Poster Presentation Australian Society for Fish Biology Conference 2024

Abundance index of adult fish stock supports the recovery of Pacific bluefin tuna stock   (#305)

Shui-Kai (Eric) Chang 1 , Tzu-Lun Yuan 2
  1. Graduate Institute of Marine Affairs, National Sun Yat-sen University, Kaohsiung, Taiwan
  2. Department of Statistics, Tunghai University, Taichung, Taiwan

Pacific bluefin tuna (PBF) stock has declined to globally concerned low levels in the 2010s. In Taiwan, the small-scale longline fishery targets adult PBF aged 5 and above in waters from the northern Philippines to southern Japan. While there was concern over the sharp decline in annual catch, the gradually increased size of the first PBF caught during the season's "First PBF Contest" led to satisfaction due to higher auction prices. Insufficient logbook data complicated the analysis of this situation. This study reconstructed logbook-like data using GIS-equipped voyage data recorders (VDR) and inspector-monitored market landing data. Catch per unit effort (CPUE) from the reconstructed data was standardized using the vector auto-regressive spatiotemporal model (VAST), incorporating age group information from spatiotemporal size data. Results indicated that the lack of young adult fish recruitment led to decreased CPUE and increased average fish size in the 2010s. However, a significant influx of young adult fish in the late 2010s suggested a stock recovery, evidenced by a sharp rise in CPUE and a reduction in average catch size as well as the Pacific-wide increase of PBF catch. This method of data reconstruction and spatiotemporal CPUE analysis proved effective for studying stock dynamics in fisheries lacking comprehensive logbook data.