Ask any taxidermist and they'll tell you that fish are the toughest animals to work with. The reason is that the fish's skin loses color once it dries out. This means that the entire body of the fish's skin needs to be completely recreated with paint. There are several ways to go about mounting a fish, and the method is typically determined by the kind of fish. Skin mounts are best for warm water fish like bass. For this method, the fish is skinned using a razor sharp filet knife or taxidermy scalpel.
The eyes are removed, and the only thing left is the skin, head and tail. The skin and remaining meat that can't be removed from the tail and head area is then preserved by injecting different kinds of salts and formaldehyde. We're talking Borax and alum, not table salt. The Borax is then spread over the inside of the skin while it's still wet. This allows the fish to dry slowly and naturally, preventing shrinkage. Then the skin is either stuffed with filler material like firmly packed sawdust or it's stretched over a mold and shaped into the desired pose.
The fins are kept wet until the fish is sewn shut, then they're spread out and pinned to a cardboard backer to keep them in place while drying. The eye is the last thing to come into play. Once the fish has dried out, which can take several weeks, a glass eye with a pin attached to the back is stuck into the socket. A little paint and varnish, and the fish is ready to be mounted onto a wood plaque.
Cold water fish like salmon and trout have thin, smooth and greasy skin. This means the stuffing would show through, so the taxidermist almost exclusively goes with a foam mold. Some taxidermists use artificial heads and attach them to the natural skin to avoid spoiling and shrinkage.
Saltwater fish are almost always recreated using entirely man-made materials. A mold of the fresh catch is made and then cast in polyester resin that's been beefed up with fiberglass. Then the taxidermist carefully recreates the coloration of the fish by painting each scale from head to tail. Many times a generic mold is used because of cost, and the fish is painted to match a photograph. Synthetic mounts are also popular with catch-and-release fishermen.