Cladistically you can decide:

a) There are fish. Sharks are fish. Trout are fish. So therefore humans are fish as we are more related to trout than we are to sharks. This is basically saying that "fish" is roughly the same as "any vertebrate" or "any vertebrate with teeth" (depending on where you draw the line).

or

b) There is no such thing as a fish. There are THREE things: sharks/rays, ray finned fishes, and lobed finned fishes (which includes humans)

That's the joke in the name of the British show.

> we are more related to trout than we are to sharks

wait what