Although they are known for being peaceful and friendly, koi carp can have quite the ravenous appetite. If you’ve ever fed them in a pond during a warm, summer afternoon, you’ll know just how eager they can be to fill their bellies with sustenance. This is one of the reasons why it feels incredibly rewarding to care for koi. When their nutritional needs are attended to, access to the right type of food goes a long way in terms of growth and survival.
Koi carp, which are scientifically referred to as Cyprinus rubrofuscus or C. carpio rubrofuscus, are domesticated descendants of the common carp (Cyprinus carpio). They have principally been cultured in captivity as a means to purify their lineages and create valuable bloodlines. As this practice has gone on for hundreds of years, we can assume that an innumerable number of koi escapees have made their way into the wild, producing generations of wild-type koi.
You may also wonder if your own captive koi can survive or learn to fend for themselves in the wild. In natural, highly productive freshwater systems, odds are that koi are able to maintain a well-balanced diet. Those born in the wild would undoubtedly be experts at searching for the right types of food. Those bred in captivity may initially struggle, only to become potentially invasive once they learn to take wild-sourced nourishment.
The Natural Koi Habitat
Koi and other Asian carp species were introduced into several countries outside of their native range, Far East Asia, due to the ornamental fish industry. Escapees from fish farms and private or commercial freshwater systems eventually established wild populations of the species. In some areas, hobbyists (e.g. aquarium amateurs) released koi carp into public water bodies without anticipating their dispersion and establishment in the wild.
As a result, C. rubrofuscus now occupies many aquatic habitats, and its populations continue to spread and coexist with native fauna. To fully comprehend the koi carp diet and their food preferences from place to place, it is important to review the features of their natural environment. Koi carp favor cool lakes, rivers, and streams with a natural benthic component. As this component becomes more diverse, the likelihood of koi encountering desirable prey increases.
Although pond parameters for ornamental koi must be strictly maintained to ensure good health, wild koi are less sensitive to turbid waters. They prefer slow-moving freshwater systems where they need not swim against a current. They are quite tolerant of turbid or murky waters, including those in degrading wetland systems where their generations can persist for as long as food is present.
Koi as Omnivores: Captive vs Wild Diet
Koi carp are able to safely consume a wide variety of food types. They can survive on a vegetarian diet, but will also benefit from protein-rich sources. Those situated in ornamental ponds tend to rely on fish feeds as these are consistently provided and nutritionally rich. Nonetheless, they may occasionally nibble on plants and gobble up unsuspecting fish fry, insect larvae, and tadpoles.
A free-for-all food situation rings true for koi carp that find themselves in the wild. They can usually survive on whatever is plentiful in their environment. In this way, they manage to find constant food sources throughout the year. When protein-rich choices are scarce, koi carp feed on algae, floating seeds, and plant matter. During their peak metabolic periods, however, they’ll search for more meat-based food types. These include aquatic insects and insect larvae, crustaceans, snails and slugs, worms, and fish fry.
Koi carp generally won’t go out of their way to chase after fish. They aren’t inherent carnivores, choosing instead to feed on whatever is readily available. They do consume smaller fish and tadpoles if these are easy prey or are abundant, such as after breeding seasons.
Koi will also feed on a huge percentage of their own young. Filial cannibalism is true for most freshwater fish species as their young resemble other types of small prey. This natural phenomenon also aids in compensating for energy lost to breeding. Moreover, it helps weed out the weak from the strong, increasing the robustness of the next generation of breeders.
Feeding Habits of Koi Carp in the Wild
In an article on koi fish populations that have become naturalized in New Zealand’s waters, the author compares them to vacuum cleaners, with bottom-feeding habits that make them “sound like hydrophones”. These opportunistic fish have the amazing ability to sift through the benthic zone. They tend to suck up bottom substrates and then orally expel unpalatable materials. Koi carp are thus comparable to filter feeders as well, but this isn’t their only means of obtaining food.
Koi are usually able to explore all water levels in pond-like environments. They can take food on the water’s surface and in the water column. They can easily swim down to depths of 20 feet (6 m) or more in search of food. In addition, staying close to the bottom of natural water features helps protect them from potential predators.
Like other fish species that prefer mildly cool temperatures, where koi carp feed is also associated with seasonal changes. In summer, they may retreat to cooler areas where the temperature ranges from 18 – 23˚C (64 – 73˚F). They can easily tolerate lower temperatures, though these compromise their metabolic rates.
Ecological Impact of Koi Carp
Koi carp have the tendency to disturb bottom sediments while feeding. An actively feeding population can significantly increase water turbidity, thereby creating poor aquatic conditions that can affect the health of native species. As sediments get lifted, they can release noxious gases, resuspend fish waste, block out light, and become trapped in the gills of other animals. Their feeding behavior can also uproot many submerged plants and erode water banks, leading to the destruction of vital microhabitats.
As koi are able to feed on so many aquatic food types, their presence may dramatically alter the environment, creating an imbalance in the natural food web. In most areas where large amounts of koi have deliberately or accidentally been introduced into natural water systems, they become pest species. In just a short amount of time, their ecological footprint mimics that of other invasive fish.
How Often Do Wild Koi Eat?
In captivity, koi are usually trained to take food at specific times of the day. As they will likely rely on fish food, they’ll quickly rise to the surface to take just enough pellets to meet their metabolic requirements. There is no such thing as a scheduled feeding session in the wild, so feral koi may feed throughout the day. They do feed at night as well, especially if nocturnal insects or macroinvertebrates are in season.
Outside of their resting hours, which would take place whenever they feel sheltered and satiated in the wild, they feed whenever the opportunity arises. In especially productive lakes and ponds, koi can feed pretty much all day. They aren’t able to eat in large quantities, however, as their guts are only able to digest a little bit of food at a time. They benefit from a higher feeding frequency and smaller servings.
Temperature and Age-Dependent Metabolism
Koi carp have metabolic demands that are altered by seasonal trends in temperature. Their nutritional needs also change over time as young koi have a higher capacity to assimilate protein-rich food types. During the warmer seasons, you can expect wild koi to have increased feeding rates. This dips down in fall and winter, during which poikilothermic fish eat sparsely.
Wild koi may have an advantage over captive koi in that they are able to maneuver through larger bodies of water in search of areas with more agreeable conditions. This means that, even as captive koi are no longer able to take up food in outdoor, ornamental ponds, their wild counterparts may still manage to metabolize food if they find warmer pockets of water.
In the wild, older koi can thrive on diets with a lower ratio of proteins. This applies to most aquaculturally-grown species. They may opt for more fiber and carbohydrate-rich food sources. In contrast, younger, rapidly growing fish can be expected to take up higher amounts of meaty prey if their environment allows it.
