In fish, carbohydrates are stored as glycogen that can be
mobilized to satisfy energy demands. They are a major
energy source for mammals, but are not used efficiently by
fish. For example, mammals can extract about 4 kcal of
energy from 1 gram of carbohydrate, whereas fish can only
extract about 1.6 kcal from the same amount of carbohydrate.
Up to about 20% of dietary carbohydrates can be
used by fish.