Sturgeon

From PenguinWiki

Born a human in 1964, Miles Pattinson realised at the age of 12 that he was a fish trapped in a man's body. He saved up until he could afford the genus-change operation and has lived as a sturgeon ever since. The fact that he is also a surgeon causes an amount of confusion from people that mistakenly think that "sturgeon" is just a typo.

see also Da Vinci cod