TAG BluefinScience in the Pacific

Tagging Locations & Deployment Statistics

Tags Deployed
Japan 2009 Lift pole
Mexico (Ensenada) 2002, 2005-07 Pen
New Zealand 2006-08 Rod & reel
U.S. (Monterey, CA) 1999-2002 Aquarium
U.S. (San Diego, CA) 1999, 2001-05,07-09 Rod & reel

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Biology of Pacific Bluefin (Thunnus orientalis)

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Identification: Until the late 1990s, Pacific northern bluefin were thought to be a subspecies of Atlantic bluefin (Thunnus thynnus) because of their similar appearance. Their dorsal sides are black or dark blue, with a gray or green iridescence. Their bellies are silvery with gray spots or bands. They have a series of yellow finlets running from the second dorsal fin to the tail, which are edged in black. It is sometimes difficult to distinguish bluefin from other tunas, particularly at small sizes. Most notably, the tips of the pectoral fins do not reach the front of the second dorsal fin in bluefin tuna, and the eyes are relatively small.

Size & Age: Pacific bluefin grow to a maximum size of 550 kg (~1,200 lbs.) and have a life expectancy of approximately 15 years.

Range map courtesy of Pacific Fishery Management Council.
Range: Bluefin tuna are found from 20ºN to 42ºN (Baja California to Oregon) in the East Pacific, and from New Zealand north to Japan in the West Pacific. All northern bluefin in the Pacific are thought to belong to the same population.

Reproduction: All Pacific bluefin spawn in the western basin between the Philippines and Japan during the months of April through August. Most individuals reach maturity at age 5, at a size of approximately 60 kg. Female bluefin tuna can produce over 10 million eggs in a single spawning season.

Feeding: Northern anchovies are the primary prey of bluefin off the California coast. Other foods include pelagic red crabs, market squid and sauries.

Did you know? Many Pacific bluefin make the lengthy journey from their birth place near Japan to the U.S. West Coast when they are one or two years old. They spend a few years feeding in the fertile California current before an unknown trigger reminds them that it is time to return to the West Pacific to spawn.

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Summary of Tagging Results

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TAG scientists use two main electronic tag technologies to study bluefin tuna - archival and pop-up satellite tags. Click here for a detailed look at the technology.

Fueled by the tagging success in the Atlantic, TAG researchers moved into the Pacific in 2002. The effort is part of the Tagging of Pacific Predators (TOPP) project, a large-scale initiative to tag 21 different marine animals, ranging from squid to whales, simultaneously gathering data on the lives of the tagged species as well as the oceanography of the Pacific Ocean. We are now learning details of the lives of Pacific bluefin for the first time.  Key findings include:

  • Most young bluefin tagged off the West Coast are recaptured along the coastline, suggesting resident feeding in the productive California Current.
  • Several tagged bluefin have made the trans-Pacific trek to their spawning grounds near Japan. 
  • Recapture rates of tagged bluefin are as high as 74%.

Click here to view a narrated slideshow of archival tagging methodology and associated data analyses on Pacific bluefin tuna.

This KQED report aired in May 2008 features TGF Scientific Director Dr. Barbara Block tagging Pacific bluefin as part of the TOPP project.

Two hundred fifteen bluefin tuna carrying archival tags have been recaptured. The fish were at large for an average of a year, while one fish's tag collected data for 1,815 days (>5 years!) before the fish was recaptured. Over 52,000 geolocations have been calculated (click here for details about the technology), allowing for a thorough examination of the seasonal movement of bluefin tuna off the west coast of North America.

The tagged bluefin exhibited repeatable seasonal movements, being farthest south in the spring (off southern Baja California) and farthest north in the fall (off central and northern California). North-south movements coincided with bursts of coastal upwelling, which leads to high productivity and thus abundant food. Sea surface temperature did not appear to limit movement. In the winter months, tagged bluefin tuna were found in areas with lower productivity compared to other regions along the coast, perhaps because they were feeding on spawning sardines and anchovies that preferentially spawn in areas of low coastal upwelling. Learn more about bluefin in the eastern Pacific in a new TAG paper published in Fisheries Oceanography in summer 2007.

Kernal density plots of Pacific bluefin tuna seasonal distributions in the California Current.

Fifteen bluefin tagged in the eastern Pacific have been recaptured off Japan, detailing the trans-Pacific migration. The figure bleow includes the tracks of eight of these fish. Interestingly, the fish traveled the same path at the same time of year. The westward migrations were initiated primarily in the winter and early spring along a narrow corridor along the northern edge of the North Pacific Gyre. The bluefin moved relatively fast across the eastern Pacific, traveling approximately 2,500 nm in 31-45 days, and then slowed down just east of Hawaii, timing their arrival to correspond with the spring pulse of productivity in this area. This area is also popular with loggerhead turtles and albatross. 

Positions of 143 Pacific bluefin tuna tracked with archival tags color-coded by month, illustrating residency along the West Coast of North America and the trans-Pacific crossings.

New analyses are studying foraging behaviors. Feeding can be observed in fish with internally-placed archival tags because the ingestion of cold seawater with prey cools the internal body temperature temporarily and is recorded by the temperature sensor on the body of the tag. An increase in body temperature follows the initial decrease because digestion increases metabolism.  Preliminary findings indicate that feeding is most frequent when fish exhibit temporary residency to an area and when diving is common.

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