Rigging a shark?

I have a shark model and I want to make it move organicly. I want to make it move it like it is using its spine and not just big bones. Is there a way to do it without just adding a bunch of bones to simulate a spine? Thanks.

You might try a curve modifier or a latice
I have never used either, but I have seen then used in this manor

Sweet model by the way

You could probably change your bones to bendy-bones (B-bones) and then in pose mode increase the number of segments in each bone (Segm in the editing panel).