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Conventional stock assessment methods require a lot of scientific knowledge and a large quantity of catch and effort data to assess the status of a simple fishery. The Indian Ocean yellowfin tuna (Thunnus albacores) fishery is not a simple ‘fishery’; it is an assemblage of fisheries. The complexity, uncertainty and data deficiency inherent in these fisheries makes it is difficult to predict the impact of a fishery on the stock. Five length-based assessment methods were applied to size frequency data for T.albacores caught using five fishing methods every five years between 1955 and 2015. The resultssuggest that different fisheries are likely to have different impacts on the stock. Longline, hand lineand trolling lines fisheries closely corresponded to the target reference points for sustainable fishing.Pole and line and purse seine fisheries generated the least favourable results for all five simpleassessment methods. Equipped with such knowledge fishery managers can formulate locallyappropriate harvest control management tools to reduce a fishery’s impact on the stock. Extrapolatingfrom the results suggests that 207,170 MT of the T. albacares harvested in 2015 (407,573 MT) wereimmature and only 47,147 MT were caught at optimum length. The annual yield of YFT in 2015 wasnumerically similar to the IOTC’s estimate of MSY (403,000 MT), but given the composition of thecatch (i.e. 52% immature / 12% optimum length) it is unlikely that this yield was sustainable. Theresults cast doubt on whether the harvest control management tools proposed by analyst’s andlobbyist to improve the status of the stock proposed (i.e. catch reductions of 5% - 25%) will beeffective, if the impact of fisheries that harvest IO YFT remains unchanged.
