Changes in size, age, and sex ratio composition of Pacific bluefin tuna (Thunnus orientalis) on the northwestern Pacific Ocean spawning grounds

Abstract

The Pacific bluefin tuna (Thunnus orientalis, PBF), an iconic fisheries species, is heavily exploited and its biomass is at a historically low level. To enhance effectiveness of management and conservation, a study was initiated to examine the demographic changes of the PBF between 2002 and 2015. Annual tuna age compositions were estimated by applying an empirical age-length key, based on the ages and lengths of 892 fish, to the length frequencies observed among annual catches in the northwestern Pacific spawning grounds located near Taiwan. Ages of the spawners concentrated in 7–20-years old (range = 4–28 years), corresponding to fork lengths between 200 and 240 cm. The size and age compositions were influenced by the strong 1994 and 1996 year-cohorts, which showed a relatively high abundance in most years except 2007–2009. Between 2013 and 2015, recruits of 2005–2009 year-cohorts (age 6–10) increased in relative abundance while the strong 1994 and 1996 year-cohorts decreased as they neared the end of their lifespan. PBF smaller than 200 cm were rarely found in the tropical areas and the mean sizes of the PBF caught south of 24°N were significantly larger than those caught north of 24°N. These results suggested that the choice of spawning latitudes of the PBF was influenced by the size of spawning adults. The sex ratio of smaller (<200 cm) and intermediate size classes (200–230 cm) was female biased (50–75%) while that of larger fish (>240 cm) were male biased (62–100%). The size and age composition of the PBF was mainly influenced by the recruitment of young spawners and was less sensitive to the catch of longline fisheries. However the impact of longline fisheries on the reproductive potential of this species warrants further investigation.