Stock assessment of yellowfin tuna in the western and central Pacific Ocean

Yellowfin tuna, an important component of tuna fisheries throughout the WCPO, are harvested with a diverse variety of gear types, from small-scale artisanal fisheries in Pacific Island and southeast Asian waters to large, distant-water longliners and purse seiners that operate widely in equatorial and tropical waters. Purse seiners catch a wide size range of yellowfin tuna, whereas the longline fishery takes mostly adult fish. Since 2000, the total yellowfin tuna catch in the WCPO has varied between 370,000 and 440,000 mt. Purse seiners harvest the majority of the yellowfin tuna catch (53% by weight in 2007), with the longline and pole-and-line fisheries comprising 16% and 4% of the total catch, respectively (source: WCPFC 2007 Yearbook). Yellowfin tuna usually represent approximately 20–25% of the overall purse-seine catch and may contribute higher percentages of the catch in individual sets. Yellowfin tuna is often directly targeted by purse seiners, especially as unassociated schools which accounted for 56% of recent (2000–2005) yellowfin purse-seine catch (by weight). Longline catches in recent years (70,000–80,000 mt) are well below catches in the late 1970s to early 1980s (which peaked at about 110,000 mt), presumably related to changes in targeting practices by some of the larger fleets. The domestic fisheries of the Philippines and eastern Indonesia catch yellowfin using a variety of gear types (e.g. pole-and-line, ringnet, gillnet, handline and seine net). Catches from these fisheries have increased over the past decade and are estimated to represent approximately 25–30% of total WCPO yellowfin tuna catches. This paper presents the 2009 assessment of yellowfin tuna in the western and central Pacific Ocean. The assessment uses the stock assessment model and computer software known as MULTIFAN-CL. The yellowfin tuna model is age (28 age-classes) and spatially structured (6 regions) and the catch, effort, size composition and tagging data used in the model are classified by 24 fisheries and quarterly time periods from 1952 through 2008. The spatial and fishery structure is equivalent to that used in the 2007 assessment and the data sets have been updated to include the catch, effort, and size composition data from the last two years. However, there have been a number of significant changes to the model inputs, in particular the adoption of an alternative catch history for the purse-seine fleet that includes a substantially higher level of catch for the associated purse-seine fishery. There have also been refinements to the catch histories from the …