STATUS OF BIGEYE TUNA IN THE EASTERN PACIFIC OCEAN IN 2007 AND OUTLOOK FOR THE FUTURE

1. SUMMARY This report presents the current stock assessment of bigeye tuna (Thunnus obesus) in the eastern Pacific Ocean (EPO). As in the last assessment, this assessment was conducted using Stock Synthesis II (SS2; Methot 2005). The assessment reported here is based on the assumption that there is a single stock of bigeye in the EPO, and that there is no exchange of fish between the EPO and the western and central Pacific Ocean. The stock assessment requires a substantial amount of information. Data on retained catch, discards, catch per unit of effort (CPUE), used as indices of abundance, and size compositions of the catches from several different fisheries have been analyzed. Several assumptions regarding processes such as growth, recruitment, movement, natural mortality, and fishing mortality, have also been made. Catch, CPUE, and length-frequency data for the surface fisheries have been updated to include new data for 2007 and revised data for 2003-2006. Analyses were carried out to assess the sensitivity of results to: 1) a stock-recruitment relationship; 2) use of the southern longline CPUE data only; 3) using two time blocks for the size selectivities of the floating-object fisheries, separated by the implementation in 2001 of IATTC Resolution C-00-08, which prohibited discards of tunas in the EPO. There have been important changes in the amount of fishing mortality caused by the fisheries that catch bigeye tuna in the EPO. On average, since 1993 the fishing mortality of bigeye less than about 15 quarters old has increased substantially, and that of fish more than about 15 quarters old has increased slightly. The increase in the fishing mortality of the younger fish was caused by the expansion of the fisheries that catch tuna in association with floating objects. Over the range of spawning biomasses estimated by the base case assessment, the abundance of bigeye recruits appears to be unrelated to the spawning potential of adult females at the time of hatching. There are several important features in the estimated time series of bigeye recruitment. First, estimates of recruitment before 1993 are very uncertain, as the floating-object fisheries were not catching significant amounts of small bigeye. There was a period of above-average recruitment in 1995-1998, followed by a period of below-average recruitment in 1999-2000. The recruitments have been above average since 2000, and were particularly large in 2005. The most recent recruitment is very uncertain, due to the fact that recently-recruited bigeye are represented in only a few length-frequency samples. The extended