Have you ever seen notices alongside natural lakes warning the fish not to eat when the temperatures are low? Me neither.
Yet still throughout koi keeping circles there are warnings that feeding your koi when water temperatures are lower than about 10°C will do them harm or possibly kill them.
Fish in the wild are not constrained by such thinking. Obviously their appetite will be much less in winter than in summer but they eat when they want to eat and they have been doing this since fish first evolved. To understand why fish need to eat during winter it might be easier to first understand what happens to the food they eat in summer.
Carp are omnivorous; they will eat whatever they can find. The food they find to eat in summer will include a good proportion of small aquatic creatures such as bloodworms, insects and insect larvae. These will be high in protein. Protein is required for growth, so part of this food will go towards increasing the fish’s size. There will also be some plant material which is a source of carbohydrates. The residual protein and the carbohydrates are used as a source of energy. The greatest demand on this energy source is the energy used for swimming. There is also energy that is used for simply staying alive. The heart needs energy for every single beat, the gills need energy, even the optic nerve requires a small amount of energy to transfer light into the brain from the eyes in the form of electrical impulses.
In winter, metabolism is slower, growth ceases and the energy expended in swimming is very small but this energy and the energy required to stay alive cannot be disregarded. Wild carp will occasionally feel the need to find something to eat. With no high protein insects about, they will eat the odd scrap of plant material. In a pond in winter, if this small amount of energy isn’t replaced by feeding, it will be replaced by burning stored fat. As this is depleted, energy will eventually have to come from burning muscle protein to stay alive. Over an extended period without food, your koi will be slowly wasting away.
Be sensible about winter feeding. Wheat-germ with low protein content is the obvious choice for koi during winter. Offer small amounts but only when they are obviously hungry and remove any that is not immediately eaten.