1. How Carp Search for Food by Putting Their Snouts Into Sand and Mud
Have you ever seen a carp in a park pond moving slowly with its mouth pointed toward the bottom? According to a fish guide from Hokkaido, when carp look for food on the bottom, they put their snout into the sand or mud, push their upper jaw forward, and suck in the sand and mud together [1]. After sucking it in, they separate the food from the mud inside their mouths, as noted in materials from the Nagano Prefectural Fisheries Experimental Station [2]. They do not just pick out food; they take in mud first and sort it out inside their mouths. So, what exactly are they sorting out from the mud?
2. Carp Are Omnivores That Eat Shellfish, Worms, and Plants
The Nagano Prefectural Fisheries Experimental Station states that carp are omnivores, eating shellfish like river snails, tubifex worms, and aquatic plants [2]. Fishing guides list crustaceans, insects, and small fish as examples of food, introducing carp as fish that will eat almost anything [4]. Because they eat both animals and plants, they are called omnivores. However, the sources used here do not provide numbers showing exactly what percentage of each food type wild carp eat. It is best to understand "eats anything" as meaning they have a wide range of foods. Aquatic plants and mud worms are soft, so they can be sucked in. But what about hard shells?
3. Why Carp Have Teeth in the Throat Instead of the Mouth
Materials from the Ibaraki Prefectural Inland Water Fisheries Experiment Station state that carp have no teeth in their upper or lower jaws. Instead, they crush shellfish like river snails using pharyngeal teeth located in their throat [3]. Pharyngeal teeth are teeth that line the bones deep in the throat. Nagano materials explain that this is where they grind food, similar to how human molars work [2]. By separating mud and food in the mouth and crushing hard items in the throat, the carp’s eating method divides tasks between the mouth and the throat. Carp also have four barbels around their mouth: one pair behind the upper jaw and one pair at the corners of the mouth [3]. The sources do not say if the barbels are used to find food, so this article does not discuss that. After crushing food in the throat, how does it move through the body?
4. Why Carp Without a Stomach Keep Eating All Day
The Hokkaido fish guide explains that carp do not have a stomach. Food goes directly into the intestines, so they are said not to become full unless their esophagus is filled with food [1]. As a result, they are described as having a tendency to keep eating all day [1]. This lack of a stomach is common in the carp family, not just carp. Therefore, rather than viewing carp as "greedy" based on personality, it is closer to the data to view this as a combination of body structure and eating method. According to Ibaraki materials, carp grow about 20 to 30 cm in one year and about 50 cm in two years, maturing in about three years [3]. 20 cm is about two-thirds of a 30 cm ruler, and 50 cm is almost two rulers connected. The sources do not state a cause-and-effect relationship between eating continuously and growing fast, but it serves as a guide for growth speed. So, what kind of presence is a carp that eats all day while digging through mud?
5. How Carp Stirring Up Mud Affects Pond Water
A page on invasive species from Tochigi Prefecture states that carp not only eat native aquatic life but also poke and stir up the bottom mud, leading to water quality deterioration and eutrophication [5]. The National Institute for Environmental Studies' invasive species database also lists damage to aquatic plants and invertebrates, and the stirring up of bottom mud which worsens water quality and increases nutrient salts [6]. It can be read that the action of sucking in mud to sort out food also stirs up the pond water. Carp are native species in Japan, but people have released them in various places since ancient times, and overseas carp were also released after the Meiji era. Therefore, in waters where carp did not originally exist, released carp are treated as invasive species [5]. It is important to note that we cannot simply say "carp are invasive species" because there are native ones and ones released by people. So, what actually happened in a pond where carp were removed?
6. How Aquatic Insects Rose From 6 to 36 Species in a Reservoir Survey
A report from the Hyogo Prefectural Museum of Nature and Human Activities describes a survey of a reservoir in western Hyogo. A pond that had carp since before 2000 was drained in November 2022, and the carp were removed. Aquatic insects increased from 6 species before removal to a total of 36 species by December 2023 [7]. In June 2023, two egg masses of giant water bugs were confirmed. Calculating 36 divided by 6, the number of species increased sixfold. This survey covers the progress of one specific pond. It cannot be said that the same result will happen in every pond. Aquatic plants were not seen in the pond during the time carp were present. The report states that there had been no study quantifying the impact of carp on aquatic insects, but it suggests that species using aquatic plants and those burrowing into the bottom are susceptible to carp's influence [7]. "Suggested" means it is thought to be so, not a definitive statement that carp were the only cause. It seems we can read that there is a connection between the carp's mud-digging eating method and the number of insects in the pond. So, what should you look for in a pond near you?
7. How to Watch Carp Eat Safely From the Edge of a Pond
If you go to a park pond or a nearby reservoir, look at the carp with an adult, being careful not to get too close to the water. Do not enter the water or reach out with your hands. Follow the rules of that pond regarding whether feeding is allowed. Look for three things: Is the carp moving its mouth near the bottom? Is the water in that area cloudy? Are there aquatic plants at the edge of the pond? These are clues to check the relationship between carp eating habits and the pond's condition with your own eyes. Since each pond is different, it is useful to note the name and date of the pond you visited so you can compare it with others. The movement of the mouth and mud is explained in fish guides from Hokkaido and Nagano's "Fish of Shinshu," while teeth and growth are detailed in Ibaraki's materials. Changes after removing carp from a drained pond are in the report from the Museum of Nature and Human Activities. All are available on the internet. It might be interesting to look at carp mouths in a pond and try to find explanations for the movements you notice.