Spatial and Temporal Variation in Adult Age Structures of Pacific Bluefin Tuna Thunnus Orientalis

Adult age structures are a critical parameter determining reproductive potential for the long-lived iteroparous fishes. To enhance understanding of the Pacific Bluefin tuna (PBF) population dynamics, we explored spatial and temporal variation in adult age structures (indexed by mean age) and evaluated how this parameter varied with recruitment (indexed by catch at recruiting ages) and older-age year-class effects (i.e., age-specific residuals derived from cohort catch-curve analysis). We used catch data of three fisheries that harvest adult PBF on different spawning area (i.e., in the west Pacific: Taiwanese longline, TWL, 1993-2013, and Japanese coastal longline, JCL, 1952-1966 and 1994-2009; in the Sea of Japan: Japanese purse seine, JPS, 1987-2010; available from ISC reports). Adult mean age varied considerably among the three fisheries (in years): TWL (11.8) > JCL (8.2) > JPS (5.0). Temporally, adult mean age increased for the TWL and JCL but decreased for the JPS during 2003-2010. A stepwise multiple regression revealed negative effects of recruitment and positive effects of age-specific residuals, with differential levels of significance and effect sizes among fisheries. The spatial and temporal heterogeneity of adult age structures reveals the non-equilibrium patterns of PBF population structure, potentially challenging stock assessment and forecasting.