Finally managed to get some progress on this over the holidays. The fish is now finished and ready for the resin pour.
Here are the progress pics :
First up I made the spine by bending some M4 bolts and welding them to the body :

Next I bent some 1.6mm welding rods to form the ribs and welded them to the M4 nuts on the spine : 
Then I cut a face and jaw (not pictured) out of 0.5mm thick stainless steel and welded it to the body along with some m6 Nyloc nuts for eyes : 
Finally , I made a tail , lamp/antenna thingy and sharpened/shaped some welding rods into teeth :

With everything assembled , the fish was complete :

At this point , I decided to deviate from the concept art a little as I felt that the perfect stainless steel finish did not work well with the rusty porthole so I sprayed the fish black and then rubbed it down so it left the black in the crevices a bit like a cavity map :

This thing was a seriously fiddly project. The photos make it look bigger than it is. The overall length of it is only approx 50mm and the wire for the teeth and ribs is 1.6mm thick and the lamp/antenna thing is 1mm welding rod so it was a bit of a mission to weld it all together!
The next stage is to make the mold and set the fish into resin so that it can be set into the porthole. The LED kit has also arrived which will illuminate the fish.