Why Is the Tuna Critically Endangered and Still Sold in the Market

In 1996, the International Union for Conservation of Nature (IUCN) designated southern bluefin tuna as a critically endangered species, which is the highest endangered category of threatened species. Threatened species in IUCN’s Red List is determined based on five quantitative criteria, which are based on precautionary principle for organisms whose populations are not well known. When these criteria are applied directly to a commercially exploited species, it meets the criteria even if it is apparently secure. This section introduces the concept of judgment criteria for endangered species and its concerns. Threatened species is listed if the reduction rate of population size is sufficiently large, irrespective of whatever large population size. This is because for most unexploited threatened species the population size is completely uncertain. Therefore, many marine fish species, including tuna, which are exploited by commercial fishing are listed as threatened species. For many fish species that are commercially exploited, the absolute number of mature individuals and the variance in its yearly reduction rare are often known. Extinction risk depends on the absolute population size, population reduction rate, and its temporal variation. Red List lists many species that are apparently secure, based on the precautionary principle. We propose a method for estimating the extinction risk of tuna based on the variance of the reduction rate. We investigated the sensitivity in the uncertain parameters involved in the models and concluded that tuna is unlikely to be listed as critically endangered but that southern bluefin tuna may be listed as vulnerable. We also propose the probability that the southern bluefin tuna is recovered by the year 2020, which is not optimistic. Finally, we obtain the optimal age-specific fishing policy by using the Lagrange method.