The kind of hibernation that fish enter is referred to as "torpor," and is only primarily dormant -- fish will still occasionally begin to move around again to find food or warm up. Goldfish, koi, and other fish will naturally come out of torpor once temperatures warm back up. This could be springtime, or if there is a mild period during the winter. It’s also natural for fish to come out of torpor periodically throughout the winter, as torpor is just a temporary state.
Don't feed your fish during this time! They may wake up temporarily if temperatures get warm enough for a period of time, but they are still in a dormant state. Their digestive system isn't operating as normal, and food may harm them or restart their metabolisms before they are ready. When leading up to winter, begin feeding your fish a diet with less protein, as protein is more difficult for fish to digest than other nutrients.
