| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 103 === Adult demographic structure is 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 size structures (indexed by mean lengths) and evaluated how adult mean lengths varied with young-of-the-year (YOY) abundance (indexed by PBF troll fishery catch-per-unit-effort, CPUE) and average fishing mortality at juvenile and adult stages. From various sources, we compiled adult lengths based on catch data of three fisheries operated on the PBF spawning grounds, i.e., the Taiwanese and Japanese coastal longliners (TWL and JCL, respectively) at the southern spawning ground in the western Pacific and the Japanese purse seine (JPS) fisheries at the northern spawning ground in the Sea of Japan. In addition, we obtained time series of YOY abundance and fishing mortality rates from the stock assessment reports. Adult mean lengths (in cm) varied considerably among the three fisheries: TWL (219.6) > JCL (188.1) > JPS (139.6). Temporally, adult mean lengths increased since mid-1990s for the TWL and JCL but not JPS. Analysis based on generalized least squares regression (GLS) revealed that juvenile and/or adult fishing morality had significant effects on annual mean lengths for these adult fisheries. In addition, the effect of YOY abundance on mean lengths was significant only for the TWL. Our results provide important information to enhance precision of the PBF stock assessment (e.g., estimation of spawning stock biomass, SSB). To achieve effective fisheries management, we suggest continuing conducting long-term monitoring of demographic changes and studying fishing-induced effects on such changes for the PBF.
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