Migration study finds that sweeping management changes are needed to protect Atlantic bluefin tuna

A team of marine scientists has mapped the undersea journeys of Atlantic bluefin tuna and concluded that tighter restrictions should be placed on commercial fishing to protect the feeding and breeding grounds of this top migratory predator–one of the most commercially valuable fish in the sea.

Researchers from Stanford University and the Monterey Bay Aquarium say that their new study, published in the April 28 edition of the journal Nature, offers substantial evidence that significant changes need to be made in how Atlantic bluefin tuna fisheries are managed internationally and in the United States.

“In my lifetime we’ve brought this majestic species to the doorstep of ecological extinction in the western Atlantic Ocean,” says Barbara A. Block, the Charles and Elizabeth Prothro Professor in Marine Sciences at Stanford’s Hopkins Marine Station and lead author of the Nature study. “Electronic tagging provides the best scientific information we’ve ever had to properly manage these tuna and we must, as an international community, start to act responsibly to ensure the future of this species.”

An expert on large migratory fish, Block is a founder and the co-director of the Tuna Research and Conservation Center (TRCC), a joint collaboration between Stanford and the Monterey Bay Aquarium. For the past 10 years, she and her colleagues have braved the rough waters of the Atlantic Ocean to carry out an unprecedented study of bluefin tuna migrations. Working with sport and commercial fishers in the Carolinas and New England, as well as commercial fleets in the Gulf of Mexico, TRCC researchers have placed electronic tags on hundreds of wild bluefin tuna ranging in size from 150 to 900 pounds. The tags track individual fish as they travel thousands of miles across the sea, to depths below 3,000 feet, in search of food and mates. Each tag records the tuna’s migration pattern, diving behavior, body temperature and the temperature of the surrounding water.

Separate stocks

In the Nature study, Block and her colleagues analyzed electronic tagging data over a period of nine years. The results showed that bluefin have a complex migratory life cycle that varies depending on the season as well as the age and body size of the fish.

The study confirmed that the North Atlantic is home to at least two populations of bluefin–a western stock that spawns primarily in the Gulf of Mexico, and an eastern stock that breeds thousands of miles away in the Mediterranean Sea.

Tagging data also revealed that western bluefin from the Gulf spawning grounds routinely swim together as adolescents and adults with their eastern cousins that were spawned in the Mediterranean. The discovery of extensive mixing between the two populations challenges a basic assumption long held by the International Commission for the Conservation of Atlantic Tunas (ICCAT), which has managed Atlantic bluefin since 1969. According to ICCAT, the two populations rarely mix–a finding that led the commission to establish separate quotas for bluefin caught in the western and eastern Atlantic.

But the Nature study suggests a more likely mixing scenario: that large western fish are actually being caught as adults in eastern waters, then mistakenly counted as part of the eastern quota. Likewise, fish of eastern origin spawned in the Mediterranean are probably hunted in the west as adolescents and wrongly included in the western quota. Therefore, say the authors, unless there is a major overhaul of ICCAT’s management system, both populations could suffer declines, particularly the smaller western stock, which has been decimated by the high consumer demand for bluefin tuna.

“Our study indicates that there are management steps we could take today to ensure that giant bluefin tuna are swimming on the western side of the Atlantic basin tomorrow,” Block says. “We believe that it’s time for ICCAT to introduce management measures that recognize the fact that there is a complex spatial and temporal mixing of the two populations in both the west and east Atlantic, except on the spawning grounds. Our data indicate measures, such as fisheries closures in the central Atlantic and on the breeding grounds, that could be taken immediately to protect the weaker of the two stocks–the western Atlantic bluefin.”
Popular delicacy

Renowned for their powerful, warm-blooded physiology, bluefins are among the biggest bony fish in the sea. A mature animal can live to age 30, grow 10 feet long and weigh 1,500 pounds. Bluefin tuna is consumed worldwide, especially in Japan where sushi and sashimi consumers have been known to pay up to $45 a pound for the popular delicacy. In fact, the price for a single giant tuna in the Tokyo fish market has even exceeded $100,000. Little wonder that these animals continue to endure relentless hunting pressure from fishing fleets around the world.

ICCAT estimates that the western population has fallen more than 80 percent since the 1970s, and that the eastern stock also has declined. Today, the commission limits the annual tuna catch in the western fishery to about 3,000 tons–a fraction of the 32,000-ton quota in the east. However, the actual tuna catch in the east is likely to far exceed the allowed quota.

How did ICCAT determine the range of each population? In 1982, armed with the best scientific data available at the time, the commission set up two management zones separated by a line in the mid-Atlantic Ocean at longitude 45 degrees West (45W). The location of this boundary was established at a time when local commercial fisheries on both sides of the Atlantic were separate.

However, recent harvest data from offshore longliners–commercial boats that catch fish on lines 20 to 30 miles long and set with up to 1,200 hooks–indicate that the populations are continuous across this poorly enforced boundary. Similarly, the Nature study demonstrated that electronically tagged bluefin frequently ignore the boundary and travel back and forth across the Atlantic, raising serious questions about whether ICCAT quotas protect vulnerable western bluefin that cross longitude 45W into productive eastern waters to feed. This may be less of a problem for adolescent fish that show a preference for the North American coastline, say the authors. However, bluefin weighing 300 pounds or more, which have a remarkable capacity to stay warm internally, have expanded their range into productive, subpolar seas throughout the North Atlantic, and thus are more likely to cross the boundary, exposing them to oceanic longlines as well as fishers from both sides of the Atlantic.

“Our science doesn’t support maintenance of a management system that assumes tuna from the eastern Atlantic remain in the eastern Atlantic, and tuna from the western Atlantic remain in the west,” Block says. “Managers must incorporate into their models how bluefin as adolescents prefer coastlines and continental margins in the western and eastern Atlantic, and as adults forage as far north as the sub-Arctic seas.”

Tagging technology

In their study, TRCC researchers analyzed data from electronic tags placed on 772 bluefins in the western Atlantic between 1996 and 2004. “It takes a village to tag this many bluefin, and we primarily have the fishers of North Carolina and Massachusetts to thank for helping our team round up the giant tunas in winter seas for tagging,” Block says.

A total of 499 tags were “archival” tube-shaped devices with external sensors that the scientists implanted inside individual fish during shipboard surgeries. A $1,000 reward was offered to anyone who recovered an archival tag, and 88 eventually were returned by commercial tuna fishers throughout the North Atlantic and the Mediterranean. Each tag recorded precious data documenting individual migrations lasting as long as 4.6 years.

“We’ve gotten tags back from fishing fleets based in Canada, Tunisia, Libya, Cuba, Italy, Spain, Japan, Morocco, the United States and other countries working in the North Atlantic–a real international effort,” says Stanford graduate student Andre Boustany, co-author of the study. “Often, the tag returns were accompanied by extensive notes and drawings from fishers indicating how and where the fish were caught.”

The research team also deployed 273 pop-up satellite tags–external devices that detach and float to the surface at a programmed date after collecting around-the-clock data on the animal’s location, diving depth and surrounding water temperature. The vast majority (89 percent) of the pop-up tags successfully transmitted data via satellite back to the Stanford lab.

Separate breeding, mixed feeding

“We observed that Atlantic bluefin tuna in North America sort into two discrete breeding populations–one that spawns in the Mediterranean and the other in the Gulf of Mexico,” Block says. “It appears that some adolescents from the east feed and frolic in the western Atlantic until they’re old enough to become breeders, at which point they go back to the Mediterranean spawning grounds and are unlikely ever to return to North America.”

Tagging data showed that while eastern and western tuna do not visit each other’s breeding grounds, adolescents from both populations regularly forage side by side in specific areas of the North Atlantic.

“We found that North Atlantic bluefin go to the best restaurants in the west and the east Atlantic,” Block says. “It’s not much different than going to a good place to eat in New York and finding a New Yorker and a diner from London at tables that are close to one another. In North America, those restaurants happen to be along the coastline, with aggregations in places like the Carolinas, New England and the Gulf Stream waters. But if you’re big enough and have enough endothermic capacity, or ’internal warmth,’ you go to subpolar seas in the central Atlantic, east of the Flemish Cap off Newfoundland. This area is rich in food and is very productive. And to our surprise, large giant bluefin tagged in the western Atlantic remain in these cold waters seven to nine months of the year.”

The researchers also were surprised to learn that some eastern and western bluefin dine together throughout the central Atlantic in a vast region that extends hundreds of square miles east and west of longitude 45W.

“Right now, any western tuna that swims to the east of the 45 meridian can end up as part of the vastly larger eastern catch,” says graduate student Steven Teo, co-author of the study. “What we’re suggesting from our data is that ICCAT establish a new central Atlantic management zone–a separate area between longitudes 35W and 50W–with an extremely low quota. That way we can reduce the mortality of giant western tuna that regularly forage there. While there may only be a few that actually get caught in these waters, they are the largest of the western Atlantic giant bluefins and, therefore, the fish with the largest reproductive potential.”

Adds Block: “We cannot conserve the western Atlantic population without protecting these fish in the central Atlantic. Or put another way, eastern fishers, particularly the high-seas longliners, may be impacting western recovery.”

Breeding “hotspots”

The Nature study also raised important questions about the protection of western spawning grounds in the Gulf of Mexico. “Relatively little is known concerning where and when bluefin spawn,” Block explains. “It has long been an area of debate among bluefin tuna scientists.”

To find out, she and her colleagues carried out five research cruises in the Gulf of Mexico aboard U.S. commercial longliners. The trips were conducted approximately 180 nautical miles offshore in spring, when stormy conditions make it especially challenging to tag and release a 400- to 1,000-pound giant tuna. Histological examinations of mature bluefin females captured in the Gulf during these trips revealed that most were ready to spawn or were close to peak spawning condition, and that a few already had spawned. These findings “are consistent with previous data indicating that spawning occurs in the Gulf of Mexico in April, May and June,” the authors wrote.

Electronic tracking data revealed that, during spawning season, giant bluefin were primarily found over the continental slope of the Gulf–a region with powerful eddies that bring nutrients to the surface, creating “hotspots” for spawning fish. These breeding hotspots, identified for the first time in the Nature paper, parallel the coasts of Texas, Louisiana, Alabama and Florida. Because they are located inside the Exclusive Economic Zone (EEZ) of the United States, they are subject to regulation by the U.S. National Marine Fisheries Service (NMFS). “Our tagging data, along with NMFS observer data and longliner catch records, leave no doubt that we have a unique population of tuna spawning in the Gulf,” Block says. “These are Earth’s largest giant bluefin.”

Because of the decline of the western stock, ICCAT has closed the Gulf breeding grounds to direct (“targeted”) bluefin tuna fishing since 1981. “However,” Block notes, “the United States allows pelagic longlines within the EEZ, and international longliners continue to harvest yellowfin tuna and other pelagic species in other parts of the Gulf and the Caribbean.”

The problem, she notes, is that the longliners sometimes unintentionally snag bluefin tunas during the breeding season, and it’s likely that many bluefins die before they can be released back into the sea. The researchers observed this firsthand when they tried to catch bluefins from commercial longliners. “We found it difficult to tag and release live bluefin tuna off longlines, which created frustration for all involved,” Block recalls.

The TRCC lab has conducted cardiac physiology studies that highlight the physical limitations of these fish. “Warm water holds less oxygen than cooler water,” Block says. “We hypothesize that large endothermic bluefin are physiologically stressed during the hot breeding season when their need for oxygen is greatest. Being caught on a longline may be too much for breeding bluefin tuna to tolerate, male or female. We found the bluefin were easily killed on our scientific longlines in warm Gulf waters when the hooks were soaked for more than two hours. Lab studies suggest that the limitations to the tuna may be that its cardiac system cannot be pushed any further at the warm end of the performance curve.” Capture on a hook must be stressful, she adds, so the fish simply die from lack of ventilation.

No one knows how many breeding-age bluefin are accidentally killed by pelagic longliners in the Gulf of Mexico, but the Nature study offers a simple solution–establish an extended time-area closure in the Gulf to all commercial longliners from January to June, the period that Block and her colleagues recorded visitations to the spawning hotspots along the continental slope waters. Since the primary western feeding grounds in the Gulf are under U.S. jurisdiction, the authors recommend that NMFS move rapidly to create a protected region for spawning giant bluefin to reduce mortalities. This is a crucial step, they add, because international longliners from Cuba and other countries with vessels in the Gulf and the Caribbean also are catching bluefin on the breeding grounds, according to data from electronic tags returned from fishers in these regions.

In addition to snagging bluefin, Block’s team caught juvenile swordfish and even a leatherback sea turtle that the team successfully released, an indication that the productive hotspots, which attract tunas, also attract other pelagic species.

“There are two ways to save the Atlantic bluefin tuna stock–protect them in their breeding grounds and in their feeding grounds,” Block says. “This will require immediate action in both the central Atlantic, to reduce mortality of giant bluefin while foraging, and in the Gulf of Mexico and Mediterranean Seas, where bluefin breed as discrete populations.”

Block’s team also found evidence that bluefin tuna linger on the Carolina slope waters in summer, a finding that could extend known western spawning areas to more northern waters. However, more work needs to be done in the Carolinas before researchers can say that bluefin tuna spawn outside the Gulf of Mexico and the Mediterranean, Block says.

The state of the bluefin is precarious, Block adds, and many questions remain. “But through the persistence of the TRCC’s Tag-A-Giant project, enough tagged fish have been put in the ocean, and the cooperation of North Atlantic fishermen has been successful enough, that we have pieced together a story of how these fish use the ocean,” she concludes. “It’s really a triumph of technology, human effort and cooperation on an ocean-basin level that has provided the empirical data to improve management decisions for the future of bluefin tuna in the Atlantic. It would be extraordinarily disappointing if ICCAT and the United States government did not use the best available science and take immediate action to improve the status of the western stock.”

Other co-authors of the Nature study are graduate student Kevin C. Weng and research associates Andreas Walli and Heidi Dewar of Stanford; graduate student Michael J. W. Stokesbury of Dalhousie University, Nova Scotia, Canada; and Charles J. Farwell and Thomas D. Williams of the Monterey Bay Aquarium. All are affiliated with the TRCC.