For over two decades, Monterey Bay Aquarium and Stanford University have partnered to study some of the world’s most mysterious ocean predators at the Tuna Research and Conservation Center (TRCC). Some of the latest work to come from the TRCC include an innovative tuna tag design, and a paper recently published in the journal Sciencedetailing the discovery of a hydraulic mechanism in tuna dorsal fins, which helps them swim with speed and precision.
In his office at Stanford University’s Hopkins Marine Station in Pacific Grove, California, Dr. Vadim Pavlov holds a pale, sleeve-like device. Its smooth lines and soft edges make it seem more like a child’s toy than a high-tech scientific product. He slips the device over a model of a dolphin dorsal fin and “swims” it around his office, mimicking a dolphin’s movements as it leaps and twists out of the water.
The device is a prototype of a new tag design intended to track top ocean predators, such as sharks and tunas, without using pins and bolts that penetrate the fin.
“Even when the dolphin leaps, the tag stays on,” Vadim says. “But, how did we do it?”
Form and function
Vadim is one of the world’s top experts in biomimetics: the science of translating natural phenomena, such as the flow of water over a dolphin’s dorsal fin, into useful technology.
For years, he’s been tackling the challenge of tagging and tracking wildlife in the open ocean. He wanted to provide “animal-friendly” tags as an alternative to the invasive bolt tags anchored into the fins of apex marine predators such as sharks, dolphins and tunas. For Vadim, that’s not just a scientific goal; it’s personal, inspired by his experience as a free diver. “I don’t like swimming with lots of gear, so I don’t think [animals] do either,” he says. “They are very sensitive to anything on their bodies.”
Traditional bolt tags, a key tool in marine animal field studies for the last half century, are kind of like an ear piercing. Researchers punch through the cartilage and collagen in the dorsal fin and attach tags that can help track the animals, or collect environmental data such as salinity, temperature, and depth.
“But over time, these bolt tags do not move with the animals,” Vadim explains. “They can alter the flow of water around the animal’s bodies, and can even cause animals to turn more in one direction over time,” he says. “The faster the animal swims, the greater the energy needed to override the drag.”
Smaller animals, such as harbor porpoises and juvenile dolphins and sharks, are especially susceptible to the pitfalls of traditional bolt tags. “There’s a conflict between the animal’s biology and the technological requirements of the tag,” says Vadim. “So my job became how to reconcile that disconnect.” Continue reading Designing an animal-friendly fin tag
The Monterey Bay Aquarium is a science-driven organization, and rigorous science underpins all of our public policy, research and education programs. Much of our research centers on marine life that visitors can also see in our exhibits – from sea otters to sharks and tunas, even our giant kelp forest. Here’s some of what we’ve learned over the past 30-plus years that is contributing to conservation of key ocean species and ecosystems.
Sea otters crack open tool-use secrets
Revolutionary female scientist Jane Goodall was the first person to discover that chimps use tools and live within complex social systems. Our team of female researchers are walking in Jane’s footsteps with their recent studies on use of tools by another mammal: the sea otter. When observing sea otters along the Monterey Peninsula, sometimes we can hear a “crack, crack, crack!” above the roar of the tide. That sound comes from sea otters using rocks and other tools to open prey items, such as crabs or bivalves, as they float on their backs. Sea otters are avid tool users, but until recently not much was known about how sea otters choose their tools, what aspects of their environments influence tool use, or whether they teach tool use to other otters. The Aquarium’s decades of research into sea otter behavior provided years of observations of sea otter foraging and tool-use behavior, including sea otter pups pounding empty fists against their chests. Could such activity be instinctual? Research Biologist Jessica Fujii has devoted much of her young career to studying the frequency and types of tools used and whether tool use can be coded in sea otter genes. Jessica is looking ahead to see how sea otters learn, teach, and eventually master tool use in the wild.
Sea otter surrogacy helps restore Elkhorn Slough
With 15 years of experience rescuing, rehabilitating, and then releasing surrogate-reared sea otters into Elkhorn Slough, an estuary near Moss Landing, California, the sea otter research team at the Aquarium began to wonder how and if their work was affecting the otter population there. Does releasing a few animals into the slough each year really make any difference? After crunching some serious numbers from the surrogacy program and the U.S. Geological Survey’s (USGS) annual sea otter census, the researchers discovered that it did. Nearly 60 percent of the 140 or so sea otters living in Elkhorn Slough today are there as a result of the Aquarium’s surrogacy program. While we’d known that sea otters served as ecosystem engineers for the giant kelp forests in Monterey Bay, we have now documented that sea otters in Elkhorn Slough are restoring the health and biodiversity of the estuary. This gives us further insights into how sea otters may contribute to coastal ecosystem resilience. Continue reading Using science to save ocean wildlife
The idea seemed like a long shot: Build a video camera that could attach to a great white shark for months at a time, withstand ocean depths of more than 3,000 feet, and sense the shark’s movements to selectively capture footage of its behavior.
But Monterey Bay Aquarium Senior Research Scientist Salvador Jorgensen, a white shark expert, thought it might have a chance if he joined forces with the talented minds at the Monterey Bay Aquarium Research Institute (MBARI).
“Some of the engineering team said it was an impossible job,” MBARI Engineer Thom Maughan recalls with a smile. “But I’m attracted to those opportunities.”
So Thom and Sal teamed up on a high-tech mission: to capture video footage of great white sharks in their most mysterious habitat.
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.
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.
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. Together, the participants are working to create a roadmap toward healthy and sustainable wild bluefin tuna populations across the world’s ocean. Aquarium Executive Director Julie Packard shared these thoughts to open the symposium.
All of us at this important gathering are here because we have a stake in the future of these remarkable fish — whether bluefin tunas are an important part of your country’s culture; as economic opportunity for your communities and nations; or as vital components of a thriving, healthy ocean. Since opening, the Aquarium has helped inspire a transformation in public understanding of the sea, and its role in our future. We know now that living and healthy ocean ecosystems are critical to enabling life on Earth to exist. Their future will determine our future — and in fact, our very survival.
We depend on the ocean in so many ways. It’s our pantry, our lungs, our playground, a massive driver of global commerce, a storehouse for innovation to meet human needs, and a source of inspiration and joy. And, we now know that the ocean is changing. Overfishing, pollution and habitat change — from the coast to the deep sea — have escalated at a dangerous pace. And the vast impacts of carbon pollution, from rising sea levels to acidifying waters, are no longer hypothetical. They’re here now, from Miami to Mumbai.
Today, here on the coast of the continent, at this moment in time, we are in fact, both literally and figuratively, on the edge. We’ve accomplished an amazing array of effective models for ocean governance and conservation, and there’s lots to celebrate. But we’ve got to step up our pace. The collective action of everyone in this room, whatever we decide to do in the next few years, will shape the future for critical species like bluefin tunas.
Examples of success
The good news is that I’m confident we can turn the tide — by investing in people and ideas to demonstrate solutions, and to do the hard work required to reach agreement on a path forward. Why do I think this? Because we have examples right in front of us.
Just one month ago, leaders from around the world gathered in Paris to tackle another, seemingly insurmountable challenge: to put the world on a path to a clean energy future that will spare us from the devastating consequences of unchecked global climate change. Faced with the gravity of the threat, the delegates to COP 21 were able to overcome their disagreements and their differences. They crafted an accord that puts our planet on course to a sustainable future — for people, and for the living ecosystems on which our survival depends.
They didn’t solve the problem in one series of meetings, but the COP 21 negotiations mark a turning point on climate change. We have the opportunity, in the next three days, to mark a similar turning point for the future of bluefin tunas. And there is no better place on Earth for this to happen than here in Monterey, California.
The Monterey Bay Aquarium stands on the site of what was once the largest sardine cannery on Cannery Row — an industrial district that was home to the largest fish-packing operations on the planet until the sardine fishery collapsed because of a combination of mismanagement and ecosystem change.
From exploitation to recovery
Sardines weren’t the only ocean life targeted by humans. People hunted sea otters, sea lions and elephant seals to near extinction right off our shore. Once-thriving whale populations were decimated as their blubber was turned into oil to light our homes. With the loss of sea otters, kelp forests disappeared — in turn affecting habitats for other commercially valuable fish, and altering the very structure of the ecosystem here.
When the Aquarium opened, Monterey Bay was already recovering. How did that happen?
Because people took action to protect and recover wildlife and ecosystems. It started in 1911, with the International Fur Seal Treaty that outlawed commercial hunting of seals, sea lions and sea otters in the North Pacific. Decades later, a devastating oil spill just south of here, in Santa Barbara, triggered a flood of environmental protection legislation in the United States: The Clean Water Act, the Clean Air Act, creation of the National Oceanic and Atmospheric Administration and the Environmental Protection Agency, and many more. In the 1970s, as these laws were being enacted here in the U.S., countries around the world began to focus more attention on conservation of our ocean resources, including the creation of The International Commission for the Conservation of Atlantic Tunas and measures to protect marine mammals. And the wildlife here in California’s coastal ocean began to recover.
Today, when you look out on Monterey Bay, you’ll see the surface waters of one of the largest national marine sanctuaries in the United States. California, in the past decade, created the largest statewide network of marine protected areas anywhere in our nation. And U.S law now mandates science-based catch limits and rebuilding of fish stocks. Our local fisheries are coming back, along with the fishing fleets that rely on healthy fish populations to make a living and to feed our people.
Public inspiration — and scientific rigor
The Aquarium has been a big part of that story because we are more than simply a place for people to come and see living ocean exhibits. Founded by marine biologists, the Aquarium has always had a larger mission: To inspire conservation of the ocean.
Today, we are not only the most innovative and admired public aquarium in the nation. We are also a leader in ocean science — notably in our two decades of research at the Tuna Research and Conservation Center. There, with our partners at Stanford, we have been doing some of the most advanced and innovative science in support of bluefin recovery. We’ve tagged and tracked bluefin tunas in the Atlantic and Pacific — developing an unprecedented body of basic science to inform management of these fish. Because we can keep bluefin and yellowfin tunas in our Open Sea exhibit and in holding tanks at the TRCC, we have also generated information about tuna physiology and biomechanics that is unique in the world. More than 200 scientific papers have come out of the work at the TRCC — with more to come.
Our goal is to share our data in scientific journals and with the modelers, managers and stakeholders who will determine the future of bluefin tunas. We know that developing the science, and adhering strictly to science-based advice in managing these fish, is the key to recovery of their stocks. Put simply, in looking at the health of fisheries around the world, one thing is clear. Well-managed fisheries all have a common ingredient: good scientific data. Where data are poor, it is impossible to establish catch levels that are sustainable in the long term.
Working across ocean basins
We’re working here in the U.S. and internationally to ensure that management decisions are informed by the best available science, and that we share a common vision of sustainability. But this is only possible when we share information with one another, in forums like this. We’re working toward the kind of recovery for bluefin tunas that we’ve seen for other animals and ecosystems here in Monterey Bay, and in California’s coastal waters. But we know that it is much more challenging to orchestrate recovery with species that migrate across political and management boundaries.
We also know that business leadership and engagement are essential to solve the challenges we face. Indeed, the Aquarium was created through collaboration between a technology business leader and conservation scientists.
That’s why I’ve encouraged international business leaders to embrace new approaches and commitments that will ensure economic prosperity and ocean health. It’s a message I’ve brought to The Economist World Ocean Summit and the World Trade Organization. And it’s an approach we’ve advocated in our work with the national ocean commissions in the U.S., and through international initiatives to help Aquarium business partners use their purchasing power to shift seafood production toward a more sustainable future.
We need the same level of commitment in our approach to bluefin tunas.
Cooperation and collaboration
Bluefin tunas are the perfect example of species that can achieve sustainability only when we cooperate and collaborate. Local fleets could head offshore in California this year and catch fish from the population that is here. But those fish will only be here for a few years, at most, before they travel across the Pacific to spawning grounds near Japan. We can’t manage them sustainably just by taking action here in California, in U.S. waters, or even in the Eastern Pacific. The solution requires cross-Pacific collaboration involving all the nations involved, and by two Regional Fisheries Management Bodies.
Southern bluefin tuna have their own unique set of stakeholders and challenges, and the situation in the Atlantic is particularly complex given the scientific uncertainties and number of countries involved. In every case, though, the need for action is urgent — and there are plenty of opportunities for transformational success.
Globally, where bluefin tuna populations are recovering, it’s the result of hard work, investments in research and willingness by all parties to turn things around. The next three days are just the beginning of a new conversation and what it will take, from all of us, to assure the sustainability of bluefin tunas in the future.
If we’re successful — and I’m confident we will be — our commitment to worldwide bluefin tuna recovery will create a lasting model of how to achieve sustainability for a species that is both iconic and a vital player in our global ocean ecosystems. By collaborating, in all the diversity of our perspectives and points of view, we can achieve great things and, together, shape a bright future for 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.
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.
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.
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.
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.
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.
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.
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.
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.
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?
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.
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’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.
“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?
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
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.
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 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.
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.
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.
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 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.
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.
“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.
“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
Reference: Rebecca E. Whitlock, Elliott L. Hazen, Andreas Walli, Charles Farwell, Steven J. Bograd, David G. Foley, Michael 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
Bluefin tunas are among the most remarkable fishes in the ocean – apex predators that migrate across ocean basins for long distances at top speeds. Biologically, economically and culturally significant at a global scale, they have long been the target of lucrative fisheries. Today, they face a range of threats worldwide.
From January 18-20, 2016, this three-day gathering will bring together the world’s foremost bluefin science and management experts to discuss issues that will shape a sustainable future for the planet’s bluefin tuna populations, and to consider a future global vision for bluefin tunas.
The program will cover the latest scientific knowledge for all three species, current and new fisheries management tools, the economics of the bluefin tuna industry and trade, the emerging role of tuna aquaculture, and the impacts of climate change. It’s all with the goal of shaping a vision for healthy, sustainable bluefin populations.