You Know, The spots on top of the head look a lot like an alantic, but the ones on the back look like a king. Every Alantic I have seen has those big round spots on its cheek. Rather than a King/Coho it looks a little like a king /alantic mix, which could make sense since any pen stray would have a hard time finding a mate. Otherwise I would actually take the logical conclusion and call it a coho. Really does look like it could be a hybrid though.
I just saw a WDFW poster that claims the definitive character between chinook and coho is the gum line, so I would say its a definite keeper under the states eyes (when open). Since it would ahv eto be a jack anyway if it was a king, I wouldn't feel real bad about keeping it anyways (again, if open for coho).