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In Tiny Ear Bones, the Life Story of a Giant Bluefin Tuna

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The Atlantic bluefin tuna, Thunnus thynnus, is one of the fastest, most powerful and most beautiful of fish. It is also the most expensive. Highly prized by sushi connoisseurs, a single giant fish of 1,400 pounds may sell for $40,000.

The tuna’s high price has led the fishery to the brink of collapse. In 1981, in response to declining numbers of tuna, the International Commission for Conservation of Atlantic Tunas (ICCAT) introduced a strict management policy for Atlantic bluefin that rapidly developed into one of the most controversial and politically charged issues in fisheries management.

The policy controversy, familiar to both commercial and recreational New England fishermen, centers on the assumption that there are two discrete and independent North Atlantic populations. The two populations are arbitrarily divided into eastern and western territories at the 45°W meridian. Each presumed stock is subject to different management restrictions, the most striking of which is the imposition of a strict, near-zero harvest quota for the western stock and the absence of country-specific quotas for the eastern stock.

There is considerable debate concerning the appropriateness of the two-stock division because evidence is lacking to support its two key assumptions. The first is that eastern and western tuna populations reproduce separately in separate spawning grounds, with western fish spawning in the Gulf of Mexico and eastern fish in the Mediterranean. The second is that the tuna populations do not migrate across the Atlantic and intermingle.

A large research effort is currently underway to test these assumptions by tracking the movements of individual fish across the North Atlantic and studying their spawning behavior. Much of this effort—led by Barbara Block of Stanford University and Molly Lutcavage of the New England Aquarium—has involved the use of sophisticated pop-up satellite tags.

Pop-up satellite tags currently have limited life-spans, ranging perhaps from months to years. At Woods Hole Oceanographic Institution, we are investigating the feasibility of using chemical signatures in the otoliths, or ear bones, of giant fish to obtain information about trans-Atlantic migrations, stock mixing, and spawning habitats. The entire and detailed life history, from birth to death, of a giant 30-year-old bluefin is contained within a single otolith, or ear bone, less than one inch long.

Our approach is based on the premise that differences in water chemistry and temperature experienced by fish during their oceanic travels will be recorded as distinct and predictable changes in the trace elements of aragonite, the mineral that makes up the otolith. This approach differs from most previous otolith studies in our use of microbeam technology to track chemical changes at weekly to daily resolution, within a single ear bone.

Using the micron-scale sampling capabilities of the Cameca 3f ion microprobe and techniques developed to study coral skeletons, we have been able to analyze the chemical composition of the primordium, a region of otolith just 20 microns in diameter. The primordium forms when the fish is still in the larval stage, and its chemical composition contains a record of where the fish was born.

Our initial results are promising and show that we may be able to use chemical signatures in the primordium to distinguish different populations of bluefin tuna—in their first years of life when the primordium is being formed. With conventional bulk sample analyses, we are not able to distinguish between different stages (i.e. larval, juvenile, adult) of otolith formation. By contrast, our approach gives us the ability to reconstruct the life history of the fish from birth to death.

Because we can obtain a daily, rather than an average, record of the tuna’s travels, we may be able to tell when, during the tuna’s long life, it swam in which waters. We can potentially discern whether the tuna was born in the west and migrated east, or was born in the east and migrated west, instead of knowing only that it was in both areas sometime during its life. This will improve our ability to manage populations of this magnificent fish.

Images

Chemical clues within the 2-centimeter-long ear bones of bluefin tuna are giving scientists the ability to track which waters the fish swam in and reconstruct their life histories. (Illustration by E. Paul Oberlander, WHOI)

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