Tuna is not freshwater fish, they are marine fish and live mainly in the ocean. They are distributed in a wide range, including Atlantic Ocean, Pacific Ocean and Indian Ocean. The fundamental difference between freshwater fish and marine fish is that they live in different environments. Freshwater fish live in rivers, lakes, ponds, streams, reservoirs and other waters, while marine fish live in the ocean. So tuna is a standard marine fish.

Although tuna are widely distributed, they can't live in all sea areas. They like the warm environment, so the sea areas where they live are all low latitudes. There is no tuna distribution in the Arctic Ocean, where the water temperature is low.

Tuna is a fish of teleost, perciformes and mackerel family, which is larger in size, thicker in head and thinner in tail. This size makes tuna swim faster and reduces the pressure of seawater on them. The life span of tuna can reach about 50 years old. Their body shape is metallic, and different species have different colors. The common tuna colors are blue and silver, and of course there are some rare gold. Tuna is an edible high-grade food. Because they move in the deep sea, there are generally no parasites in the body, and the taste is the best when eaten directly.