Of Carp and Koi

This write-up was inspired by an idea from Kodama Koi Farm.

Background

I was born in France (where carp fishing is as popular as bass fishing in the US). I fished for carp for 25+ years and I used to be the vice-president and treasurer of the Carp Anglers Group, which promotes carp fishing in the US. My wife and I now live north of New York City and I fulfilled a lifelong dream of having a large koi pond. The Mid-Atlantic Koi Club was extremely helpful in the process. Our pond is a little peculiar because we have koi, goldfish AND a few common carp and even two grass carp.

My koi pond (~13,000 gallons)	Oompa and Loompa, my koi twins

Carp in the US: short history

By the end of the 19th century, waves of immigrants were seeking an easy food source. The US government selected common carp (Cyprinus Carpio) as a potential answer, as carp are highly adaptive and sturdy fish, which can grow fast. Common carp were imported from Germany and a well organized effort soon led to carp being introduced in most states of the US. Europe (notably eastern Europe) and Asia (notably China) had for centuries a culture of eating carp. Carp remains a traditional dish for Christmas in eastern Germany to this day. 

Carp aren't terribly tasty though and as freezing and transportation systems developed, allowing seawater fish to be available beyond the coasts, the use of carp as a food source was quickly forgotten by most communities. The industrial period and its increasing levels of pollution then led to a negative perception of carp in the US which lasts to this day. Carp are bottom feeders and highly tolerant of murky or polluted waters—more so than trout or bass. Over time, people wrongly assumed this meant carp caused the poor conditions, and they became labeled as 'trash fish'. If carp could speak, they would ask who created such pollution in the first place! Nowadays, wild carp are very widespread all over North America in ponds, lakes and rivers. I have caught (and released) them in 45 states at this point. Some of those waters remain highly polluted, others are crystal clear and carp just adapt. 

A common carp in its natural environment

Many people call common carp invasive, but they often overlook that most fish species in the US -including brown trout, salmon, and muskies- were also introduced by humans. These introductions aimed to recreate European ecosystems, where carp coexisted peacefully with other species for thousands of years. Serious biologists now refer to "adapted species" to speak of non-native species which found a balance in a new environment and both common carp and trouts are a good example. The primary issue actually comes from confusion with other (unrelated) species, which we'll discuss later. Bass anglers often complain about carp, thinking they destroy bass nests (which I have never seen), but I sure did see bass and muskies feasting on carp eggs and small carp fry. Nature is a little harsh, but it finds a balance (when human beings don't overly interfere, that is). If you're not convinced, just travel to Europe, take a boat trip and speak with local anglers!

Carp and koi: patterns and history

Common carp have been bred in Europe and Asia (notably China) for more than a millennium, essentially as a food source. They have been present on both continents for millions of years. To be more specific, the current scientific consensus is that there are actually two species of common carp. Cyprinus carpio is native to Europe and Central Asia, while Cyprinus rubrofuscus is native to Eastern Asia (the latter being the ancestor of Japanese koi). The evolutionary divergence between Cyprinus carpio and Cyprinus rubrofuscus is estimated to have occurred roughly one million years ago, due to geographic isolation. Both species can easily interbreed and are hard to distinguish from each other. Subtle differences do exist, e.g. the number of lateral line scales and the number of dorsal fin rays.

Common carp have this interesting characteristic of having occasional minor mutations. Every koi enthusiast knows about mirror koi ("Doitsu" in Japanese terminology) with bare skin and oversize scales. In the Middle Ages, monks in Europe got into the habit of selectively breed common carp to get more mirror carp because they were much easier to scale before cooking them! A rarer mutation lead to some colorful fish, which didn't seem terribly useful until Japanese breeders honed in on it in the early 19th century, for aesthetic pleasure as well as symbolism (carp leisurely swimming made them a symbol of wisdom, and some viewed them as semi-divine creatures). After a couple of centuries of selective breeding, we now enjoy incredible color and scaling varieties of koi. And this all started from the lowly common carp and its rather dull brown colors! Unsurprisingly, koi actually means 'carp' in Japanese (as well as 'joy' and 'love'). 

Coming back to common carp, there are three main varieties in the wild, due to recurring genetic mutations, regular common carp with small scales covering their entire bodies, mirror carp with large scales and patches of bare skin, and leather carp with a small row of tiny scales on top and bare skin bodies. In the US, leathers do not seem to exist, while they are quite common in Europe. Fin colors are typically similar to their body, but sometimes display shades of orange or red, depending on what they eat. Fantails (long fin) carp are quite rare, but can be found in some areas. We also occasionally catch carp with split colors (e.g. light brown vs. darker brown), dubbed two-tones by carp anglers. Koi enthusiasts will of course recognize such patterns which are often sought for their visual beauty. I have caught a few colorful koi in the wild, but it was usually in a pretty urban environment where somebody might have released a Japanese koi which overgrew its aquarium. Check the last picture below though, this is a regular common carp with one small spot of orange, a a subtle reminder that the genetic roots of koi beauty still exist in wild populations.

A large common carp from the Delaware river	A mirror carp from a reservoir in Tennessee

 

A fantail carp from the Connecticut river	A common carp with a spot of koi!

 

Carp and Koi: a few differences

To be clear, selective breeding doesn't change biology, it just enhances some physical traits (e.g. colors), therefore koi and common carp will coexist, display similar behaviors and interbreed. As previously mentioned, carp are very adaptive creatures though and will learn new behaviors to adjust to their environment.

Case in point, a common carp is essentially a bottom feeder. Its mouth can extend downwards like a vacuum cleaner and they use their pectoral fins to lightly stir up the bottom to extract nutrients while their barbels help detecting edible food. If you pay attention to your kois, they probably do the same thing, vacuuming the pond's bottom, even if there is not much to find. Koi typically feed on floating food, and it's just them adapting to their environment. The few wild carp I have in my pond got used to it too. Wild carp sometimes do the same, e.g. sucking a snail from under lily pads

Another difference is how they respond to noise and human presence. Like many fish, carp and koi have a lateral line organ that detects vibrations. Try stomping on the ground close to a natural pond and you might very well frighten some carp which were cruising close to shore. One would say that koi do the exact opposite, being attracted to the sound and sight of human beings bringing food. And it's just another form of adaptive behavior. There are various marinas in the US where local owners got in the habit of feeding wild carp pretty much every day and then you will see hundreds of wild carp acting like hungry koi. The wild carp in my pond aren't as tame as my kois, but I think it's just a matter of time. 

Koi and carp feeding together (pond in South Korea)

About size, the average wild carp size in the US is around 10 pounds, but it varies a lot (I've caught them up to 50lb) and it is really a function of the environment. The same applies to koi, of course, depending on how well they are fed and how well they are taken care of. At the end, we tend to see bigger koi than wild carp and it's basically about food quality. Carp anglers are actually well aware of the advantages of using protein-rich bait (in Europe, common carp over 80 or even 100 pounds have been caught in special venues where big carp are fostered - yes, like koi). Note that wild carp will often get their protein fix from swallowing clams, mussels and crayfish, crushing shells with the powerful teeth they have in their throat. Did you know that your kois have the same pharyngeal teeth? They sure do!

Where there is a more significant difference is the way wild carp and koi react to parasites and bacterial infections. Common carp are incredibly robust fish and can survive not only murky and polluted environments, but also big scars due to boat propellers or various predators, and subsequent infections. Koi, as a consequence of selective breeding and living in very contained environments, tend to be more fragile. I think this is why we see wild carp jump more often than koi, probably trying to clean their gills from parasites. They live in a tough world and the bottom of wild waters is nowhere near as clean as our koi ponds! Also, by nature, wild carp will be very opportunistic and will eat pretty much anything, including some pretty unsavory stuff. This contrast highlights just how different the life of a pampered pond koi is from its resilient wild cousins.

Let's touch on seasonal considerations to finish. Koi keepers usually avoid feeding under 50°F. This point puzzled me to no end as wild carp tend to be ravenous mid to late fall when the water temperature drops to the mid 40s or lower. And yes, they do digest their food as the whole point is to fatten up for the winter. For sure, koi would do the same if given the opportunity. It slowly dawned on me that the main reason might be more about the nitrification cycle, as corresponding bacteria do seem to significantly weaken under 50°F and this is problematic in a small contained environment like a koi pond (and much less so in a larger ecosystem). Early spring, wild carp tend to stir up and seek shallow sunny areas for some light feeding, and their timing seems pretty similar to the koi in my pond. Once again, their biology is identical—the real difference lies in how carp and koi are cared for, or left to fend for themselves. 

A carp is a carp, isn't it?

You may have noticed that I often used the 'common carp' terminology instead of just saying 'carp'. For a reason which escapes me, unrelated species are called 'something carp' in the US, apparently just because of their size and bulk. In Europe or Asia, a grass carp is called a white amur and the scientific name is Ctenopharyngodon idella. You will occasionally find grass carp roaming in Japanese koi ponds. A grass carp is a long silvery fish with a short dorsal fin and is often introduced in natural ponds and lakes for its ability to eat a lot of weeds. I have one in my koi pond which is 4 feet long! Grass carp are omnivorous, like common carp (and koi), even though they sure like weeds. Koi food works fine for them, they are actually more natural surface feeders than common carp. Note that there is another species called black amur (Mylopharyngodon piceus) or black carp, which looks very similar, albeit darker and more carnivorous. 

In the south and central parts of the US, we can find buffalo carp. Smallmouth buffalo (Ictiobus bubalus) and black buffalo (Ictiobus niger) can grow to 60 pounds or more, their general shape does look similar to a bulky common carp, but their snout and eyes are very distinctive. Another variety, the bigmouth buffalo (Ictiobus cyprinellus) can live over 100 years, barring human interferences. Buffalo carp are actually native to the US and now coexist peacefully with common carp, with fairly similar feeding habits. I was recently traveling in Mississippi, and one morning, I caught two common carp and two buffalos, same bait, same rig, very distinct fish!

My first grass carp over 50lb!	A smallmouth buffalo from Oklahoma

Then we have the real troublemakers, the only ones which really deserve the 'invasive' moniker. First, the bighead carp (Hypophthalmichthys nobilis) which can grow enormous (over 100 pounds) and then the dreaded silver carp (Hypophthalmichthys molitrix). The latter received a lot of media coverage for their leaping abilities, notably when startled by boat engines. Both species are often referred to as 'Asian carp', which is rather silly as they have nothing to do with (common) carp and have their own very distinct shapes. It is true that both species originated from Asia, but then common carp and grass carp are widespread in Asia too. Both species are ravenous filter feeders and tend to consume a lot of plankton, which can severely impact the entire ecosystem. History will tell how nature will find its balance again (it did in Asia after all), but it is certainly true that bighead and silver carp can be extremely disruptive. 

Bighead carp (from wikipedia)

Silver carp (from wikipedia)

Finally, I didn't mention crucian carp (Carassius carassius) because their presence in the US is uncertain (if they exist at all). They look like small common carp, albeit without barbels and staying pretty small (e.g. a few pounds). They are easily confused with carp-goldfish hybrids (which can indeed be found in the US). 

To summarize

As discussed above, koi are essentially colorful common carp, selectively bred for beauty and symbolism. Any difference in behavior between koi and common carp is tenuous and usually a consequence of the environment in which they live. Koi tend to be more fragile and require more care though, while common carp tend to be very resilient. They will happily coexist in a pond and feed on the same food. Common carp should not be confused with more troublesome filter feeders like bighead carp or silver carp. Please enjoy nature and all its offerings!