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⚫ Using the SCAS software, a preliminary stock assessment was attempted by the following specification, i.e., four scenarios incorporating nine different variants for the model uncertainties.
⚫ Four scenarios are a combination of the following two assumptions; (a) two types of use in CPUEs, i.e., Western Indian Ocean (IO) (R1+R3) (say 2CPUE) or Whole IO (R1-R4) (say 4CPUE) and (b) the relative weight to CAS against CPUE (0.05 and 0.1), i.e., CASW(0.05) and CASW(0.1) for short. The four scenarios were named as 2CPUE_CASW(0.05), 2CPUE_CASW(0.1), 4CPUE_CASW(0.05) and 4CPUE_CASW(0.1).
⚫ Nine different variants for the model uncertainties area a combination of three levels of σR (0.5, 0.6 and 0.7) and three levels of h (steepness, 0.7, 0.8 and 0.9).
⚫ As convergence was not met in optimization when the original models with 11 fleet with 9 CAS (full spec) were employed, reduced 8 fleets with 8 CAS models were used with an aggregated definition of combined OT fleets (whole IO) from 4 regional OT fleets, and convergence was achieved in all four of those scenarios.
⚫ Based on the retrospective analyses, two scenarios, [A] 2CPUE_CASW(0.1) and [B] 4CPUE_CASW(0.05), were considered to be the most plausible and were used in subsequent evaluations.
⚫ Based on various comparisons of results between the two selected scenarios [A] and [B], scenario [A] seems to be more plausible than [B] by the following two reasons; (a) MSY (78K t) in [B], is likely too high comparing to the current catch (41K t), which is almost twice; and (b) the stock status [B] (SSB ratio=1.83 and F ratio=0.57) is likely too optimistic considering general decreasing/constant trends of the joint CPUE except increasing trends in recent years in R3+R4 (Eastern IO) due to the sharp increasing catch, which implied that CPUE standardization may not reflect the intrinsic CPUE trends. In fact, there are nil correlation between catch and joint CPUE in R4.
⚫ The selected best scenario [A] 2CPUE_CASW(0.1) suggests that the current stock status (2020) is in the orange zone (Kobe plot) (SSB ratio=1.26 and F ratio=1.12) (not overfished but overfishing), MSY(58K ton), and depression(0.28).
