Yellowfin tuna are a relatively large tuna, easily distinguished as adults by their large second dorsal and anal fins which, along with finlets, are typically bright yellow. However, they can be less easy to distinguish from other tuna (like bigeye) as juveniles (<70cm).
Yellowfin tuna are distributed throughout the tropical and sub-equatorial waters of the WCPO, and typically spend most of their time in the warmer mixed surface waters (above the thermocline). Small yellowfin are caught on the surface by a range of gears including handline, ringnet, purse seine and pole/line gear and are used mainly for canning, while the majority of larger/older fish are caught by both purse seine and longline fisheries, with the longline catch often shipped fresh to overseas markets.
The typical capture size for yellowfin shows two distinct modes in the WCPO, being 20 to 70cm (ringnet, handline, purse seine, pole and line) which corresponds to fish between approximately 3 months and 1.5 years of age, and between 90 and 160cm (purse seine, longline), corresponding to fish mostly between 1.5 and 6-7 years of age. Very few captured fish exceed 180cm or x kg.
Yellowfin tuna is a fast growing species (reaching > 45cm within its first year), have a life span of up to ~7 years of age and mature around 2-3 years of age. These biological characteristics promote moderate turnover in yellowfin populations.
Yellowfin are highly fecund and can spawn year round over a wide area of the tropical and subtropical Pacific, providing environmental conditions (such as water temperature) are suitable. As with many tropical tuna species, environmental conditions are believed to significantly influence recruitment levels over time.
For stock assessment purposes, yellowfin tuna are believed to constitute a single stock in the WCPO.