Many people have been asking me if I used a wire structure inside the fish to animate it, the answer is no. The actual bones of the fish provided all the structure I needed to move jaw and fins around, I just had to find a way to hold the poses still while I was shooting frame by frame. The solution I found most successful was to work with the fish in a half frozen state.
After purchase of the bass at the fishmarket, I'd stick it in the freezer until I was ready for a full night of animating (stop-motion 101: if you want consistent lighting, daylight is not your friend ;-D). I would take the stinky bastard out a few hours ahead of shooting, while setting up the lights and camera. The fish would thaw from stonehard to kind of rigid in 3 hours, and for a while, its head, fins and mouth would have the right rigidity in order to hold a pose for a while.
So I'd animate as fast as I could, until the fish thawed completely and its jaw went slack... that is when invisible thread was useful : I'd lift the slack jaw with a string which I'd attach to an overhead structure off-screen. Later I could easily mask the thread out of the frames, if it showed too much.
Gross Trivia : somehow the inner stuff of the fish started bloating after a week, and that pressure tended to push its tongue out of its mouth... I had no choice but to ram it back down its throat with my fingers, and was instantly rewarded with a sound that is too obscene for words. It was easy to forget that this was actually a slowly decaying dead body I was animating. Some orange pus oozing from underneath its gill cover during the shooting was a nice reminder of that.
