Scientific Journal Article Summary

Annual migrations, diving behavior, and thermal biology of Atlantic bluefin tuna, Thunnus thynnus, on their Gulf of Mexico breeding grounds
Steven L. H. Teo, Andre Boustany, Heidi Dewar, Michael J. W. Stokesbury, Kevin C. Weng, Shana Beemer, Andrew C. Seitz, Charles J. Farwell, Eric D. Prince, Barbara A. Block
Marine Biology (2007) 151:1-18.

Electronic tags were used to examine the biology of Atlantic bluefin tuna on their breeding grounds in the Gulf of Mexico (GOM). Mature Atlantic bluefin tuna tagged in the western Atlantic and the GOM were on their breeding grounds from February to June for an average of 39 days (Figure 1). One tag recorded a fish who visited the spawning grounds three years in a row. The bluefin tuna experienced significantly warmer water temperatures within the GOM than in other parts of their range.

There were three distinct patterns of swimming behavior among bluefin in the Gulf, corresponding to entry, breeding and exit (Figure 2).  As the bluefin tuna entered and exited the GOM, the fish made daily deep dives to depths of nearly 2,000 feet.  Their swimming routes showed rapid, targeted movement directly to and from the spawning grounds. During the alleged breeding phase, bluefin made significantly shallower dives to maximum daily depths of approximately 650 feet.  At night, they exhibited shallow oscillatory dives – a unique diving behavior seen nowhere else in the tagging studies. It is as if the bluefin were performing courtship and spawning at the surface and then diving down for a short period before returning to the surface to continue spawning. In addition, the movement paths of bluefin tuna during the breeding phase were much more sinuous than during the entry and exit phases, and the fish stayed in a much more confined region.

Due to the warm waters of the GOM combined with the energy expended while breeding, bluefin showed the highest body temperatures recorded in TAG research during the breeding phase. To avoid overheating, a calculation of body heat transfer to the surrounding water revealed that bluefin increase their heat loss at night during breeding. Remarkably, this suggests that bluefin regulate their temperature during breeding both by altering their behavior by repeatedly diving into cooler water and also by altering their physiology to increase heat loss. The thermal biology of bluefin in the warm Gulf waters may explain why bluefin tuna caught incidentally by longline fishing vessels experience relatively high mortality.  A bluefin caught on a hook would be unable to dive down to cooler waters and might die from heat stress.

This study demonstrates that the movement patterns, diving behavior, and thermal biology of Atlantic bluefin tuna change significantly at different stages of the breeding migration and can be used to define spawning location and timing. This information is critical to protecting western bluefin tuna on their spawning grounds.

Figure 1: Tracks of two bluefin tuna tagged off North Carolina that migrated to the Gulf of Mexico during the spawning season.

Figure 2: Depth (black), water temperature (blue), and body temperature (red) of a bluefin in the Gulf of Mexico. Vertical grey bars indicate nighttime.

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