| 英文摘要 |
In this project, we trained scientific observers to collect otoliths of southern bluefin tuna (SBT, Thunnus maccoyii) on Taiwanese longline vessels and also estimated the age of 61 otoliths collected in 2008. The mean values of these otoliths were 2.79 ± 0.9 years, which was approximately 1 - 2 smaller than the samples collected from 2003 - 2007. The length frequency distribution of total catch in 2008 (n = 29329) ranges from 80 - 160 cm, with most samples in the size between 100 - 130 cm. The length composition in 2008 was similar to the catch compositions in previous years. The length data were transformed to age data by the age-length key. The age composition of total catch in 2008 ranged from 0 - 10 years. The mean value was 4.3 ± 1.7 years with approximately 80% of the individuals in the range of 3 - 5.
Pacific bluefin tuna (PBT, Thunnus orientalis) has two spawning grounds. Most PBT spawn in the warm Pacific Ocean from northeastern Philippines to southern Okinawa while some PBT spawn in the Sea of Japan. We developed a new methodology to discriminate the origin of PBT in this project by analyzing otolith δ18O profile. For the PBT hatched in the Sea of Japan, their δ18O values were around -2.5? to -2.3? from otolith core to approx. 600 μm from the core. Then the values rapidly increased (enriched) to approx. -0.8? to -1? at around 600 - 1000 μm from the core. For the PBT hatched in the Pacific Ocean, their otolith δ18O values were stable and varied between -2.8? to -2.5? from otolith core to appro. 1000 μm from the core. During this stage, the otolith δ18O of some PBT might show gradual decrease from -2.5? to -2.8? then the values returned to similar level at around 1000 μm. However, the values would eventually increase to approx. -1.5? to -1? at around 1200 - 1500 μm from the core. Based on these results, we are able to discriminate the origin of the PBT either from the Sea of Japan or from the Pacific Ocean. |