Bluefin tuna are monumental in size, speed, power, value, and charisma. They are among the largest fish on earth and make trans-oceanic migrations at speeds to rival the fastest racehorses. They dive to abyssal depths of nearly a mile and ply waters from the equator to frigid polar seas. Amazingly, they have the capacity to maintain a warm, stable body temperature throughout their wide thermal niche like a mammal or bird. They have captivated humans for millennia; images of bluefin once graced the same coins as Hercules.
Fourteen years of tuna tagging has shown that there are at least two stocks of Atlantic bluefin tuna that mix significantly on North Atlantic foraging grounds but separate to distinct spawning grounds in the Gulf of Mexico and Mediterranean Sea. Fish from both populations are affected by fisheries on both sides of the Atlantic due to this mixing. DNA analyses performed by TRCC scientists have corroborated the tagging data, showing conclusively that there are two separate populations of Atlantic bluefin tuna. Armed with a better understanding of mixing rates, scientists are now collaborating with colleagues in Canada and at NOAA to revise stock assessment methodologies to more accurately determine population structure and the number of fish remaining in the two populations.
Tagging has also revealed critical breeding habitat and foraging hotspots. A unique oscillatory diving behavior exhibited by fish in the Gulf of Mexico has been interpreted as courtship or spawning. By examining the locations of the observed spawning and comparing it to remotely sensed oceanographic data, our scientists have developed a new dynamic habitat utilization model that can predict where bluefin are most likely to spawn by comparing real-time oceanographic data to habitat preferences identified through tagging; this type of model has the potential to allow implementation of a dynamic time-area closure to prevent bycatch mortality of spawning bluefin tuna. Likewise, feeding aggregations revealed by this research can be targeted for spatial management; recent research suggests that the Gulf of St. Lawrence, in particular, is an important congregation point for western bluefin prior to spawning.