TAG Bluefin Science in the Atlantic

Tagging Locations & Deployment Statistics

Location
Year(s)
Gear
Tags Deployed
Total
Canada 2005, 2007-10 Rod & reel
2
80
82
France (incl. Corsica) 2000-01, 2007 Rod & reel
0
21
21
Ireland 2003-06 Rod & reel
0
6
6
Italy 2009-10 Rod & reel
3
0
3
Spain (Cartagena) 2000, 2006 Purse seine
1
15
16
U.S. (Gulf of Mexico) 1999-2002 Longline
0
28
28
U.S. (Nantucket, MA) 2000-03 Rod & reel
2
28
30
U.S. (New England) 1998 Purse seine
0
8
8
U.S. (North Carolina) 1996-97, 1999-2009 Rod & reel
683
251
934
Total
691
437
1128

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Recent Tagging Data

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In October 2007, the TAG team released 15 GIANT Atlantic bluefin tuna with satellite tags in the Gulf of St. Lawrence off Port Hood, Nova Scotia. A couple of the fish were behemoths of over 1,000-pounds, or "granders," and included the largest fish ever tagged in TAG's 10+ years of research. The figure at right shows pop-up locations of the tags. Three fish migrated to the Gulf of Mexico - the first-ever satellite tags to report from the western bluefin's only known spawning ground. Early pop-ups included one fish in North Carolina in November, and another east of the Bahamas in December, confirming that bluefin travel long distances in short periods. These southerly movements may indicate travel to the Gulf of Mexico spawning ground by additional fish.

Data highlights are below...

 

Tracks of three fish depicting their migratory routes from the Gulf of St. Lawrence to the Gulf of Mexico.
Temperature-depth profiles showing the wide range of temperatures encountered by tagged fish in different areas and on dives. Red profiles were recorded by fish that visited the Gulf of Mexico (and are therefore warmer on average), and blue profiles are from all the other fish.


Tagging in Port Hood, Nova Scotia, Canada - October 2007


Related publication:
Block, B.A., G. L. Lawson, A. M. Boustany, M. J.W. Stokesbury, M. Castleton, A. Spares, and J. D. Neilson. 2008. Preliminary results from electronic tagging of bluefin tuna (Thunnus thynnus) in the Gulf of St. Lawrence, Canada. SCRS/2008/092. Link to PDF

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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 archival tags. Click here for a detailed look at the technology.

Information gleaned from these tags has revealed extensive insights into the lives of these ocean giants who once passed secretly below the ocean’s surface.  Key findings include:

  • Support for at least two distinct populations of bluefin tuna in the North Atlantic, one that spawns in the Gulf of Mexico and another that spawns in the Mediterranean Sea.
  • Mixing of the two populations during feeding times at productive oceanic hotspots, such as North Carolina’s Outer Banks and the Flemish Cap.
  • A behavioral signal for spawning in the Gulf of Mexico, characterized by oscillatory diving at night.
  • Identification of the spawning locations in the Gulf of Mexico, characterized by cyclonic eddies.
  • Evidence that most western bluefin spawn for the first time at age 11-12, rather than 8 years old as previously thought.

Migratory Patterns of Western vs. Eastern Atlantic Bluefin Tuna
Annual movements of tuna tagged along the East Coast vary by population (eastern vs. western) and age class. Age class has a significant influence on migratory patterns, with larger, older fish undertaking longer migrations than smaller, younger fish (Figure 1).

Figure 1. Seasonal distribution by size of Atlantic bluefin tuna that were tagged in the western Atlantic.
a–d, Less than 90 in. curved fork length (CFL). a, Winter; b, spring; c, summer; d, autumn.
e–h, Greater than or equal to 90 in. CFL. e, Winter; f, spring; g, summer; h, autumn.

TAG tagging has revealed four distinct migratory patterns:

  1. North-south movement along the East Coast of North America.
  2. Movement throughout the North Atlantic on both sides of the 45º management boundary (See Figure 3).
  3. Movements along the East Coast of North America and in the North Atlantic with a late spring/early summer trip to the Gulf of Mexico (See Figure 2).
  4. Movements along the East Coast of North America and in the North Atlantic with a migration to the Mediterranean Sea (See Figure 4&5).

Patterns (1) and (2) reveal foraging travels of juvenile bluefin tuna, both western and eastern. Pattern (3) reveals foraging and spawning behavior of mature western bluefin. Pattern (4) reveals foraging and spawning behavior of mature eastern bluefin.

Figure 2. Coastal migration with visit to the Gulf of Mexico.


Figure 3. The track of a NC-tagged fish that moved throughout the North Atlantic to feed but did not visit a known spawning ground. Color represents water temperature, and spikes represent diving depth.

Figure 4. The track of a NC-tagged fish that fed in the West Atlantic before making a trans-oceanic migration to the Mediterranean Sea to spawn. Color represents water temperature, and spikes represent diving depth.

Figure 5. Movements over 4.5 years of one individual Atlantic bluefin tuna that was tagged in North Carolina in 1999 and demonstrated site fidelity to a known spawning area in the Mediterranean Sea. Each panel shows a year of the fish’s track; a, The bluefin tuna was tagged on 1/17/99 (arrow) and showed a year of western residency. b, In 2000, the bluefin tuna showed transatlantic movement to the eastern Atlantic. ce, Three consecutive years of movements from the eastern Atlantic to a Mediterranean Sea spawning area near the Balearic Islands, during the breeding season. The fish was recaptured on 7/2/03 (yellow triangle).

Tag data illustrate that there is extensive mixing of the populations across the 45ºW longitude management divide. Western fish feed in the Central North Atlantic and even along the West Coast of Europe, and eastern fish are even more likely to visit West Atlantic waters to feed. The figures below show positions of western fish that visited the Gulf of Mexico (Figure 6), eastern fish that visited the Mediterranean Sea (Figure 7), and all fish, including those that did not visit any spawning ground during the tag period and were defined as "neutral" (Figure 8). There is clearly a great deal of overlap between the populations.

Figure 6. Positions of 36 bluefin classified as western breeders, which were tagged at three western Atlantic locations (arrows) during 1996–2004. Circles represent locations based on deployment positions, light-based and SST-based geolocation estimates, and PAT tag satellite endpoint positions. Triangles represent recapture locations of tagged fish.

Figure 7. Positions of 26 bluefin classified as eastern breeders, which were tagged at three western Atlantic locations (arrows) during 1996–2004. Circles represent locations, and triangles represent recapture locations.

Figure 8. Positions of 268 bluefin tagged at three western Atlantic locations (arrows) during 1996–2004. "Neutral" fish did not visit a known ICCAT breeding ground. Circles represent locations, and triangles represent recapture locations.


North Atlantic Foraging Hot Spots
High residence times were consistently observed in four regions of the North Atlantic on a seasonal scale. The aggregations occurred off North Carolina during winter, in the Northwest Atlantic (Georges Bank, Gulf of Maine and Nova Scotia) during summer and fall, in the central Atlantic (North West Corner) during spring to summer and in the East Atlantic (off Portugal) during spring and fall. See Figure 9 below for a representative track. The regions of aggregation were tentatively linked to areas of abundant prey and are believed to represent critical foraging habitat. The distribution patterns provide information that is important for development of future management strategies that consider the effects of targeted fishing pressure on these populations.

Figure 9. Monthly geolocation estimates and track of an individual Atlantic bluefin tuna (98-485) showing typical North Atlantic foraging movements, with corresponding depth and temperature profiles. Black boxes indicate geographic regions covered by profiles. White arrow indicates deployment location. Dark grey line in depth and temperature profiles indicates calculated depth of thermocline. Section 1: North Carolina, 2: off Portugal, 3: North West Corner, 4: northern Caribbean, 5: East Atlantic passing through Azores.

back to Summary of Tagging Results

Gulf of Mexico Behavior & Spawning Habitat

Scientists have known that the Gulf of Mexico is a spawning ground for decades, but details on where, when and how evaded researchers since spawning occurs far offshore. Electronic tagging has broken the knowledge barriers and provided a comprehensive picture of spawning.

Early in the tagging program, TAG researchers noticed a unique diving behavior in the Gulf of Mexico that was seen nowhere else in the tagging studies. Bluefin in the Gulf of Mexico exhibited a short period of significantly shallower daytime dives compared to other times in the Gulf.  At night, the fish made shallow oscillatory dives hypothesized to indicate surface courtship and spawning interrupted by short dives (Figure 10). Due to the Gulf's warm waters and significant energy expended while breeding, bluefin showed the highest body temperatures recorded in TAG research during the breeding phase. To avoid overheating, it appears that bluefin alter their swimming pattern by repeatedly diving into cooler water, a remarkable behavior.

Figure 10. Depth (black), water temperature (blue), and body temperature (red) of a bluefin in the Gulf of Mexico. Vertical grey bars indicate nighttime.

Once the breeding behavior was identified, TAG scientists could pinpoint the spawning hotspots in the Gulf (Figure 11). In marine fish, spawning in environmental conditions optimal for both adults and larvae can maximize reproductive success. To determine the environmental preferences of bluefin breeding in the Gulf of Mexico, position data of electronically tagged fish were compared to oceanographic data collected by satellites.  Water temperature was the most important factor, with the bluefin preferring moderate temperatures (75-80ºF) to ensure a warm area for the larvae without causing too much heat stress to the spawning adults. Bluefin tuna also exhibited significant preference for continental slope waters (~2 miles deep), moderate eddy kinetic energy, low chlorophyll concentrations, and moderate wind speeds. The depth and eddy kinetic energy preferences are likely related to a desire to spawn in eddies, which have increased retention of larvae and their prey and have been hypothesized to increase larval survival.

Figure 11. Spatial distribution of breeding Atlantic bluefin tuna in the Gulf of Mexico.  Warmer colors indicate a greater concentration of fish.

 

Depth & Temperature Preferences
In the North Atlantic, diving depth is significantly correlated with the depth of the thermocline (i.e., the depth at which the water temperature changes rapidly, separating the mixed surface layer from the colder, deep waters). Obviously water depth is also a major factor, and dives are much shallower when bluefin are in continental shelf areas than when they are offshore. Bluefin typically remain in the top 300 meters (984 feet) of the water column but occasionally make dives that peg the tag sensors at 1,000 meters (3,280 feet).

Electronic tagged bluefin tuna experience a wide range of ambient water temperature (0.1° - 31.0°C, or 32.2° - 87.8°F). Remarkably, a sophisticated counter-current heat exchanger allows bluefin tuna to warm cool blood coming from the gills with warm blood coming from the muscles to maintain their body temperature much higher than the surrounding water. Internal archival tags have recorded body temperatures up to 21°C (69.8°F) above the water (See Figure 12).

See Figure 9 above for a typical depth and temperature profile of a bluefin travelling the North Atlantic.

Figure 12. Tag-recorded daily mean internal peritoneal temperatures (red line) and ambient water temperatures (black line), illustrating the warm-bodiedness of bluefin tuna. Red dots show maximum recorded internal temperatures, and black dots show minimum recorded external temperatures.

Related publications:
Teo, SLH, A Boustany, and BA Block. 2007. Oceanographic preferences of Atlantic bluefin tuna, Thunnus thynnus, on their Gulf of Mexico breeding grounds. Marine Biology 152:1105–1119. Link to Full Text.

Teo S.L.H., Boustany, A. Dewar, H., Stokesbury, M.J.W., Weng, K.C., Beemer, S., Seitz, A.C., Farwell, C.J., Prince, E.D. and B. A. Block. 2007. Annual migrations, diving behavior, and thermal biology of Atlantic bluefin tuna, Thunnus thynnus, on their Gulf of Mexico breeding grounds. Marine Biology 151:1-18. Link to Full Text.

Block, BA, SLH Teo, A Walli, A Boustany, MJW Stokesbury, CJ Farwell, KC Weng, H Dewar & TD Williams. 2005. Electronic tagging and population structure of Atlantic Bluefin Tuna. Nature 434: 1121-1127. Link to Full Text.

Block, B. A., H. Dewar, S. B. Blackwell, T. D. Williams, E. D. Prince, C. J. Farwell, A. Boustany, S. L. H. Teo, A. Seitz, A. Walli, & D. Fudge. 2001. Migratory movements, depth preferences, and thermal biology of Atlantic bluefin tuna. Science 293: 1310-1314. Link to Full Text.

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