Surface-schooling, adult skipjack tuna (greater than 40 cm fork length, FL) are commonly found in tropical and subtropical waters of the Pacific Ocean. Skipjack in the western and central Pacific Ocean (WCPO) are considered a single stock for assessment purposes (Wild and Hampton 1994). In the western Pacific, warm, poleward-flowing currents near northern Japan and southern Australia extend their distribution to 40 N and 40 S. These limits roughly correspond to the 20 C surface isotherm. A substantial amount of information on skipjack movement is available from tagging programmes (Figure 1). In general, skipjack movement is highly variable (Sibert et al. 1999) but is thought to be influenced by large-scale oceanographic variability (Lehodey et al. 1997).

Skipjack growth is rapid compared to yellowfin and bigeye tuna. In the Pacific, approximate age estimates from tagging and otoliths indicate FLs of 48, 65, 75, and 80 cm for ages 1–4 (Tanabe et al. 2003); though significant differences occur between individuals. The longest period at liberty for a tagged skipjack was 4.5 years. Estimates of natural mortality rate have been obtained using a sizestructured tag attrition model (Hampton 2000), which indicated that natural mortality was substantially larger for small skipjack (21–30 cm FL, M= 0.8 mo-1) than larger skipjack (51–70 cm FL, M= 0.12–0.15 mo-1).