No Shechitah is not required: Keritot 21b
Whether fish which aren't Kosher have a different spinal structure than those which are Kosher: Succah 18a
HaShem warns us: I knewwhether an Egyptian child was a firstborn, I will also be able to tell if you mix non-kosher fish innards with kosher fish innards and market them: Bava Metzia 61b