The deep scattering layer is the largest daily movement of animals on Earth: mesopelagic fish, squid and crustaceans ascend at dusk to feed in surface waters and descend at dawn to escape visual predators. Bigeye tuna are among the predators that follow it rather than waiting above it.
Electronic tagging shows bigeye making repeated daytime descents into cool, oxygen-poor water well below the thermocline, returning to warmer surface layers between dives, apparently to rewarm. The species has physiological equipment suited to this: a large eye adapted to low light and cranial heat exchange that keeps the eye and brain warmer than the surrounding water, preserving visual and neural performance in the cold.
This vertical behaviour has a fishing consequence. Deep-set longlines target bigeye precisely where it spends the day, while surface fisheries using fish aggregating devices catch juveniles before they mature. That combination pressures the stock at both ends of its life, and bigeye is assessed as Vulnerable. How much heat and oxygen conditions limit the depth range, and how ocean deoxygenation will change it, is an open question.