What is the name of fattest fish in sea?

There isn't one "fattest" fish in the sea, as the word "fat" can be interpreted in different ways.

Do you mean:

* Heaviest: In terms of weight, the whale shark is the largest fish in the world, reaching up to 40 tons. However, it doesn't have a lot of body fat compared to its overall size.

* Most fat content: Some fish are known for being particularly high in fat content, like salmon, tuna, and mackerel.

* Most blubber: Some marine mammals, like whales, have a thick layer of blubber for insulation in cold water. This might be what you're thinking of.

Please clarify what you mean by "fattest" so I can give you a more accurate answer!