With its large size and high quality flesh, the northern bluefin tuna has long been a favourite of fishermen and a highly valued delicacy in Japan (3). Its popularity has led to severe exploitation in several areas, particularly the North Atlantic Ocean (2). Despite catch quotas being in place, limits are often not respected and catches are frequently under-reported (5). Unless adequate management measures are quickly implemented and, most importantly, enforced, the collapse of some northern bluefin tuna stocks is a real possibility (5). It is thought that the Critically Endangered western Atlantic stock may have already collapsed, the result of overfishing and poor management. This stock is now in grave danger of extinction (6). In the Mediterranean, where adult northern bluefin tuna decreased by 80 percent between around 1979 and 1999, tuna ranching now poses the greatest threat to the survival of this species. Tuna are captured alive and taken to one of the many ranches that have spread along the Mediterranean coast, where they are fed and fattened for months; the enormous amounts of fish needed to feed the tuna during this time is itself a matter of great concern. The tuna is then sold, primarily to Japan, creating a lucrative business that is considered to be the main force driving illegal and unreported fishing (7).