A large open-water tuna with very long yellow second dorsal and anal fins, seen in the blue rather than on the reef.
To about 240 cm and 200 kg. Metallic dark blue above, silver below with about twenty vertical lines. The dorsal and anal fins and all the small finlets are bright yellow, the finlets narrowly edged black.
In a mature fish the second dorsal and anal fins grow very long and sickle-shaped, sweeping back towards the tail, and that is the character at range. A bigeye tuna of the same size does not develop them.
Recorded from one to over 1,600 metres but usually the top 250, above the thermocline, with most activity in the top hundred. It is sensitive to low oxygen, which is part of what keeps it shallow. It schools by size, alone or mixed with other species, and small fish often travel with skipjack.
The association worth knowing is with dolphins. In the eastern tropical Pacific large yellowfin school beneath pantropical spotted and spinner dolphins, which is the basis of the purse-seine dolphin set and of the tuna-dolphin bycatch problem. Why the fish do it is still argued about in the literature.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.