One might wonder why such warm-water fish as tuna and dolphin fish appear in Northern Japan, even in winter. The answer is to be found in the ocean current. The Tsushima Current is a warm-surface ocean current, a northward-flowing branch of the Kuroshio (Japan) Current, which runs along the Japanese side of the Japan Sea and averages 24°C. At the northern end of Honshu, the current divides, one branch continuing northward along the west coast of Hokkaido and flowing into the Sea of Okhotsk, while the other, called the Tsugaru Current, brings its warm and relatively salty water into the Pacific Ocean through the Tsugaru Strait.
Migrations of warm-water oceanic fish follow this current. On the Asian mainland side of the Japan Sea, three relatively cold and fresh-water currents, (the Liman) the North Korea and the Mid-Japan Sea — flow southward.