||In this project, we trained scientific observers to collect otoliths of southern bluefin tuna (SBT, Thunnus maccoyii) on Taiwanese longline vessels in the Indian Ocean. The otoliths collected from SBT and Pacific bluefin tuna (PBT, Thunnus orientalis) were used for age estimation. In 2012, 33 SBT heads were collected in the central Indian Ocean and otoliths were removed in the lab. The estimated mean ages were 1.9 ± 0.7 years (range: 1-4 year). Another batch of SBT otolith were collected in the central Indian Ocean 2007 and these samples were aged this year. The mean age of year 2007 samples was 3.4 ± 0.9 years with a range from 2-5 years. The observers also collected otoliths from 168 PBT landed in domestic harbors. The estimated mean age for these PBT was 15.5 ± 3.9 years (rang: 6-26 years). The historical length data of the landed PBT measured by the observers showed gradually increasing from approximately 210 cm at year 2002 to 230 cm in recent years (2010-2013). The estimated mean age of the landed PBT also gradually increase from approximately 11 to 17 years. The decreasing recruitment of the immature PBT might be the main reason that cause the increasing trend of mean size and age of PBT landed in Taiwan although other possibilities were carefully discussed in this report.