1. Carp Spend Their Lives Near the Bottom of Slow Water
The common carp lives in the middle and lower parts of large rivers, in lakes and marshes, and even in brackish water near river mouths. It prefers slow-moving pools to fast current [1]. A fisheries center in Shimane Prefecture, Japan, adds that carp live on sandy or muddy bottoms, and that the species is spread across Japan, Asia and the temperate parts of Europe [2].
So the short answer is that a carp is usually at the bottom, in calm water. It is also a big fish: usually about 60 cm (2 feet) long, and in some places more than 1 m (3 feet) [1]. These are broad descriptions, not rules for every pond or river.
2. How Carp Eat Without Teeth in Their Mouths
Life on the bottom suits the way carp eat. Carp are omnivores that eat worms, small crustaceans, water plants, shellfish, insects and small fish [1]. Their mouths have no teeth. Instead, they have teeth in the throat, called pharyngeal teeth, and food that is sucked in gets crushed there. Shimane's fisheries center says this is why carp can even eat freshwater clams [2]. The sources do not say exactly how the tooth position and bottom feeding fit together, so treat it as a clue rather than an explanation.
That leaves a second question. Is a carp always at the bottom of the same place?
3. How Lake Biwa Carp Move Between the Shore and Open Water Through the Year
Lake Biwa is Japan's largest lake. A research group at Japan's National Institute for Environmental Studies (NIES) describes its native carp as moving with the seasons. In spring, roughly March to June, they come into reed beds along the shore, water channels and flooded rice fields to lay eggs. From summer to autumn they are thought to feed near the shore. In winter, December to February, they are thought to move out to open water and spend the cold months there [3].
Only the spring part is really seen by people, because the carp come close to the shore then. The rest is guessed from where and when fishing nets catch them [3]. Even the spawning season depends on the source and the place: NIES gives March to June for Lake Biwa, while an angling guide says about April to July [1]. Read these months as rough ranges.
Temperature may play a role. The Shimane page says carp stop moving and sit still near the bottom when the water drops to 8°C (46°F) or colder [2]. That is one source's guide number. Nothing in these sources shows that 8°C is what sends Lake Biwa carp to open water.
4. What Researchers Still Do Not Know About Carp in Winter, and One Clue in Their Bodies
The NIES group says that outside the spawning season, it is not known for sure where native carp live or what they do, even though that is a very basic question. Summer, autumn and winter lives have almost never been seen directly [3].
There is a clue in their bodies. Carp have a small tube joining the swim bladder (the gas-filled organ that helps a fish float) to the throat. In the nearly pure native carp of Lake Biwa, a valve on that tube is thick and well developed. NIES researchers infer that this suits a life of moving up and down between deep and shallow water in a short time. They also think it may stop gas from escaping when a carp dives suddenly, and help it rise again [3]. This is an inference from body shape, not something anyone has watched. The group is trying to find out by attaching small video cameras and depth recorders to carp [3].
5. Why Deep Water in Lake Biwa Holds Japan's Original Carp
The deep water matters because of what lives there. Researchers have long noticed that Japan has two body types of carp: a slender "wild" type and a taller-bodied "farmed" type. Since the Meiji period (starting 1868), huge numbers of carp had been released into Japanese waters. DNA studies by NIES researchers showed that the wild type is the original Japanese carp, while the farmed type was brought from the Eurasian continent [3][4].
In one survey of 166 carp from 11 places in Japan, many of the carp taken from lakes were of continental origin. In many lakes the two kinds had been mixing. Lake Biwa's deep layer, about 20 to 100 m (66 to 330 feet), was a rare exception, with carp close to the pure native type, and these tend to be more slender [4]. A separate research project reports the same pattern: relatively pure wild-type carp remain below about 20 m [5]. Because of this, Lake Biwa's native carp are on Japan's national Red List as a regional population at risk [3].
The percentages come from the places the researchers sampled, so they are not a count for all of Japan. Names also get tangled. Shimane's fisheries center says wild carp and farmed carp have become hard to tell apart because of releases and crossbreeding [2]. We cannot tell from the sources whether its "farmed" carp are exactly the same group as NIES's continental ones.
6. How to Look for Carp Safely Near You
If there is a pond or slow river nearby, go with an adult and stay back from the edge. Notice whether the carp are near the surface, near the bottom, or close to the bank. Write down the date and weather, and note how clear or cloudy the water looks. After several visits you can check whether their position changes with the day or the weather, using your own notes. If you can read Japanese, the NIES page on Lake Biwa carp [3] shows how researchers are trying to see what no one has seen yet.