Preliminary stock assessment of Indian Ocean yellowfin tuna using Statistical-Catch-At-Size (SCAS) (1950-2020

⚫ We conducted preliminary yellowfin tuna stock assessments (1950-2020) using the SCAS software. The primary objective of our assessments is to evaluate the current YFT stock status (2020) as a reference for SS3.
⚫ Major differences between SCAS vs. SS3 is the time unit (annual vs. quarter) and tagging data (without vs. with). Other input information is nearly identical.
⚫ We searched the current stock status (2020) using the wider ranges of grids by combining 6 factors, i.e., (a) 2 types of area models, i.e., whole (one) area (9 fleets) model and 4 sub-areas (21 fleets) model, (b) 3 types of CPUE, i.e., LL (longline), PSA (Purse Seine Adult) and PSJ (Purse Seine Juvenile), (c) 3 steepness (0.7, 0.8 and 0.9), (d) 3 σ for recruitment deviation (0.4, 0.6 and 0.8) and (e) 2 weightings for CAS (0.1 and 0.01). The total number of grids is 108, i.e., 54 each for the whole area (9 fleets) model and the 4 sub-areas (21 fleets) model.
⚫ Only 9 girds out of 108 produced convergences, i.e., 3 for the whole area model and 6 for the 4 areas model. Even numbers (9 grids) are small, this implies that 4 areas (21 fleets) model is likely more plausible.
⚫ The representative result (the median point of 9 grids) suggested that YFT stock status (2020) is very unhealthy (TB2020/TBmsy=0.74 and F2020/Fmsy=1.84) (high level of the red zone in the Kobe plot).
⚫ Considering the current situation, we considered that this result is implausible. We presume that the effort creep may affect the results, especially for F. Thus, we attempted further SCAS runs with 1% and 2% annual effort creeps of the median point of the first run. For these attempts, we incorporated 1% and 2% to standardized CPUE.
⚫ As a result, when 1%, the stock status is still unhealthy but closed to both MSY (0.80 and 1.18) (red zone) and when 2%, it is healthier than in 1% (1.09 and 1.06) (orange zone).
⚫ Considering the current situation, YFT stock status (2020) without the effort creep is implausible, thus we consider the plausible stock statuses (2020) are likely around in 1% annual effort creep, i.e., F2020/Fmsy=1.18 and TB2020/TBmsy=0.80.
⚫ We did not consider the results with 2% as it produces too high creep effect, i.e., 248% in 46 years in case for LL CPUE, while 1%, for 158%, which is likely more realistic.