| 英文摘要 |
Abundance index of bigeye tuna (Thunnus obesus) in the Atlantic Ocean for Taiwanese longline fishery are presented for the period 1968-2008. The index (number caught per 1,000 hooks) was generated from numbers of bigeye tuna caught, reported and compiled by two series, the TASK II series from 1968 to 1989 and logbook series from 1990 to 2008. Variables used in standardization are year, sub-area, and season (quarter) as fixed factors and year-sub-area interaction. Consequently, the results of abundance index were obtained from a general linear mixed model (GLMM) with lognormal error structure. The stock assessment of bigeye tuna, applying biology, historical catch and effort data and tagging data, abundance indices catch at length and length at age and the models that include production models with Bayesian approach, VPA, SSv3 and MULTIFAN-CL, indicates that the MSY estimated is about 92,000 t (78,000 – 101,000 t). The current TAC (85,000 t) was used in the futures, there are about 60% opportunity to achieve the management goal.
Catch and effort data of Taiwanese small-scaled longline fishery target Pacific bluefin tuna were used to estimate the abundance index, indicating that the index decreased from 1999, stabilized to 2003 and 2004, then dramatically declined to the 2005 minimal level, and increased slightly to 2007 level and decreased slightly to 2008 level. Data of catches, efforts and length measurements of all fishing nations were applied to estimate biological reference points. The sensitivity analysis was used to show uncertainty of biological assumption and fishery data. The results indicate that the assumption of adult natural mortality affects significantly spawning stock biomass and absolute values of fishing mortality. The spawning stock biomass decreased from 2006 and the level of historical observed was about 40-60% as the estimated in 2008. The comparison between F2004-2006 and F2002-2004 shows that the 0-age fish increased about 6%, 1-4-aged fish increased 30% and 5+ age fish increased about 6%. For the robust biological reference point, Fmed, is expected to perform the reproduction and robustness.Further, the length-weight relationship is required useful in estimating abundance. This relationship is various in time and space. Therefore, a hierarchical general linear mixed model was applied with observers’ measurements of length and weight on board to estimate length-weight relationship of bigeye tuna in the Atlantic Ocean. |