Tuna is marine fish because tuna is distributed in the ocean. Tuna is widely distributed, and they are found in low latitude waters such as Atlantic Ocean, Pacific Ocean and Indian Ocean. But there is no tuna like the Arctic Ocean, because tuna likes to live in a warm environment, and the latitude of the Arctic Ocean is relatively high, which is not suitable for tuna life.

Although tuna is distributed in different sea areas, these are all marine environments, so tuna is a marine fish. Tuna likes to live in the sea area with a water depth of about 100-400 meters. Different species live in different sea areas, but most species live in the deep sea.

Tuna can't live in fresh water, because tuna belongs to marine fish, they are used to the marine environment, and their cells have high salt content. If they live directly in fresh water, the osmotic pressure and salt content in the water are relatively low, and they are likely to dehydrate and die. In addition, tuna lives in deep water, and freshwater waters generally cannot reach a depth of 100-400 meters, so tuna cannot live in fresh water.