Unidentified Floating Object
(noun) Unidentified Floating Object or UFO is a massive shoal of fish where hundreds of millions of oceanic fish collectively swim together across areas that can be hundreds of kilometers across. It is not to be confused with an Unidentified Flying Object.
In biology, any group of fish that stay together for social reasons are said to be shoaling. If, in addition, the group is swimming in the same direction in a coordinated manner, the fish are said to be schooling. In common usage, the terms are sometimes used rather loosely. About one quarter of fishes shoal all their lives, and about one half of fishes shoal for part of their lives.