Bigeye tuna dive deeper and more persistently than yellowfin or skipjack. Tag records show daytime residence commonly below 300 metres, with excursions considerably deeper, followed by ascent into warm surface layers at night. The pattern matches the nightly rise and daily descent of mesopelagic prey.
Staying deep imposes a thermal cost, and bigeye handle it with the tuna toolkit: a counter-current vascular exchanger that retains metabolic heat in the swimming muscle, plus periodic returns to warm water. Their large eyes fit the low light at depth, which is where the common name comes from.
The depth habit has practical consequences. Because bigeye occupy deeper water by day than surface-associated tunas, they are caught by deep-set longlines and, as juveniles, in purse-seine sets around drifting fish aggregating devices. Managing catch of small bigeye taken alongside skipjack has been a persistent problem in tropical tuna fisheries.