This chu-toro Bluefin is said to be amongst the best in Tsukiji market (after Golden Week) weighing in at 240kg. The spring summer tuna flesh has a changed, a different consistency, the red is a different red color from the past months, the fat looks and tastes different, it migrates in different patterns, changing over from the winter cold waters.
Research reveals that Pacific bluefin tuna in the East China Sea has circadian cycle. Tuna stay in the mixed layers of water around the surface at night, and move to deeper water in day time. They work according to a precise biological clock, in part their flesh changes due to a warmer body temperature than surrounding water. Also, difference between body and water temperature tends to be bigger as water temperature drops. So the spring waters change, the diet fluctuates and the fish develops a different taste.
Tuna enjoy and profit from swimming in warm eddy waters, a mass of water of one temperature that breaks off from a current, spinning in a circle, and drifts intact through waters of a different temperature. Warm blooded Bluefin find eddies which can maintain their integrity for long distances. Marine Biologists know very little about eddies, but consider that they play an important role in the circulation whole Atlantic Ocean and in fish migration.