Scientific Journal Article Summary

Migratory movements, depth preferences, and thermal biology of Atlantic bluefin tuna
Barbara A. Block, Heidi Dewar, Susanna B. Blackwell, Thomas D. Williams, Eric D. Prince, Charles J. Farwell, Andre Boustany, Steven L. H. Teo, Andrew Seitz, Andreas Walli, & Douglas Fudge
Science (2001) 293: 1310-1314

The recent development of electronic implantable archival and pop-up satellite archival tags (PSAT) has revealed new information about Atlantic bluefin tuna seasonal movements, environmental preferences, and physiology. This paper documents the original findings of the Tag-A-Giant research program gathered via 377 bluefin tuna tagged between 1996 and 2001 off the east coast of North America.  Data were retrieved from recapture of 49 of 279 archival-tagged bluefin and from transmission of 90% of the 98 PSATs deployed.

While most tagged fish displayed a western resident track and were recaptured in the western Atlantic (west of 45ºW), 31% (n=15) were recaptured in the eastern Atlantic or Mediterranean Sea.  Western-tagged bluefin tuna made trans-Atlantic migrations in as little as 40 days. Seasonal movement of western residents revealed that bluefin tuna are off the Carolina coast in winter, and then move offshore in early spring along the Gulf Stream toward the New England and Canadian Shelf. They remain there from early summer to autumn, before returning to the Carolinas by winter.  These residents that did not visit a known spawning area were hypothesized to be immature.

Bluefin tuna visited known spawning grounds in the Gulf of Mexico (n=4) and eastern Mediterranean (n=7) during the known spawning periods.  A distinctive nighttime oscillatory diving behavior observed in the Gulf of Mexico was interpreted as potential spawning activity.  Occupancy of West Atlantic waters that were warm enough to support spawning (i.e., >73ºF) by several fish large enough to be mature revealed that some western resident fish may be breeding outside the Gulf of Mexico in the Bahamas, Caribbean and offshore Carolina waters.

Bluefin tuna most often occupy the upper 1000 feet of the water column but occasionally dive to depths of 3300 feet.  Individuals experienced a wide range of environmental temperatures (37-87ºF) but were able to maintain a relatively constant internal temperature in the abdominal cavity (~77ºF).  This confirms the bluefin tuna’s endothermic ability to maintain a warm body temperature.  The greatest thermal excess recorded was an internal temperature 70ºF above that of the surrounding water.

These data are critical for the future management and conservation of bluefin tuna in the Atlantic.  Western-tagged fish are susceptible to all Atlantic fisheries, and western fishers may be exploiting eastern Atlantic bluefin tuna.

Figure 1. Maximum (black) and mean (blue) daily depth of two bluefin in the western North Atlantic . (A) 9.3-year-old bluefin that displayed western residency in 1999 before a trans-Atlantic crossing in 2000. (B) 8.5-year-old bluefin that showed visitation to the Gulf of Mexico in 1999, the year of release. Breeding in the Gulf is proposed for 14 days in June where a relaxation of deep-diving behavior is evident (days 161 to 175).

Figure 2. 1.5 years of daily mean ambient water (black line) and peritoneal body (red) temperatures for a tagged bluefin tuna. Daily minimum water temperatures (black dots) and maximum body temperatures (red dots) are shown.

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