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Total Allowable Catches (TAC’s) have been implemented for numerous stocks by ICCAT. However, in the case of tropical tunas (yellowfin (Thunnus albacares) and bigeye (Thunnus obesus)), catch controls, while intended to ensure that overall fishing mortalities are not exceeded, have failed to maintain catches at the desired level because some ICCAT CPCs have exceed targets on a regular basis or were not covered by the measures. The purpose of this study is to explore how full seasonal closures (over an estimated time-frame), where vessels remain in port, may better assist surface fisheries in achieving the levels of catch reduction sought by the ICCAT. It presents a model based on parameter estimates of individual models to estimate catches by time as a function of available biomass for BET, effort by strata (month), and month-effort interactions to estimate BET catch targets (and associated YFT and SKJ as a result). The implementation of seasonal fishery closures has proved successful at the IATTC, which has been using a control rule based on this principle for over fifteen years with stocks maintained by the target reference level throughout that period. Management systems based on seasonal fishery closures have also proved to be more efficient than those based on TACs, due to the latter leading to underreporting unless extensive monitoring is in place. Some examples of how the control rule may be implemented are provided. A decision support tool is developed based on the data and proposed season closures to implement an overall target catch on Bigeye tuna, one of the stocks managed to a TAC by ICCAT.
