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Commercial Fisheries News
Volume 32 Number 12
July 2005
Block team documents east-west tuna mixing
PACIFIC GROVE, CA - Eastern and western Atlantic bluefin tuna populations spend enough time in each other’s backyards to warrant a significant change in international management policy.
That was one of several conclusions reached by Stanford University tuna researcher Barbara Block and eight colleagues in a landmark paper published in the April edition of the scientific journal Nature.
Eastern and Western Populations – This figure shows numerous data points collected from 62 fish identified as having come from western spawning grounds (orange) or eastern spawning grounds (white) and clearly shows how the eastern and western groups intermingle. (click to enlarge)
“We’re adamant that quotas must be reduced on the eastern side to aid the western recovery,” Block said in a mid-June telephone interview.
At the same time, Block concluded that there are at least two distinct stocks of bluefin tuna in the North Atlantic and that fish from each stock tend to return to their own specific spawning areas – in the Gulf of Mexico for western Atlantic fish and in the Mediterranean for eastern Atlantic fish.
According to Block, that means the onus is still on the US to take additional steps to protect spawning bluefin in the Gulf of Mexico.
“We must do more in the west to protect our western spawners,” she said.
Most East Coast industry people welcomed the first finding as the long-awaited conclusive evidence needed to force eastern Atlantic countries to take meaningful action to conserve bluefin.
However, the paper’s support for the established “two-stock theory” and Block’s advocacy for unilateral US action to seasonally close the Gulf of Mexico spawning area to pelagic longliners were more controversial.
Tagging program
The Nature paper summarized data collected through an ongoing electronic tagging program conducted by Block and the rest of the tagging team from 1996 to 2004.
As of the paper’s publication, the team had attached 273 pop-up satellite (PAT) tags and surgically implanted 499 archival tags into bluefin tuna for a total of 772 tagged fish. Most of the fish were tagged off North Carolina working with commercial and charter boat fishermen. The team also tagged fish off Nantucket and, working with commercial pelagic longliners, in the Gulf of Mexico.
The PAT tags are programmed to transmit location information to Argos satellites and typically provided one to eight months of data. The archival tags gather light level and sea temperature data, which is used to track the movements of individual fish for up to several years.
In all, the research team got back 86 archival tags. For the PAT tags, 237 out of the 273 transmitted data, a success rate of 89 percent, which Block called “remarkable.”
Altogether, the research team was able to track the movements of individual fish for anywhere from two up to 1,623 days.
By using both kinds of tags, the team was able to generate a large amount of data. In the end, the researchers reported that they were able to plot a total of “13,372 positions obtained from 330” individual tagged fish.
Another important factor in the tagging program was the size of the fish. All tagged fish were measured so that the researchers could determine their age and whether they were juvenile or sexually mature fish.
Two stocks
In the paper, the research team reported that of the 86 archival-tagged bluefin recaptured, 54 were taken in the west Atlantic, nine in the east Atlantic, and 23 in the Mediterranean.
So, a total of 32 tags were in fish captured east of the 45° meridian, which is the line used since the early 1980s by the International Commission for the Conservation of Atlantic Tunas (ICCAT) to distinguish two separate bluefin tuna stocks and allow for the establishment of two completely different management programs.
Out of the reporting PAT tags, 51 also popped up across the line.
A total of 62 tagged fish visited a known ICCAT spawning area allowing the researchers to classify them as coming from the western or eastern stock.
Western fish
According to Block, the data showed that adolescent fish less than 80" curved fork length (CFL) identified as coming from the western stock spent most of their time on the North American Continental Shelf, primarily from the Carolinas to New England and the Nova Scotian shelf.
“These fish have a probability of remaining in the west that is quite high,” Block explained. “That is, if we say you’re a western fish from six to eight years of age, then you don’t stray out of the western area very much at this size.”
However, once these fish get to be 400 pounds or more, they physically reach a point where they can handle more variation in water temperature and are freer to roam.
“When they get larger in body size, they show some increased use of the central and east Atlantic,” Block said. “They get caught by longline fishers and the tags come back to us, primarily from (Japanese longliners).”
She added that it was important to keep in mind that these fish breed at a very large size – 95" CFL or about 11 years old – and that they return to the west to spawn.
“If you’re a big western fish that spawns on western spawning grounds, you go trans-Atlantic, but only into the Atlantic, not the Mediterranean,” Block said. “You go back and forth across the (45°) line in the North Atlantic and then return to the west after you forage to spawn.”
Eastern visitors
The research team identified another population found along the North American coast as adolescent fish presumably spawned in the Mediterranean. Once these fish reached about eight or nine years of age, they swam back across the Atlantic to breed. These fish made up a large part of the team’s data set.
“It’s important to set the record straight,” Block said. “This is not western fish moving to the east. Many are Mediterranean fish returning home.
“So what we say is there are two stocks and the trans-Atlantic component is due, in many cases, to these adolescent Med fish going home to spawn after spending some youthful time in our waters,” she continued.
“And once they go into the Med, they are unlikely to come back. Thus, we are fishing on the eastern stock in the west when they are adolescent fish. I do not think we are fishing on adult eastern Med-spawned fish,” Block said.
She added that the tagging data indicate these fish appear to breed at a larger size than what Europeans typically say bluefin breed.
“Fish from our study were primarily going into the Med at eight and nine years of age,” she said.
Gulf of Mexico
While Block stressed that she has no affiliation with the Oceana petition to seasonally close bluefin spawning areas in the Gulf of Mexico, the paper strongly stated that bluefin in that area during the spring spawning months are likely to die if incidentally hooked by a longliner.
The research team theorized that the warm water temperature so important to bluefin egg development is also physiologically stressful for giant tunas.
“During our longline trips in the Gulf of Mexico, we had trouble tagging because of the mortality. It was difficult to keep a tuna alive. That same fish caught in New England is not a problem,” Block said.
The mortality problem occurred even on short sets and with circle hooks, according to the paper, raising questions over the ability of longliners to return incidentally caught fish to the sea alive.
“It behooves the bluefin tuna fishers to listen to what we’re saying,” Block said.
She added that she agrees “the central Atlantic also needs to be closed down.”
Questions remain
While the information gathered so far is compelling, the research tagging program has also generated many additional questions.
For one thing, a large number of the tagged fish – 268 – did not move into a known spawning ground. The paper noted that, in many cases, tracking time was shorter with these fish because of premature release of PAT tags and problems with the early generation of archival tags, so the fish may have traveled to a known breeding ground at a later date.
But the results left researchers wondering: Are there additional spawning areas? Are there water temperature and food availability issues? And, even, do bluefin spawn every year?
Watch for tags!
During the phone interview, Block reported that the research team tagged additional bluefin off the Carolinas over the winter of 2005, bringing the total number of tags placed in fish up to 900.
“I said at the beginning my goal was 1,000 tags. It’s taken us longer than we expected. We have 100 tags left and will finish by the end of January 2006,” Block said. “I have no idea what will happen next.”
Though that’s only in terms of new tagging efforts by the Stanford researcher.
Block said she wanted to remind Commercial Fisheries News readers that her tagging program will continue collecting and analyzing data long after all the tags are placed in fish. She urged fishermen to keep an eye out for tags and reminded readers that the program will continue to offer significant cash rewards for tag returns.
“There are a lot of fish with archival tags out there still swimming around,” Block said.
Anyone interested in looking at some fascinating examples of the PAT tag data can check it out online at <http://las.pfeg.noaa.gov/tagagiant>
Lorelei Stevens
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