Tag A Giant
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.
Bluefin tuna have also long captivated the human palate. For thousands of years, Mediterranean peoples have staged a ritual kill called the mattanza, in which a unique formation of underwater nets is used to capture and “slaughter” bluefin tuna as they enter the Mediterranean Sea to spawn. Demand for the bluefin’s ruby red flesh has grown even more fervent over the centuries, and sushi connoisseurs revere bluefin tuna above all else; a single 513 lb. bluefin sold for US$177,000 in 2010 in Tokyo’s famed Tsukiji fish market.
Industrial fishing pressure to fuel what has become known as the sushi economy has taken its toll, and bluefin populations are now severely depleted. Western Atlantic bluefin tuna are listed as Critically Endangered by the IUCN, and eastern Atlantic tuna are listed as Endangered. The western Atlantic bluefin population has shown a 90% reduction in its spawning biomass since 1970, and eastern bluefin have declined by over 50% during the same period. The declines have had dramatic impacts on both commercial and recreational fisheries. The U.S. has not met its full allocated quota for four fishing seasons. Atlantic bluefin tuna have been proposed for protections under the strictest wildlife conservation laws in the world – the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and the U.S. Endangered Species Act.
Tag-A-Giant (TAG) research is shedding light on how to improve fisheries management to ensure a future for wild bluefin tuna. Until recently, knowledge of the movements and biology of bluefin was limited due to challenges posed by studying an animal that remains completely submerged in the open ocean throughout its life history, and fisheries management regimes were therefore compromised. TAG has pioneered the field of biologging science, attaching 1,150 electronic tags to bluefin tuna over the last decade from the Gulf of Mexico to the Gulf of St. Lawrence, and from Ireland to Corsica. TAG research has allowed us to look beneath the surface and discover the elusive life of bluefin tuna, including their migratory patterns, population structure, breeding site fidelity, reproductive schedules, physiology and diving behaviors – all traits that are critical to designing effective management programs.
TAG tagging has demonstrated unequivocally 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. East to west mixing of juveniles from the Med to the western Atlantic fishery may be as high as 30% or more. Movement of adolescent and adult fish from the west to the east is less than 10%. Fish from both populations are affected by fisheries on both sides of the Atlantic due to this mixing. DNA analyses performed by TAG 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, TAG 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. TAG research has also informed the reproductive schedules used in the assessments, showing that age and size of maturation for western bluefin is much higher than previously estimated (~12 years to mean maturity) based on the age and size of tagged bluefin tuna present in the Gulf of Mexico.
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, TAG 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 TAG science 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.
TAG scientists and policy staff will continue to work closely with international and domestic fisheries managers and scientists to communicate our findings and integrate them into efforts to devise and adopt a sustainable management regime for Atlantic bluefin tuna. And we will continue to partner with commercial and recreational fishermen to help us succeed in our work of deploying and recovering tags. New data are collected and analyzed every day, getting us closer to our goal of ensuring a future for bluefin tuna.