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Bigeye tuna, Thunnus obesus (Lowe, 1839), is a large epi- and mesopelagic fish distributed in tropical and subtropical waters of Indian Ocean. The bigeye tuna (BET) resource was initially harvested by longlines since the 1950s and now is one of the main economic tuna recourses in the Indian Ocean. They are currently caught by longliners (deep-freezing and fresh-tuna longliners), purse seiners (free-school and associated school), pole and line, and other small fleets as well.
Stock assessments of BET in the Indian Ocean in recent years were conducted using forward-projecting based models such as Stock Synthesis (Shono et al., 2009; Kolody et al., 2010), Virtual population analysis (VPA) (Nishida and Takeuchi, 1999), and age-structured production model (ASMP)(Nishida and Rademeyer, 2011). These assessments suggested the Indian Ocean BET there was low probability that the stock has been overfished and overfishing is probably not occurring (Kolody et al., 2010; Nishida and Rademeyer, 2011). However, it should be cautious that the BET assessments were associated with many uncertainties according to explorations of extensive sensitivity analysis (Kolody et al., 2010).
Following the uncertainty remaining in the assessments carried out for the previous Working Party on Tropical Tunas (WPTT) meetings in 2010 and 2011, the WPTT recommended that bigeye tuna would be the priority species for stock assessments in 2013 (IOTC–WPTT14, 2012).
This working paper presented a stock assessment of Indian Ocean BET using Age Structured Assessment Program (ASAP, Version 3; NOAA Fisheries Toolbox, 2011). ASAP is a formal stock assessment model and has been used for assessing many commercially exploited stocks, e.g. red grouper, yellowtail flounder, Pacific sardine, Greenland halibut, Gulf of Maine cod, Florida lobster (see NOAA Fisheries Toolbox, http://nft.nefsc.noaa.gov).
The assessment included a base-case model and sensitivity analyses designed for the consideration of alternative key assumption regarding population dynamics (i.e., the steepness parameter of the stock-recruitment relationship), and weighting scheme for the most important two abundance indices being used to tune the model. Stock status was evaluated based on fishing mortality and spawning stock biomass based reference points. Kobe plots will be presented to show historical trends in stock status, as recommended by the Scientific Committed (SC, 2012?).
The current assessment was not considered to be final and it would be revised and improved based on the consensus in recommendations and comments reached on the WPTT15 meeting.
