To give you a better sense of how this works, try to imagine the sea as a 1,500 m- high column of water that you dive into on a summer day. On the surface, there’s more sunlight, and the temperature is higher than down at the deep end, where the light doesn’t reach and the water is darker and colder. The eggs of all tuna species need warm water, so they stay near the surface. They also float. In addition, the larvae need lots of light to spot their prey among the plankton, and warmer temperatures to survive. So, generally speaking, we could say that during the first month of their lives, all tunas live in the surface water, usually in the top 20-30 meters. Also, in general, when they breed, tunas stay in the surface water so they can spawn their eggs where the environmental conditions are best for their survival.
As they grow, tunas develop muscles and fins for swimming and accelerating quickly. They also develop the ability to raise their body temperature above the surrounding water temperature, which enables them to endure cold waters in addition to warming their eyes and improving their low-light vision. Adult tunas usually live at 100-400 meters below the surface, although the exact depth varies across different individuals and species. In general, tunas spend the daytime in deeper waters than at night. They also often go down into the deepest water in search of prey. Atlantic bluefin and bigeye tunas are the two species that dive deepest, down to 600 and even more than 1,000 meters.
