Electronic tags reveal transatlantic migrations and breeding grounds of Atlantic bluefin tuna

August 16, 2001

Bluefin tuna tagged in the Western Atlantic with electronic data recording tags are mixing on their feeding grounds but moving to distinct spawning grounds in the Western Atlantic and the Eastern Mediterranean.

These and other significant insights into the migrations, diving patterns, thermal biology and environmental preferences of the prized warm-blooded giants are described in a new study in the journal Science. The findings emerged from five years of electronic tagging by scientists with the Tuna Research and Conservation Center in Pacific Grove, California. The center is a joint project of Stanford's Hopkins Marine Station and the Monterey Bay Aquarium.

The tagging research - a collaboration of scientists from Stanford, the Aquarium and the National Marine Fisheries Service (NMFS) - was led by Barbara A. Block, the Charles and Elizabeth Prothro Professor in Marine Science at Stanford and lead author of the Aug. 17 Science study.

The majestic bluefin, which can grow to be 10 feet (305 centimeters) long and weigh 1,500 pounds (680 kilograms), is so commercially valuable that, in January, a single fish weighing 444 pounds (201 kilograms) sold at auction for $175,000 in the Tokyo seafood market. Atlantic bluefin in that market routinely sell for $8 to $45 per pound.

Commercial harvesting of bluefin and other Atlantic tuna is managed through catch quotas established by the International Commission for the Conservation of Atlantic Tunas (ICCAT) based in Madrid, Spain.

Transatlantic migrations

In the Science study, Block and her colleagues analyzed data recovered from electronic tags placed on Atlantic bluefin tuna off the East Coast of North America starting in 1996. Three hundred and seventy-seven (377) tags were deployed on fish weighing between 250 and 800 pounds (113 and 363 kilograms). To date, 49 have been recovered. Most of the electronic tags were "archival" tags that scientists surgically implanted inside each fish with the help of recreational fishers off the coast of North Carolina during the winter.

Thirty-one percent of the recovered electronic tags have been returned from the Eastern Atlantic and Mediterranean Sea by commercial fishers from Italy, Morocco, Portugal and Japan. Sixty-nine percent have been recaptured in the Western Atlantic, primarily off Maine, Nova Scotia and Cape Cod, Mass., by American, Canadian and Japanese fishers.

Researchers discovered that Atlantic bluefin - considered overexploited in the Western Atlantic since 1982 - often are traveling throughout the entire North Atlantic and, in some cases, into the Mediterranean Sea. Tuna tagged in the Western Atlantic in most cases resided for a year or more on Western North Atlantic feeding grounds. While many fish remained in the west, some of the recaptured fish had migrated to the Eastern Atlantic or Mediterranean Sea. Individuals also migrated from the Western Atlantic to the east and back again in the same year.

Most of the fish examined showed at least one year of western residency traveling between the Carolinas and New England and back again to the original release location. The fidelity to these two locations is associated with high primary productivity and represents bluefin feeding aggregations. A third aggregation site is near the Flemish Cap - a section of the Atlantic seafloor just east of the Grand Banks off Canada's Newfoundland.

Researchers also reported that the western-tagged bluefin travel to distinct spawning grounds in the Gulf of Mexico or the Eastern Mediterranean.

"The results indicate western-tagged bluefin are vulnerable to fishing mortality from all Atlantic bluefin tuna fisheries," the authors write in Science. And, they add, tagging data emphasize "the need to protect both major eastern and western spawning regions, as they directly influence the western fishery."

This is an important consideration since the 2001 catch quota for Western Atlantic fisheries (2,500 metric tons or 5.5 million pounds) is one-twelfth the quota for the Eastern Atlantic fisheries (29,500 metric tons or 65 million pounds). Current management of the fishery by ICCAT assumes that catches involve only a low level of mixing between tunas from the Western and Eastern Atlantic.

ICCAT imposes strict quotas on fish caught in the Western Atlantic and much more liberal quotas on bluefin landed in the Eastern Atlantic and Mediterranean Sea. The Western Atlantic management is aimed at recovering the breeding population to levels that produce "maximum sustainable yield. The Eastern Atlantic breeding population is considered to be in decline and overfished, although it is of greater overall abundance than the western breeding population.

According to the Science study, "The electronic tagging data indicate that mixing between the two management units exists at a higher level than ICCAT has incorporated into...stock assessments. However, movements to distinct breeding grounds are apparent, suggesting a mixing of stocks on feeding grounds and a separation to distinct breeding localities. Future assessment of stock status should evaluate the new information and reassess the management strategies applied to Atlantic bluefin tuna."

"From the data, it's clear there are two breeding populations of bluefin tuna that spend considerable time together on the rich Western North Atlantic feeding grounds," says Block. "Our results demonstrate that bluefin tuna are capable of ranging widely throughout the North Atlantic without regard to the stock boundary in the mid-Atlantic. That means efforts to bring about a recovery of bluefin tuna populations will require increased cooperation among all nations fishing for bluefin tuna.

"Importantly," she adds, "our data indicate that bluefin are sorting to distinct breeding grounds where efforts to protect the breeding aggregations should be increased."

Remarkable data

The deployment of electronic tags that can record and archive data eliminates the need to track individual fish acoustically from a ship. The tags collect data on depth, light and external and internal temperature every two minutes. They also use sunrise and sunset data with sea surface temperature to compute a daily location, yielding tens of thousands of data points.

More than 15,000 days of archival data have been collected in the past five years, and records on individual fish span as much as 3.6 years. Previously, the longest duration scientists had followed large open-ocean fish and obtained similar types of data was six days.

The research team also deployed pop-up satellite tags. These tags, which are attached externally to the fish, detach and float to the surface at a pre-programmed date. Data on depth, temperature and location were stored on the tag during the mission and transmitted back to the lab via the Argos environmental satellite system. Pop-up tags were deployed in New England, North Carolina and in the Gulf of Mexico.

The researchers demonstrated that bluefin dive to remarkable depths, at times exceeding the 3,281-foot (1000-meter) pressure sensors in the tags. Data from the tags show that bluefin spend most of their time closer to the surface, in the top 984 feet (300 meters).

Bluefin also display a remarkable range of temperature preferences, from near-freezing waters when feeding (37 F or 2.8 C) to very warm temperatures when breeding (86 F or 29.5 C). The Atlantic bluefin is one of only a handful of warm-blooded fishes. Block and her colleagues demonstrated that bluefin maintain a high internal temperature, around 77 F to 80 F (25 C to 26 C) even when swimming in the most frigid waters.

The researchers also reported the first descriptions of what they believe to be breeding behavior in giant tunas. Most surprising is that the spawning period occupies a short interval in the yearly activities -- less than one month. A unique behavioral repertoire recorded by the electronic tags suggests that the giant tunas breed at night in very warm waters, swimming up and down in the water column for hours. In daytime this behavior is not apparent.
Other co-authors of the Science study are:The research was primarily funded by NMFS, the Packard Foundation and the following organizations: National Science Foundation, Pew Foundation, MacArthur Foundation, the Disney Conservation Fund and the National Geographic Society. The Tag-A-Giant campaign also promoted individual sponsorships of tags.

This article was written by Ken Peterson of the Monterey Bay Aquarium.

Mark Shwartz, News Service
(650) 723-9296;

Ken Peterson, Monterey Bay Aquarium

Barbara A. Block, Department of Biological Sciences
(831) 655-6236;

Relevant Web URLs: http://www.tunaresearch.org

Stanford University

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