Field notebook
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Animal profile
Thunnus obesus
The bigeye tuna is a large streamlined tuna distinguished from yellowfin by its larger eye and stouter body. It dives deeper and stays deeper by day than most tunas, following the deep scattering layer. Adults can exceed two metres and 200 kilograms.
Every night a vast layer of small fish and squid rises toward the surface, and every dawn it sinks again. Bigeye tuna go down with it, spending daylight hours in cold, dim water hundreds of metres deep that other tropical tunas avoid.
Archival tags show bigeye tuna spending daylight hours several hundred metres down in cold water and rising towards the surface at night, tracking the vertical migration of the prey layer rather than the sun.