skin bubble, ideas on a fix??

Once it's good and dry... grind or cut it out and sand edges. Then just fill, transition and texture with an epoxy sculpt.

 

Just to add with what Jeff says who has probably done more coldwater fish in a year than I will probably do in a lifetime, can you rehydrate that spot (shouldn't take long with a thin skin using a wetted balled up piece of paper towel with saran wrap over it) , then prick the bubble with a tiny dress maker's pin, push the air out and fill in the hole with a filler after the skin dries?

Not unusual with the thin skinned salmonids like a small brook trout. Obviously need to prevent this before it becomes a problem on your next fish.

I used to have an issue with this, but I now religiously check the fish, and move any air bubbles to the ventral surface and then push out of the back seam, after mounting, and continue checking for a short period of time. Or make a small hole on the belly somewhere with a pin where it can easily be filled in and and paint, and direct the air bubble there. Also you can direct the air bubble to the area where you attached the head.

The wetted palm of my hand seems to work as good as anything to move air bubbles if done gently.

Something which can lead to this are using too much glue under the skin. Years ago I also went to putting the glue on the inside of the fish skin vs. the form itself.