No, that’s not a goldfish! It’s a smallmouth bass (Micropterus dolomieu) with a pigment mutation called xanthism. Xanthism is one of a suite of pigment mutations that affect lots of different animals. Others include melanism, leucism, and albinism.
The coloration of a fish is determined by a few different kinds of cells, and certain mutations can affect the expression of those cells, resulting in dramatic pigmentation or complete lack of pigmentation.
Melanism is the result of over-expression of melanophores, cells controlling the concentration of melanin in the skin. This typically presents as either splotchy black, or completely black coloration, as seen in this alligator gar caught in Texas.
Xanthism is the result of a lack of melanophore expression, which allows their neighboring cells, xanthophores, to show through. Essentially, without the expression of other pigments, the yellow-pigment producing xanthophores dominate the coloration of the animal, resulting in bright yellow or orange fish like the smallmouth above.
Leucism is the result of another mutation where color pigments are not completely expressed. This can be the result of a lot of different conditions, including piebaldism. A very famous example of leucism in fishes is the “palomino trout”, rainbow trout (Oncorhynchus mykiss) that have been selectively bred in hatcheries to express a leucistic mutation.
Xanthism is sometimes confused with leucism because of their similar symptoms, but xanthic fish are typically orange, while leucistic fish are typically pale yellow or white, although coloration varies.
Finally, albinism is the complete lack of pigmentation. Animals with this mutation have no color-production in their skin whatsoever, and typically present as completely white. Often times in mammals, eyes will be reddish in color. True albinism in fish is extremely uncommon, and leucism is typically the mutation present in aquarium fish that are marketed as “albino”.