INTER-AMERICAN TROPICAL TUNA COMMISSION COMISIÓN INTERAMERICANA DEL ATÚN TROPICAL WORKING GROUP ON STOCK ASSESSMENTS 6 TH MEETING LA JOLLA, CALIFORNIA (USA) 2-6 MAY 2005 SAR-6-07b BET DRAFT.doc DOCUMENT SAR-6-07b DRAFT STATUS OF BIGEYE TUNA IN THE EASTERN PACIFIC OCEAN IN 2004 AND OUTLOOK FOR 2005 Mark N. Maunder and Simon D. Hoyle CONTENTS 1. Executive summary .............................................................................................................................. 1 2. Data ...................................................................................................................................................... 3 3. Assumptions and parameters ................................................................................................................ 7 4. Stock assessment ................................................................................................................................ 10 5. Stock status......................................................................................................................................... 17 6. Simulated effects of future fishing operations ................................................................................... 22 7. Future directions................................................................................................................................. 24 Figures ............................................................................................................................................... 27 Tables ................................................................................................................................................. 65 Appendices ........................................................................................................................................ 72 1. EXECUTIVE SUMMARY This report presents the current stock assessment of bigeye tuna (Thunnus obesus) in the eastern Pacific Ocean (EPO). This assessment, and the previous ones, were conducted with A-SCALA, an age- structured catch-at-length analysis. The current version of A-SCALA is similar to that used for the most recent previous assessment. The assessment reported here is based on a single stock in the eastern Pacific Ocean. Its results are consistent with results of other analyses of bigeye tuna on a Pacific-wide basis. The stock assessment requires a substantial amount of information. Data on retained catch, discards, fishing effort, and size compositions of the catches from several different fisheries have been analyzed. Several assumptions regarding processes such as growth, recruitment, movement, natural mortality, fishing mortality, and stock structure have also been made. The differences between the assessments for 2004 and 2003 are as follows: Revised inputs for the prior on length at age from counts of daily rings on otoliths were included in the analysis. The length at age data were also used to covert maturity, fecundity, and age-specific proportions of females in the population from length to age. Length-specific proportions of females in the population we updated using data from the Japanese longline fishery. Changes in the age-specific proportions of females in the population and age-specific maturity were used to update the estimates of natural mortality. The changes in length at age required the model to start at the age of one quarter to cover the range of lengths observed in the fisheries.