TGF Bluefin Science in the Atlantic
Biology of Atlantic Bluefin Tuna (Thunnus thynnus)
| Identification: As members of the suborder Scombroidei, bluefin are closely related to other tunas. They also are in the same suborder as the mackerels, bonitos and billfishes. Their dorsal sides are dark blue, with a green/yellow iridescence. Their bellies are silver with 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. |  |
The shaded area depicts the known range of Atlantic bluefin. The 45ºW management boundary and two known spawning areas are also shown. (Graphic courtesy of National Research Council) |
Size & Age: Atlantic bluefin tuna are among the largest bony fish on earth. The rod and reel world record topped the scales at 1,496 lbs. in 1979. So-called "giant bluefin" refer to fish greater than 300 lbs. These giants can live up to 30 years. Click here for a table of approximate age at length.
Range: Atlantic bluefin tuna are found from the Gulf of Mexico to Newfoundland, Canada in the West Atlantic, and from the Canary Islands off Africa to Iceland in the East Atlantic. There are at least two separate spawning populations, one in the West Atlantic and one in the East Atlantic. |
| Reproduction: Bluefin tuna are thought to show spawning site fidelity, that is, they return to the place they were born each year to reproduce. The western Atlantic population spawns each late spring/early summer in the Gulf of Mexico. Larvae have been recovered in the Florida Straits and Bahamas, suggesting spawning may also be occurring in these regions, although the larvae may have drifted there from the Gulf. The eastern bluefin stocks spawn in the Mediterranean Sea. Interestingly, western bluefin show a median maturation age of 12, while the median bluefin tuna spawning age in the Mediterranean is variable and can occur as young as 5 years of age or even earlier. Female bluefin tuna can produce over 10 million eggs in a single spawning season. |  The egg-laden ovaries of a ripe female bluefin in the Gulf of Mexico. |
Feeding: Bluefin prey upon squid, mackerel, herring, small tuna and other small schooling fish. Whereas young tuna themselves fall prey to many species of seabirds and small fish, giant tuna are preyed upon only by top predators like whales, dolphins, and large sharks.
Did you know? Bluefin can withstand a large range of water temperatures, from 1-31ºC, because of counter-current heat exchangers that render bluefin endothermic, or warm-blooded, a very rare trait in the fish world. As a result, their body temperature stays stable and warm, even when diving to depths of more than 1,000 m.
Tagging Locations & Deployment Statistics
Location |
Year(s) |
Gear |
Tags Deployed |
|||
Archival |
PAT |
Total | ||||
 |
Canada | 2005, 2007 | Rod & reel | 1 |
21 |
22
|
 |
France (incl. Corsica) | 2000-01, 2007 | Rod & reel | 0 |
21 |
21
|
 |
Ireland | 2003-06 | Rod & reel | 0 |
6 |
6
|
 |
Spain (Cartagena) | 2000 | Purse seine | 0 |
15 |
15
|
 |
U.S. (Gulf of Mexico) | 1999-2002 | Longline | 0 |
29 |
29
|
| U.S. (Nantucket, MA) | 2000-03 | Rod & reel | 2 |
28 |
30
|
|
| U.S. (New England) | 1998 | Purse seine | 0 |
9 |
9
|
|
| U.S. (North Carolina) | 1996-97, 1999-2008 | Rod & reel | 601 |
250 |
851
|
|
| Total |
604 |
379 |
983
|
|||
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Recent Tagging Data
| The pop-up tags deployed in Canada in October 2007 have begun to report. As seen in the figure at right, three fish migrated to the Gulf of Mexico - the first-ever satellite tags to report from the Gulf of Mexico. 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. One of the fifteen tagged fish reported via a mortality program where the tag releases from the fish if it stays at the same depth for a long time. The fish was noted to have been deeply hooked upon capture, which likely contributed to its death. |
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Summary of Tagging Results
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.
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 12, rather than 8 years old as previously thought.
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.
TGF Tagging has revealed four distinct migratory patterns:
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. |
Figure 3. 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. c–e, 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 4), eastern fish that visited the Mediterranean Sea (Figure 5), and all fish, including those that did not visit any spawning ground during the tag period and were defined as "neutral" (Figure 6). There is clearly a great deal of overlap between the populations.
Figure 4. 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 5. 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 6. 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 7 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 7. 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.
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).
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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 8). See Figure 7 above for a typical depth and temperature profile of a bluefin travelling the North Atlantic. |
Figure 8. 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. |
Molecular Genetic Studies of Population Structure
In addition to electronic tagging efforts, population structure has been studied through genetic analysis of bluefin tuna DNA. Population subdivision was examined through sequencing of 860 base pairs of the control region of the mitochondrial genome. A total of 178 samples obtained from the known spawning grounds in the Gulf of Mexico and Mediterranean Sea were analyzed. Bluefin tuna populations from the Gulf of Mexico and the Mediterranean Sea were found to be genetically distinct based on Φst, sequence nearest neighbor and AMOVA analyses, supporting the hypothesis that these two major spawning areas are independent stocks. These data are in agreement with electronic tagging data that indicate distinct migratory paths of the Gulf of Mexico and Mediterranean Sea spawning populations.