Pulse of the Planet: Wait a Second Before You Bite Into That Tuna Sandwich

Many of us love bluefin tuna – think sushi and sashimi – a fish so in demand that just one can fetch upwards of $10,000. But there are reasons to think twice about eating it. Mercury contamination is one. Another is declining numbers. For 40 years, despite catch regulations, Atlantic bluefin tuna populations have declined  precipitously. Some estimates show western Atlantic bluefin tuna down by 90 percent from preindustrial levels. Overfishing may have already pushed bluefin beyond the brink of return. What’s gone wrong?

The problem may stem from one of the fundamental assumptions used to develop tuna catch limits. Atlantic bluefin tuna (Thunnus thynnus) have two distinct populations: a western one originating in the Gulf of Mexico and an eastern one from the Mediterranean Sea.

The management models used by the International Commission for the Conservation of Atlantic Tunas (ICCAT), the body that regulates fish catch, assume there is limited mixing between the two populations and so catch limits are set for each side of the Atlantic independent of the other population. But what if parts of these populations are being counted on the wrong side of the ocean? Wouldn’t that make it appear as though there were more fish to be caught than there actually are?

The Otolith Story Shows Holes in Catch Limit Assumptions

Figuring out the migratory patterns of oceanic fish is no easy task, but scientists have been creative in developing tools. One of these is the otolith, an organ in our inner ear that helps us sense up from down and keeps us on an even keel so to speak.

Fish have them too, and their calcium carbonate composition (formed from the calcium and carbonate ions dissolved in the ocean) can help us track whither they come and go. Analyzing the isotopic composition of bluefin otoliths allows scientists to tell whether the fish were spawned in the Gulf of Mexico or the Mediterranean.

A new study using the otolith method, by Jay Rooker from Texas A&M and his colleagues, provides stark evidence that ICCAT's assumption is flawed.

Their data, published in Science, show that about half of the adolescent tuna sampled in some western Atlantic foraging grounds were from the Mediterranean. But upon reaching reproductive age, more than 95 percent of them returned to their own spawning location and stayed on that side of the Atlantic. So, counting all the adolescent tuna found in the western Atlantic as tuna originating in that fishery would tend to overestimate the fishery's health and in turn lead to higher catch limits – unsustainable ones.

Tagging Fish Strengthens the Case for Better Counting Methods

Stanford University researchers tag bluefin tuna to see where they breed. The hope is that this information can then be used to create better management tools. (Credit: Tag-a-Giant/tagagiant.org)

The results of this otolith study fit rather nicely with the bluefin tagging program called TAG-A-Giant, a collaborative effort between program co-founder (and Duke alumna) Barbara Block and Andre Boustany, of Stanford University, and Pat Halpin, a researcher at Duke’s Nicholas School of the Environment. 

In Tag-A-Giant, bluefin tuna swimming off the North American coast are tagged with small radio transmitters that allow scientists to track their paths from satellites.

Like the otolith data, the Tag-A-Giant program, which just tagged its thousandth tuna last month, has found that fish tagged off North America travel to both the Mediterranean Sea and Gulf of Mexico, suggesting that these fish represent both populations.

The tagging data also agree with the Rooker study, that once a fish is old enough to spawn it tends to stay on its side of the Atlantic.

Genetic Studies Offer Yet More Evidence of Tuna Populations Mixing

Studies looking at the genetic composition of bluefin tuna add another important piece of the puzzle. Genetic work by Jens Carlsson and colleagues while at William and Mary College and Andre Boustany and his Stanford colleagues, published in Marine Biology, shows distinct differences between the gulf and Mediterranean populations. Such differentiation would be expected to disappear if even small amounts of the adolescent bluefin found in the western Atlantic but originally from the Mediterranean stayed in the western Atlantic to spawn.

So while large numbers of adolescent bluefin tuna manage transatlantic travel to foraging grounds, multiple studies show that the vast majority of adults of spawning age tend to return to their own birthplace to spawn. It seems therefore inappropriate to count Mediterranean bluefin found in the western Atlantic as part of the western Atlantic’s fishery.

A change in the methods used by ICCAT may be in order. As my colleague Pat Halpin points out, “This is an excellent example of how new and emerging scientific techniques, in this case involving genetics, satellite tracking, animal physiology and stable isotope analysis, can help inform critical international management practices.” Let’s see if it does.