Updated Age of Maturity of Pacific Bluefin Tuna

Abstract

Although Pacific Bluefin Tuna (PBF) range throughout the Pacific Ocean, there are two main spawning grounds located in a geographically limited region, one in the area between the Philippines and the Ryukyu Islands (southern) and the other in the Sea of Japan (northern). As of now the age of maturity used in the stock assessment is based on histological observations from only the northern spawning ground. Using this maturity schedule might result in an underestimation of the age of maturity, since all the fish in the population might not be present on the spawning ground and the northern spawning ground has a younger distribution of fish than the southern spawning ground. In this paper, we compare the age distribution of PBF caught on the spawning grounds to the expected age distribution of fish in the overall population to determine the proportion of fish that are spawning in each age class and the probability of maturity at each age class. We found a large discrepancy between the spawning grounds with age of 50% maturity for the Taiwanese spawning grounds at approximately age 11 while in Sea of Japan there was a 50% probability of maturity between age 3 and 4. To explore the effects of estimated age of maturities, a simple age-structured model was created, and different scenarios were projected. An older age of maturity has serious repercussions for spawning stock biomass. With no reduction in fishing mortality, 3.10% of the population in the northern spawning ground reached maturity in comparison to 0.419% of the population in the southern spawning ground.