Stock assessment of bigeye tuna in the Indian Ocean for 2012

The distant-water longline fishery commenced operation in the Indian Ocean during the early 1950s. Bigeye tuna represented a significant component of the total catch from the longline fishery and catches increased steadily over the subsequent decades, reaching a peak in the late 1990s-early 2000s. The purse-seine fisheries and fresh-chilled longline fisheries developed from the mid 1980s and total bigeye tuna catches peaked at about 150,000 mt in the late 1990s. Since the mid 2000s, the total annual bigeye catch has declined considerably, primarily due to a decline in the longline catch in the western equatorial region in response to the threat of piracy off the Somali coast. The total annual catch declined to 87,235 mt in 2010 and then subsequently recovered to 115,793 in 2012 (Herrera et al 2013). A detailed description of the Indian Ocean bigeye tuna fishery and available fishery data are presented in Herrera et al (2013).
Recent assessments of the Indian Ocean bigeye tuna stock have been conducted using Stock Synthesis (Kolody et al. 2010 and Shono et al. 2009) and ASPM (Nishida & Rademeyer 2011) software. The two most recent assessments (2010 SS3 and ASPM 2011) form the basis for the current management advice for bigeye tuna (IOTC 2011). Both assessments methodologies estimated that recent fishing mortality rates were below the FMSY reference level (Fcurrent/FMSY < 1) and that the stock was not overfished (SBcurrent/SBMSY > 1). However, Kolody et al. (2010) conducted a thorough examination of the sensitivity of the assessment to a range of the key model assumptions. The resulting model scenarios highlighted the high level of uncertainty associated with the stock assessment, encompassing a range of contrasting estimates of stock status.
Langley et al. (2013) updated the IO bigeye assessment, extending the model time period to 2011. The modelling investigated a wide range of model sensitivities particularly regarding the spatial configuration of the model, the treatment of the available tagging data and the relative influence of the individual length frequency data sets. The assessment was updated again during the WPTT15 meeting, including catch, CPUE indices and size frequency data to the end of the 2012 year. This report documents the updated assessment of the Indian Ocean bigeye tuna stock. The assessment formed the basis for the bigeye tuna management advice from WPTT15.