Longtail tuna (Thunnus tonggol) stock assessment in the Indian Ocean by ASPIC (A Stock-Production model Incorporating Covariates) using available CPUE information

We attempted the stock assessment for longtail tuna in the Indian Ocean by ASPIC using nominal catch and four available CPUE (1950-2013). We assume that longtail tuna in the Indian Ocean is a single stock. Results of the ASPIC analysis suggested that longtail tuna stock status (2013) is in the overfishing phase (orange zone in the Kobe plot) (F/Fmsy=1.43 and TB/TBmsy=1.01), i.e., high F (high fishing pressure, 43% above the Fmsy level), while the TB is about in the TBmsy level. Uncertainty around the 2013 point estimate in the Kobe plot is covered by 54% in the red zone, 25% in orange and 21% in green. In addition, the direction of the stock status trajectory vector is toward the red zone. These facts suggest that the 2013 stock status has the high probability in the red (overfished) zone. The risk assessment (Kobe II) suggests that if the current catch continues (159,313 t), there are high risks (100%) for both TB and F to violate their MSY levels. If the current catch level is reduced by 30%, then risk probabilities for both TB and F will be reduced by 50% in three years later (2016).