The yellowfin tuna is a schooling fish, and has a strong tendency to aggregate with fish of the same size, rather that just school with other yellowfin tunas (2). They can often be seen swimming near the surface with other tunas, such as skipjack tuna (Katsuwonus pelamis) and bigeye tuna (Thunnus obesus). Large yellowfin tunas have also been seen with porpoises or dolphins, particularly the spotted, spinner and common dolphins (2) (4). Schools of yellowfin tuna under floating debris are also often observed (4).
This powerful swimmer is an opportunistic predator, feeding on a wide variety of fish, squid, cuttlefish, octopus, shrimp, lobster and oceanic crabs (2). Some of the fish consumed include pilchard, anchovy, mackerel, and even other tunas (2). They apparently locate their food simply by sight, as they search for food primarily in the surface waters during daylight (2).
Spawning in yellowfin tuna populations can occur at any time of the year, but is most frequent during the summer months in each hemisphere (4). Each female releases several million eggs each year into the ocean (2), which are fertilised by the sperm released by the males. Yellowfin tuna juveniles grow quickly, reaching a weight of 3.4 kilograms in 18 months (2).