I sure hope you don't dry out the outside of the fish while the interior is still cold. Whole fish is just not something I do low and slow. If you filleted it no problems. If you cook at the higher temps then you shouldn't have a problem. I'm thinking 275 or 300 min. But like I said when we smoke redfish it is filleted with the scales on, scale side down on the grill, patted with butter, garlic and lemon juice.
Fish will spoil quick so do you think a 15 lb fish will smoke in 4 hours? SmokinAl has done it but I just don't believe they catch 15lb fish in Florida, at least that's what I have heard
If you do it post some Qview. If it works I'll have to try it but like I said I think it is to big for low and slow. You may consider cutting it in 1 - 2 inch thick slabs, laying them on some aluminum foil on the smoker grill. It should cook all the way through, you'll have nice color on both sides and your seasonings will penetrate better.
Just my thoughts
Al