Scientific Journal Article Summary

Electronic tagging and population structure of Atlantic bluefin tuna
Barbara A. Block, Steven L. H. Teo, Andreas Walli, Andre Boustany, Michael J. W. Stokesbury, Charles J. Farwell, Kevin C. Weng, Heidi Dewar & Thomas D. Williams
Nature (2005) 434: 1121-1127

Atlantic bluefin tuna are thought to have two populations – a western that spawns in the Gulf of Mexico and an eastern that spawns in the Mediterranean Sea .  Is this hypothesis true? If so, to what extent, if at all, do they mix on North Atlantic feeding grounds?  Is the 45ºW meridian population boundary used by fisheries managers realistic?  To answer these questions, the Tag-A-Giant team deployed 772 electronic tags on bluefin tuna in the western Atlantic Ocean between 1996 and 2004. 

Accurate location data were retrieved from electronic tags for 330 individual bluefin tuna, revealing extensive oceanic migrations.  The results were consistent with the two-population hypothesis.  Migration data showed that fish tagged in the western Atlantic traveled to spawning grounds in the Gulf of Mexico and Mediterranean Sea , but no fish traveled to both areas (Figure 1). Furthermore, bluefin tuna that traveled to the western spawning ground moved to central and eastern Atlantic foraging grounds, crossing the 45ºW meridian into the eastern management unit. As eastern spawning fish were also present on western Atlantic foraging grounds where they were tagged, the study confirmed frequent movement of fish across the 45ºW meridian and extensive mixing on feeding grounds in the North Atlantic despite separation during spawning.  The calculated mixing rate of 47% is much higher than the 1-5% mixing rates used by fisheries managers and raises questions about the utility of the 45ºW management line.

Movements varied by body size, with larger fish undertaking longer migrations (Figure 2).  Bluefin that visited a spawning ground were at least 90 inches long (measured as curved fork length), and fish in the Gulf of Mexico were about 11 years old (compared to the 8-year old maturation age currently assumed by fisheries managers).

When combined with fisheries data, the tagging data suggest that western Atlantic bluefin spawning is concentrated in an area in the northwestern Gulf of Mexico . While directed bluefin fishing is prohibited in the entire Gulf, it is likely that fisheries targeting other species are killing spawning bluefin at a potentially harmful rate.  Restricting fishing for a limited time in this area could reduce incidental catch mortalities of western spawning bluefin tuna.

Figure 1.  Positions of Atlantic bluefin tuna electronically tagged at three western Atlantic locations (arrows) during 1996–2004. Circles represent daily locations based on deployment positions and tagging data. a) Fish classified as western breeders – 36 fish; b) Fish classified as eastern breeders – 26 fish; c) Fish that did not visit a known breeding ground – 268 fish. Triangles represent locations where tagged fish were caught by fishermen. The dashed line indicates the current management boundary (45ºW meridian).

Figure 2. 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.

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