National Geographic : 2014 Mar
86 national geographic • March 2014 been solved by tagging technology pioneered by Block and others. The interior of Block’s lab makes a sort of gal- lery. The walls and cabinet doors, plastered with charts, maps, and illustrations from scientific journals, amount to an exhibit. If it had a title, it might be called “State of the Bluefin.” The state of the bluefin is not good. One post- er, “Estimated Spawning Stock Atlantic Bluefin Tuna (1950-2008),” shows a graph of the spawn- ing biomass of Gulf of Mexico breeders atop a similar graph for the Mediterranean breeders. Both populations are represented by lines in the shape of eels, and both eels are diving toward the bottom of their graphs. They have plunged past the dotted line representing sustainable yield and are headed for a spot where the kilotons of spawning biomass read zero. The art on the maps is a kind of pointillism. The locations of bluefin, as reported by the many electronic tags deployed by the lab over the years, are represented as a proliferation of small circles in many colors. The maps of most interest to Block show the distribution of bluefin in relation to something called the ICCAT line. The fisheries for the Atlantic bluefin tuna are managed by the International Commis- sion for the Conservation of Atlantic Tunas. ICCAT stock assessment models make use of a dotted line dividing the North Atlantic verti- cally. Drawn in 1981, this demarcation follows the meridian at 45° west longitude and divides the western stock of Atlantic bluefin from the eastern. The lab’s pointillist maps show a curi- ous thing. The positions of electronically tagged western bluefin, represented as reddish-orange circles, pack the Gulf of Mexico, the spawning grounds for this stock, and from there spill east- ward into the Atlantic. They cross the ICCAT line with impunity and spread all the way to Por- tugal and Spain. The positions of tagged eastern- breeding bluefin, represented as white circles, pack the Mediterranean, spawning grounds for this stock, and from there spill westward, cross- ing the ICCAT line to fill the coastal waters of the United States and Canada. The ICCAT line, the maps testify, is a fiction. Scientists once believed that each stock stayed on its own side of the ocean, but no one be- lieves that now. Everywhere in the Atlantic, all across the feeding grounds of this species, the eastern and western stocks mix. It seems that only in their respective spawning grounds are they separate. The fact of this mixing was well established by Block, other taggers, and DNA researchers more than a decade ago. It has yet to be incorporated in ICCAT models. The best estimates today are that around half of the bluefin caught off the eastern shores of North America originated in the Mediterranean, yet any of these tuna, if caught in the west, are still counted as fish of western origin. The ICCAT line is not just a dull management tool—it is no tool at all. The ICCAT model fails, as well, to make any allow- ance for illegal fishing, though studies indicate that the illegal take is large. For most of its history, ICCAT has ignored the advice of its scientific panel, the Standing Committee on Research and Statistics (SCRS). For the eastern stock that breeds in the Medi- terranean, the much larger population, ICCAT has routinely set quotas far higher than sci- ence recommends. In 2008 the SCRS issued its most alarming assessment yet on the status of the eastern stock. The actual catch, the scien- tists reported, was likely more than double the 28,500 metric tons of the allowable catch, and more than quadruple a level that was sustainable. They recommended closing the fishery through The good news is that Atlantic bluefin populations, if allowed to rebound, could grow to five times their present size.