleagues to tag bluefin tuna in the Sea of Japan. Tuna Research and Conservation Center Research Technician Ethan Estess, working with Program Manager Chuck Farwell, is chronicling his experience in the field. This the first dispatch in his series.

Tags are laid out on a tatami mat, prepped and ready for use when the tagging team heads out to sea.
Tags are laid out on a tatami mat, prepped and ready for use when the tagging team heads out to sea.

The alarm buzzes beside my head and, opening my eyes, I have no idea where I am.

I’m lying on the floor of a room covered wall-to-wall in woven straw mats, with rice paper windows and a table rising a foot off the ground. Right. Japan. Sado Island in the Sea of Japan, where I’m sleeping on a traditional tatami mat. Yesterday’s cannery whistle is blowing back home at the Monterey Bay Aquarium at noon, but my 4 a.m. alarm tells me it’s time to get up and find some Pacific bluefin tuna.

One year ago, I sat in a third-floor office at the aquarium with my colleague Chuck Farwell and Dr. Ko Fujioka from Japan’s National Research Institute of Far Seas Fisheries as we went through the steps necessary to deploy a satellite tag on a bluefin tuna. We discussed the tag attachment system that anchors the small electronic device to the animal for up to a year at a time, as well as the process for programming the tag’s onboard computer to record the whereabouts and behaviors of these wide-ranging fish. Continue reading Dispatch from the Sea of Japan: Tagging takes teamwork

It takes a global village to conserve bluefin tuna

From Jan. 18-20, 2016, Monterey Bay Aquarium and Stanford University convened many of the world’s leading bluefin tuna researchers, policymakers and stakeholders for the Bluefin Futures Symposium in Monterey. Together, this diverse group of experts explored opportunities for international collaboration with a common goal: healthy and sustainable wild bluefin tuna populations across the world’s ocean.

Aquarium Executive Director Julie Packard welcomes participants to the Bluefin Futures Symposium.

The future of bluefin tunas is in the hands of the global community. It depends on our collective ability to work together across sectors — including scientists, governments, businesses and non-governmental organizations — to improve fisheries management and rebuild bluefin tuna populations to sustainable levels.

Nearly 200 experts, representing every
region where bluefin tunas are found, came to Monterey to participate in this unique forum.

“This symposium has filled a clear need for a time and a place where we can have open discussion and inform each other about techniques and strategies that link official science and management decisions with key academics, experts and stakeholders,” said Margaret Spring, the Aquarium’s Chief Conservation Officer and Vice President of Conservation and Science.

Panelists discuss tuna management at the Bluefin Futures Symposium.

The symposium’s first day featured scientific experts from around the world, presenting their latest research on all three bluefin species—Atlantic, Pacific and southern. On the second day, discussions turned to best management practices, exploring how fisheries managers and scientists can work together. Day three focused on key challenges and opportunities, including breakthroughs in bluefin aquaculture, the economics of the tuna trade, and the potential impacts of climate change on bluefin populations.

Read our news release to learn more about the big questions tackled at the symposium.

Featured photo: Bluefin tuna art, on display at the symposium, celebrates the beauty and bling of these powerful ocean predators.


Meet our new director of science: Dr. Kyle Van Houtan

Dr. Kyle Van Houtan, a conservation ecologist with expertise in marine biodiversity and global change, has been named director of science at the Monterey Bay Aquarium.

Dr. Kyle Van Houtan
Dr. Kyle Van Houtan

In his new position, Dr. Van Houtan will manage, coordinate and strengthen our science programs and partnerships. This includes conservation research focused on sea otters, great white sharksPacific bluefin tuna and other iconic California Current species and ecosystems.

For the past six years, he has led several initiatives in global change and protected species from the director’s office at the National Oceanic and Atmospheric Administration’s (NOAA) Pacific Islands Fisheries Science Center in Hawai’i. His research and teaching focus on multi-faceted approaches to marine biodiversity conservation, and his work spans a range of topics from animal behavior, foraging ecology and physiology, to fisheries stock assessments, climate change and ecosystem-based management.

His work includes studies using bomb radiocarbon from Pacific nuclear tests to help in sea turtle conservation.
His work includes studies using bomb radiocarbon from Pacific nuclear tests to help in sea turtle conservation.

His latest research paper uses bomb radiocarbon from Pacific nuclear tests to aid in the conservation of critically endangered hawksbill sea turtles. He has also spoken and written widely about issues of environmental policy and ethics.

Dr. Van Houtan earned his undergraduate degree at the University of Virginia, a master of science from Stanford University and his Ph.D. from Duke University, where he serves as an adjunct associate professor in the Nicholas School of the Environment.

A passionate science and conservation communicator, his research has been featured on National Public Radio, in the New York Times, Nature, Science, National Geographic, WIRED andSmithsonian. He is also a recipient of the Presidential Early Career Award for Scientists and Engineers from President Obama for his pioneering research into how climate influences sea turtle populations.

When he received the award, NOAA Fisheries Chief Science Advisor Richard Merrick, noted that Dr. Van Houtan “has shown how a deep understanding of biology, ecology, and climate science can provide answers to the important question of how climate change can affect animal populations over decades and vast geographies.”

“We are fortunate to have Kyle Van Houtan as our director of science,” said Margaret Spring, vice president of conservation and science, and chief conservation officer for the aquarium. “He brings new perspectives to our work on behalf of iconic ocean wildlife at a time when marine ecosystems face unprecedented challenges from climate change and ocean acidification.”

Learn more about our Conservation Research initiatives.

Talking tuna with Maria Damanaki

From Jan. 18-20, Monterey Bay Aquarium and Stanford University are convening the world’s leading bluefin tuna researchers, policymakers and stakeholders for the Bluefin Futures Symposium in Monterey. Using the power of its global expertise and diverse perspectives, the group is exploring opportunities for international collaboration toward healthy and sustainable wild bluefin tuna populations across the world’s ocean.

One of the symposium’s keynote speakers is Maria Damanaki, global managing director for oceans at The Nature Conservancy and former European Union Commissioner for Maritime Affairs and Fisheries.

Maria Damanaki calls bluefin tunas “silver bullets.” The massive fish are some of the fastest creatures in the sea, tearing through the water in shimmery flashes beneath the waves.

There is, unfortunately, no silver bullet to ensure that bluefin tunas are managed sustainably. A long history of overfishing the three major bluefin species reduced their numbers to very low levels, and regional management efforts over the past 50 years have had mixed results in recovering these stocks. The Pacific bluefin tuna population is still in decline, while the southern bluefin, and some populations of Atlantic bluefin, are seeing some improvements.

Atlantic bluefin tuna_NOAA
Illustration of an Atlantic bluefin tuna. Courtesy of NOAA.

Damanaki is optimistic that the Bluefin Futures Symposium will facilitate an honest dialogue among the diverse international interest groups and governments with stakes in bluefin tuna management. At the end of the day, she hopes there will be an encouraging outcome.

“This positive message has to be spread, because we need hope,” Damanaki said. “We need to believe that we can do it, and then we’re going to deliver.”

Strict European Union quotas for bluefin tuna and other fisheries sparked a recovery for many overfished species. Photo © EU - Jari Leskinen
Strict European Union quotas for bluefin tuna and other fisheries sparked a recovery for many overfished species. Photo © EU – Jari Leskinen

As European Union Commissioner for Maritime Affairs and Fisheries from 2010 to 2015, Damanaki implemented a strict monitoring program for Atlantic bluefin populations in the Mediterranean Sea. Under her leadership, the European Commission cut the eastern Atlantic bluefin tuna catch quota by over 50 percent and restricted fishing to just one month per year. As a result, the population of eastern Atlantic bluefin tuna have shown signs of recovery.

Still, Damanaki says, sustainable bluefin tuna management has a long way to go. The symposium represents an important step to establish effective and lasting international strategies by bringing together the key people who represent what Damanaki considers the five pillars of bluefin management: the fishing industry, science, policy, non-governmental organizations and technology.

Global distribution of bluefin tunas. Courtesy Marine Conservation Science Institute.
Global distribution of bluefin tunas. Graph courtesy Marine Conservation Science Institute.

Damanaki hopes the science presented at the symposium will inspire additional research. One of the most pressing needs for each of the three bluefin species: improved  population estimates. It’s not easy to count fish that move at fast speeds and travel long distances through the open ocean, even with help from advanced computer modeling. But it’s essential data for policymakers considering tradeoffs between short-term economic interests and long-term recovery of bluefin populations.

Of course, a sustainable future for bluefin tunas depends on more than just data. Damanki says it’s critical for scientists, politicians, NGOs and industry to work together on this issue. And she stresses the importance of partnering with fishers by giving them incentives to cooperate.

“We can’t do this without them.” Damanaki says. “We need all of them to come to a good solution.”

— Lisa Marie Potter

A powerful collaboration to ensure the future of bluefin tunas

From Jan. 18-20, Monterey Bay Aquarium and Stanford University will convene the world’s leading bluefin tuna researchers, policymakers and stakeholders for the Bluefin Futures Symposium in Monterey. Using the power of its global expertise and diverse perspectives, the group will explore opportunities for international collaboration. Together, the participants will work to create a roadmap toward healthy and sustainable wild bluefin populations across the world’s ocean.

 Here, the Aquarium and three symposium sponsors share their thoughts and hopes for Bluefin Futures.

Margaret Spring

Vice President of Conservation & Science and Chief Conservation Officer, Monterey Bay Aquarium

For decades, scientists around the world have been on a quest to understand bluefin tunas, some of the ocean’s most fascinating, powerful and mysterious top predators. Through our 20 years of collaboration at the Tuna Research and Conservation Center, Monterey Bay Aquarium and Stanford University have made significant scientific contributions to our understanding of these amazing animals.

Margaret Spring
Margaret Spring

But given increasing global threats, a future with abundant bluefin tunas will require internationally coordinated research, conservation and management. In a global first, the Bluefin Futures Symposium will bring together many of the world’s leading experts on all three bluefin tuna species in advance of international scientific and management meetings. During the symposium, we’ll be sharing the latest research, science-based management approaches and opportunities to define a sustainable path forward for these iconic and ecologically important species.

Monterey Bay Aquarium is honored to co-host this gathering of world experts, and we’re grateful to our many sponsors. I look forward to hearing from our distinguished speakers and participants on how, working together, we can achieve conservation success for bluefin tunas.


Campbell Davies

Senior Principal Research Scientist, Marine and Atmospheric Research, Commonwealth Scientific and Industrial Research Organisation

Bluefin tuna are iconic species which have captured the imagination of people around the globe for millennia. CSIRO has a long history of association with southern bluefin tuna. Our multi-disciplinary research program, over many decades, has made substantial contributions to our knowledge of the species, methods for studying them (such as archival tagging technology), and development of the early cooperative science and management arrangements required to manage the international fishery.

Campbell Davies
Campbell Davies

We are very pleased to support the Bluefin Futures Symposium. The meeting provides the first opportunity for the global bluefin tuna community to come together and share their understanding of the current state and potential futures for these spectacular fish and the diverse range of fisheries they support.

I’m looking forward to an insightful and productive meeting. I hope it will stimulate the next phase of international cooperation required to conserve the world’s bluefin populations and the fisheries, societies and cultural traditions that depend on them.


Amanda Nickson

Director of Global Tuna Conservation, The Pew Charitable Trusts

This important symposium is a global call to action. We must work together to identify conservation solutions for bluefin tuna that are grounded in good science. With such a large representation of viewpoints and expertise from the world’s foremost bluefin scientists, managers and stakeholders, there is an opportunity to complete a roadmap for ending overfishing of all bluefin populations, rebuilding them,​ and putting safeguards in place to ensure bluefin are never overfished again.

Amanda Nickson (Photo by The Pew Charitable Trusts)

High human demand coupled with insufficient management has put at risk all bluefin tuna species at some point during the last 50 years. There are still populations that have been fished to the brink. The Pacific bluefin tuna catch has been so high that just 4 percent of the population remains today. Current management measures won’t do enough to reverse the decline.

All bluefin species should be valued and managed as much for their ecological importance as they are​ for the price they command at market. It’s especially critical to cooperate and collaborate to save Pacific bluefin.


Russell F. Smith III

Deputy Assistant Secretary for International Fisheries, National Oceanic and Atmospheric Administration

The Bluefin Futures Symposium provides a unique opportunity for us, as scientists and managers, to consider how best to manage bluefin tuna stocks in order to ensure their long-term sustainability.

Russell Smith
Russell F. Smith III

Working through the regional fishery management organizations, we should develop harvest strategies that take full advantage of innovations – for example, through use of management strategy evaluation – that help us respond to what we know about changes in the stocks and the fisheries.

At the same time, we should apply a precautionary approach to take uncertainties into account. We must also adopt objectives that lead to meaningful progress in rebuilding depleted stocks. Like the joint tuna regional fisheries management organization process (also known as the Kobe process), this symposium allows us to compare experiences in different oceans and consider how lessons learned might apply elsewhere.

Tirelessly tracking bluefin tunas

Bluefin tunas are among the ocean’s most fabulous fish. Sleek and strong, they cross oceans in mere weeks, warm their bodies by capturing their metabolic heat,  and live for decades. They’re also prized commodities, especially as sushi in restaurants around the world. Given bluefin’s high cultural and economic value, overfishing has driven some populations of these prized ocean predators into steep decline.

How to rebuild bluefin populations remains a critical question — one science can help us answer.

Tuna swim in a flume, also known as a “tuna treadmill,” at the Tuna Research and Conservation Center.

Researchers and fisheries managers around the world are working to protect and recover bluefin tuna populations. But conservation efforts must be informed by basic science: When do bluefin mature? Where do they travel in the ocean? When do they stop to eat?

From Jan. 18-20, Monterey Bay Aquarium and Stanford University will convene the world’s top bluefin researchers, policy makers and stakeholders to share cutting-edge data and new approaches to conserving these iconic species. Together, they’ll look to identify areas for international collaboration.

The Tuna Research and Conservation Center, a partnership between the Monterey Bay Aquarium and Stanford University.
The Tuna Research and Conservation Center is a partnership between Monterey Bay Aquarium and Stanford University.

The upcoming meeting is the brainchild of Stanford University professor Barbara Block, who’s devoted her research career to the hows and whys of bluefin tunas. It also reflects three decades of collaboration between Barbara and Chuck Farwell, the Aquarium’s Tuna Research and Conservation Manager.

In 1993, Barbara was recruited to Stanford from the University of Chicago. During the visit, she and Chuck hatched a plan to join forces and build a tuna facility at Stanford: the Tuna Research and Conservation Center (TRCC). They hoped to jointly accomplish two missions: to help the Aquarium exhibit tunas, and to start a research facility specializing in the biology of these Olympic-caliber athletes.

The science of “fish and chips”

For more than 20 years, the TRCC team has focused on big-picture tuna challenges. First up was learning how to keep yellowfin and bluefin tunas in captivity — research that eventually enabled the Aquarium to display the sleek predators in the Open Sea exhibit.

Tuna in a TRCC tank. Photo ©Monterey Bay Aquarium, by Tyson V. Rininger
Tuna in a TRCC tank.

In 1996, the TRCC team began asking where tunas go in the wild. Barbara had worked with the father of tuna biology, Dr. Frank Carey (to whom the TRCC lab is dedicated), to track tunas with telemetry. Using tracking technology, the team  has explored questions of where tunas travel in the ocean and how their bodies handle the extreme conditions they face on their migrations — between continents, from subtropical to temperate waters, and to depths of more than 6,000 feet. Their findings are helping inform conservation practices that could help bluefin tuna populations recover in years to come.

The TRCC team has mapped the migrations of hundreds of Pacific bluefin tuna.
The TRCC team has mapped the migrations of hundreds of Pacific bluefin tuna.

The TRCC team’s research has been especially challenging and transformative for one reason: It’s difficult to understand where animals go, and what they do, when they’re underwater and far from shore.

“Most of us from [a] ship — even I — look out at the ocean and see a homogeneous sea,” Barbara explained during a 2010 TED talk. “We don’t know where the structure is. We can’t tell where the watering holes are, like we can on an African plain.”

Using the “fish and chips” strategy, TRCC scientists have uncovered critical information about where tunas travel. In the early 2000s, they documented tunas making transoceanic journeys. Some of the bluefin born in Japan travel to the California coast, and some born in the Gulf of Mexico travel to the European coast. The discovery of these fishes’ highly migratory behavior has greatly improved our understanding of all three bluefin species, and informs international negotiations on conserving bluefin tuna populations.

Warm-blooded but cold-hearted

Other studies have uncovered where bluefin tunas eat and where they spawn — two crucial bits of information when it comes to protecting them and essential tuna habitats. A recent paper in the journal Science Advances identified key bluefin tuna feeding locations in the Pacific, and determined they prefer searching for food in specific conditions.

Chuck Farwell at the Tuna Research and Conservation Center. Photo ©Monterey Bay Aquarium, by Tyson V. Rininger
Chuck Farwell checks in on fish at the Tuna Research and Conservation Center.

“They tend to select a certain temperature range to live in,” Chuck explains. “They also have the ability to dive and explore in very warm or very cold water, for short periods of time.”

In collaboration with tuna researchers in Japan, Chuck and the TRCC have been working in the Sea of Japan to find out where Pacific bluefin spawn, and what habitat the young fish utilize as they develop. Their work should be published later this year.

The TRCC team is making important discoveries about bluefin physiology, too. Unlike most fishes, tuna are warm-blooded, or “endothermic,” meaning they can heat their bodies above the temperature of the surrounding ocean. But not every body part gets warmed equally. Bluefin maintain heat in their eyes, brain, swimming muscles and guts. But their hearts are cold, experiencing temperature drops of tens of degrees Celsius during deep dives. How do tuna manage to keep their hearts pumping at temperatures that would stop a human heart?

Barbara Block and other TRCC researchers have perfected the art of tagging giant bluefin tuna at sea.
Barbara Block and other TRCC researchers have perfected the art of tagging giant bluefin tuna at sea.

In 2015, Barbara and colleagues published a paper in the Proceedings of the Royal Society of London B that answered this question. They found that adrenaline was the secret. Cold temperatures trigger an adrenaline rush, which helps maintain the level of calcium in tuna hearts. Without calcium, the heart would not be able to beat normally at extremely cold temperatures.

In May, Barbara will receive the 2016 Peter Benchley Ocean Award for Excellence in Science. The award is just one of several she has earned over the past two decades — including a MacArthur Foundation “genius grant” — but her tireless work is far from finished. There are still hundreds of questions to be answered, more bluefin to track, and populations to preserve.

A chance to inspire change

Illustration of an Atlantic bluefin tuna – one of three bluefin tuna species, along with southern and Pacific.

By tagging bluefin tuna in the wild and learning more about their physiology in captivity, the TRCC team is producing data crucial to sustainable management. Barbara hopes that by bringing together global scientists, fishers, managers and policymakers, we can ensure that collaboration increases, transfer of knowledge improves, and the steep decline of bluefin populations in the Pacific and the western Atlantic reverses in her lifetime.

Chuck has high hopes the Bluefin Futures Symposium will bring the science to bear on management solutions. “Everyone at the Aquarium that’s involved in this has high expectations there will be positive outcomes,” he says.

Learn more about the Bluefin Futures Symposium at

— Diana LaScala-Gruenewald

Featured photo: Dr. Barbara Block was honored in the Rolex Awards for Enterprise in 2012. Photo by Rolex.

Chuck Farwell: Tuna Scientist, Mentor, Fish Whisperer

No two days are the same in the life of Chuck Farwell, manager of the Aquarium’s Tuna Research and Conservation Program. Some days he’s helping the husbandry team maintain our stock of Pacific bluefin tuna. Other days he’s on a boat at sea, surgically implanting electronic tracking tags into the bellies of fish. And some days he’s in Japan, advocating for the conservation and preservation of the Pacific bluefin tuna.

Chuck Farwell at the TRCC
Chuck Farwell at the TRCC

Chuck has been working with tuna since the 1960s, when he first surveyed albacore tuna ranges for the California Department of Fish and Wildlife. He joined the Aquarium before it opened in 1984, with a long-term vision of developing husbandry techniques to allow us to keep and maintain tuna. At the time, no aquarium outside Japan had ever kept tuna on permanent exhibit. In 1996, Monterey became the first, with displays of yellowfin and bluefin tuna.

Now, Chuck focuses on Pacific bluefin tuna, large predators that can migrate across ocean basins in a matter of weeks. They’re beautiful, lightning-fast and as majestic as they are delicious. The species is prized among seafood enthusiasts – primarily for the high-end sushi trade.

Dramatic population collapse Continue reading Chuck Farwell: Tuna Scientist, Mentor, Fish Whisperer

Feeding hotspots for Pacific bluefin tuna – discovered

After chowing down a big meal, you might feel your belly warm as your stomach muscles and digestive organs set to work breaking your food into smaller and smaller pieces rich in nutrients. A bluefin tuna’s stomach experiences a similar spike in temperature when it gulps down a mouthful of juicy sardines.

Chuck Farwell of the Monterey Bay Aquarium and Barbara Block of Stanford University tag a Pacific bluefin tuna for the study. Photo courtesy Tuna Research and Conservation Center
Chuck Farwell of the Monterey Bay Aquarium and Barbara Block of Stanford University tag a Pacific bluefin tuna for the study. Photo courtesy Tuna Research and Conservation Center

Now, scientists at Stanford University, Monterey Bay Aquarium and the National Oceanic and Atmospheric Administration (NOAA) have devised a way to measure that internal temperature increase in the fish – and connect it to how much the tuna ate and where it consumed its meal.

This is the first work to measure how much energy aquatic animals consume in the wild, and has allowed researchers to identify favorite dining spots for Pacific bluefin tuna along the North American coastline. The findings are published online in Science Advances, and could help develop conservation policies to protect a species that’s in steep decline.

Pacific bluefin tuna are superbly streamlined, bullet-shaped fish, with powerful swimming muscles capable of powering transoceanic travels. Unlike most other bony fishes, they are warm bodied, able to elevate their internal tissue temperatures above that of the surrounding water.

“Bluefin tuna are the pinnacle of bony fish evolution, endothermic or warm-bodied in a manner that rivals the metabolic performances of birds and mammals,” said senior author Barbara Block, a professor of marine sciences at Stanford’s Hopkins Marine Station and a senior fellow at the Stanford Woods Institute for the Environment.

Following the fish

Bluefin tuna remain warm by capturing the metabolic heat produced as their swimming muscles contract with every tailbeat. This happens via specialized net-like blood vessels, called counter-current heat exchangers, in their muscles and digestive organs that prevent heat loss through the gills. Maintaining warmer-than-water body temperatures allows the fish to swim more efficiently and spend less energy digesting food, and enables them to thrive in a wide range of ecological niches.

At the Tuna Research and Conservation Center, scientists learned to correlate digestion of precise quantities of food with changes in tunas' internal temperatures. Photo Monterey Bay Aquarium/Tyson Rininger
At the Tuna Research and Conservation Center, scientists learned to correlate digestion of precise quantities of food with changes in tunas’ internal temperatures. Photo Monterey Bay Aquarium/Tyson Rininger

The researchers focused on this thermal characteristic to measure energy intake, and from that to surmise the animals’ daily foraging habits. Researchers implanted small data-logging tags in more than 500 tunas off the coast of southern California and Mexico, and recorded the fishes’ body temperature, ambient water temperature, and their locations and diving patterns as they searched for prey. With the help of fishers, the researchers recovered more than one-third of the tags, containing data records as long as three years as the fish made seasonal migrations from the waters off Mexico to Oregon.

Previously, observation work led by Rebecca Whitlock, a postdoctoral scholar at Stanford, made with fish held at the Tuna Research and Conservation Center, had created a model to translate changes in tuna heat signatures into caloric intake. At the center – a partnership between Stanford and the Monterey Bay Aquarium – researchers could count single sardines or squid consumed by individual tunas and match the warming signal in the stomach to the energy value of the prey item digested.

Documenting their dining

The thermal data showed exactly when the tunas ate a meal, and the researchers estimated how much energy a free-swimming bluefin receives per unit of time, as well as how temperature changes impact that energy intake.

Researchers fit a data tag in a Pacific bluefin tuna, one of 500 implanted as part of the study. Photo Monterey Bay Aquarium/Andre Boustany

“We’ve been able to follow what Pacific bluefin tuna do in the open sea and record their feeding and meal size, every day for up to three years,” said Whitlock, the lead author of the new paper. “Combining laboratory observations with electronic tagging can provide amazingly rich data and insights into the life of a wild marine predator.”

Tag data showed that wild tunas consumed prey on 90 percent of the days observed during the study. The empirical data analyses and energetic model output allowed scientists to chart precisely how much the fish ate – typically sardines and squid – and the total energy they consumed as they journeyed through the ocean.

A roadmap in the ocean

From this, the scientists mapped the position data from the tags to satellite observations of sea temperature, chlorophyll levels and ocean currents – all factors that can combine to create nutrient-rich feeding grounds. The location of these feeding grounds coincided well with successful tuna feedings, though interestingly the fish didn’t always camp out at places with the best conditions to take advantage of the buffet.

“Foraging success was correlated to environmental features,” said co-author Elliott Hazen, a research ecologist with NOAA’s Southwest Fisheries Science Center. “Tuna may use the oceanography as a roadmap to move from hotspot to hotspot, and temperature appears to be the most important environmental cue.”

Interestingly, the study demonstrated a potential tradeoff between feeding in the richest areas and avoiding the physiological constraints associated with feeding in waters that are either very cold (which slows the heart) or very warm (which are energetically taxing). This answered a long-standing question about the species’ traditional range limits, from north of Oregon to south of the Baja Peninsula, despite the fact that close relatives of bluefin (yellowfin and albacore tuna) thrive outside of those latitudes.

Good places to digest

Tag data showed that tuna tend to stay in waters where they can remain at an optimum temperature to promote rapid digestion. Too high or too low an ocean temperature, and the increased demands of digestion can strain the cardiovascular system.

Preferred feeding habitats varied by season and by size of the fish.
Preferred feeding habitats varied by season and by size of the fish.

“Digestion is metabolically costly, and the bluefin are doing it most efficiently,” Block said. “Our results suggest that physiological constraints on the tunas’ whole organismal performance constrain their thermal distribution, and thus the latitudinal distribution of the fish.”

Block calls this portion of the Pacific Ocean the “Blue Serengeti,” an open ocean ecospace where currents concentrate nutrients and plankton, attracting forage fish such as sardines or anchovies, which in turn lure larger predatory fish such as bluefin tuna.

Understanding the locations of Blue Serengeti “watering holes” for these large migratory fish remains a mystery, but is a key part in planning better conservation efforts that protect essential habitat. Linking the regions both physiologically and to environmental correlates has been an objective of the research team.

The new work helps close that gap by identifying feeding hotspots (areas of highly successful feeding) for Pacific bluefin tuna: along the Baja Peninsula in June and July, off Northern California from October to November, and near Central California in January and February.

“Our results add to our understanding of predator-prey dynamics in the California Current,” Block said. “By understanding where bluefin forage most, we can help protect these places and improve efforts to rebuild Pacific bluefin tuna stocks.”

– Bjorn Carey is a life sciences public information officer for Stanford News Service

Learn more about the aquarium’s efforts to recover Pacific bluefin tuna.

Reference: Rebecca E. Whitlock, Elliott L. HazenAndreas WalliCharles FarwellSteven J. BogradDavid G. FoleyMichael Castleton and Barbara A. Block. “Direct quantification of energy intake in an apex marine predator suggests physiology is a key driver of migration.” Science Advances  25 Sep 2015: Vol. 1, no. 8, e1400270 DOI: 10.1126/sciadv.1400270

Tuna Commission fails to adopt new conservation measures for highly depleted Pacific bluefin tuna

Statement of Margaret Spring, Vice President of Conservation and Science and Chief Conservation Officer, Monterey Bay Aquarium

“The Monterey Bay Aquarium is disappointed that nations at the 89th meeting of the Inter-American Tropical Tuna Commission were unable to reach agreement on any new measures to conserve Pacific bluefin tuna populations. Pacific bluefin tuna are key top predators in the ocean, but the population’s breeding stock has been depleted to approximately 4% of historic levels. As the population continues to decline, we need all Pacific nations to collaborate and commit to a science-based, long-term recovery plan that will result in a healthy, sustainable Pacific bluefin population.

Chuck Farwell of the Monterey Bay Aquarium and Barbara Block of Stanford University tag a Pacific bluefin tuna as part of a collaborative long-term research effort. Credit: Monterey Bay Aquarium/Tyson Rininger
Chuck Farwell of the Monterey Bay Aquarium and Barbara Block of Stanford University tag a Pacific bluefin tuna as part of a collaborative long-term research effort. Credit: Monterey Bay Aquarium/Tyson Rininger

“We applaud the United States’ leadership in advancing a proposal at last week’s meeting to support new scientific analyses and collaboration across international science advisory bodies that could improve future conservation and management decisions. We strongly support the United States’ effort to include a science-based recovery target, known as maximum sustainable yield, as an indication of what measures are needed to ensure the long-term conservation and sustainable use of the species.

“Despite significant support from most Member nations, the Commission could not reach a consensus on the U.S. scientific analysis proposal, and unfortunately did not adopt any new measures.

“Now is the time for all nations fishing in the region to think beyond purely domestic concerns and commit to a Pacific-wide plan to reverse the decline of Pacific bluefin tuna in a meaningful, responsible and cooperative manner. This must start with a commitment to strictly adhere to scientific advice in establishing rebuilding targets. And it must include serious consideration of other conservation measures, such as protection for bluefin spawning areas.

“The next opportunity for action comes when the Northern Committee of the Western and Central Pacific Fisheries Commission meets in Japan at the end of August. We urge all parties to pledge their support for new international research investments, including electronic and emerging techniques for tagging and tracking bluefin tuna populations across the Pacific. This commitment is essential to strengthen conservation measures, and to advance a science-based, long-term rebuilding plan that will recover the species to sustainable levels.”

Learn more about the Bluefin Futures Symposium we’re hosting in January 2016