Evaluating harvest control rules for bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) fisheries in the Indian Ocean

Bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) support two of the most important fisheries in the Indian Ocean. However, there is little research evaluating harvest control rules (HCRs) for their management. In this study we evaluated four HCRs, 'knife-edged', 'linear', 'convex', and 'concave', for these two species. These four HCRs defined management rules for how annual fishing mortality should be adjusted based on perceived stock status. Fishing mortality was adjusted linearly, convexly and concavely for the 'linear', 'convex', and 'concave' HCRs, respectively when the current spawning stock biomass (SSB) was between the limit and target SSB-based biological reference points (BRPs). Two age-structured operating models were developed to simulate fisheries managed under these HCRs for a 25-year management period. Implementation and process errors, and uncertainties in key fisheries parameters were considered as sources of uncertainty in this study. All four HCRs were found to be effective in driving both stocks to the status defined by maximum sustainable yield-based BRPs. The 'knife-edged' HCR, which has constant fishing mortality but switches fishing mortality to 0 when stock biomass is below the limit SSB-based BRP, led to relatively poor performance. Our results indicate that a simulation study is needed to evaluate the performance of BRPs and HCRs in managing bigeye tuna and yellowfin tuna fisheries in the Indian Ocean.