Can Koi and Bass Live Together? (Pros & Cons)

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Can Koi & Bass Live Together? Pros & Cons of Koi & Bass Sharing Ponds.

Koi fish at the water surface
It’s possible for koi & bass to live together in a well-maintained system, though they aren’t the most compatible. xflickrx / CC BY-SA 2.0

Unless you’re the type to simply accept the loss of a few fish, you’ll take time to reconsider rearing bass and koi alongside one another. These fish may be as different in terms of behavior as they are in terms of appearance. While one can be the very symbol of peace, the other has a reputation for aggression.

This is why it’s important to do thorough research on each species before going out to purchase them. Although it can be extremely tempting to simply buy a wide variety of fish and stock up your pond, you must consider how each species would affect the other. For pond fish to live in harmony despite their differences, your pond environment should be in tip-top shape.

It’s certainly possible for koi and bass to live together in one well-maintained system. These species are not necessarily the most compatible, however, so it may be best to leave this possibility to those with at least a few years of experience raising pond fish. With a few considerations for their biology and behavior, they can be reared together with minimal incidents of koi loss.

Bass Species & Features

The inside of a largemouth bass' mouth
Largemouth bass have distinctly large mouths with needle-like teeth. Fredlyfish4, CC BY-SA 3.0, via Wikimedia Commons

Bass are not often reared in ornamental fish ponds, but these two species are the most common: largemouth bass (Micropterus salmoides) and smallmouth bass (Micropterus dolomieu). Both are native to the freshwater systems of North America. On average, both species tend to grow to 15 inches (38 cm) long. They have distinct jaw structures that allow their mouths to open remarkably wide (more so for M. salmoides). In the wild, these distinctly large mouths, filled with needle-like teeth, place these species high up the food chain.

Behavior of Koi vs Bass

A largemouth bass eating another largemouth bass
Bass have the tendency to eat anything that can fit into their mouths. They’re sometimes even cannibals, as seen above! USFWS Mountain-Prairie / No copyright

Koi are known for being friendly and peaceful. They like to stay out in the open. Though they may occasionally dive to the pond bottom, koi will generally swim through the upper sections of the water column. In the wild, they are bottom feeders. In a pond, they readily take food on the water’s surface. Unless stressed or scared, koi will rarely show instances of aggression.

In contrast, bass don’t exactly have a reputation for being the most peaceful of fish. Confined environments can cause them to become territorial and aggressive to both smaller or larger species. They are also less likely to stay out in the open as they are ambush predators. Bass have the tendency to eat pretty much anything that can fit into their mouths. If they’re hungry, they won’t hesitate to eat other fish. In the wild, bass will even eat waterfowl or small mammals.

Unlike koi, bass are undisputedly partial to live food and may not be trained to take fish feeds. With regard to behavior and preferences, these two species definitely clash. Avoid unfriendly encounters by making sure all needs are met and the comfort of both species is rarely compromised.

Important Considerations for Keeping Bass & Koi

1) Size of koi

A Japanese koi fish swimming underwater
Japanese koi are less likely to be eaten as they are larger, but they may still face aggression if they come into close contact with bass. turquoise field / CC BY-SA 2.0

Your koi should be at least twice as big as the bass in your pond. Any smaller, and they might, unfortunately, end up as chum. Bass do attempt to eat fish that are larger than their own mouths! They can easily consume koi fry, juveniles, and domestic koi that are on the small side or have yet to reach sexual maturity (less than 10 inches). Japanese koi or jumbo koi are less likely to be eaten. They may still be victims of aggression, however, if they are forced to come into close contact with your bass.

2) Hiding places

Cod and striped bass swimming past a rock
You can provide your bass with hiding places & shelter by using rocks, logs, vegetation, and more! Becky Schaefer / CC BY 2.0

Make sure that your bass feels comfortable by providing many hiding places or makeshift shelters. This can be a great avenue for creativity! Make use of vegetation, artificial caves, large pipes, logs, and rocks. You can also create sections with varying depths. Structural diversity that mimics features found in the wild is great at serving as a protective barrier. Your bass is likely to select one of these features and constantly return to it, establishing its territory. Those not chosen by your bass should also benefit koi and any smaller fish that may feel bullied or threatened by potential predators.

3) Pond size

Koi pond in Japan
Your pond should be wide enough so that your koi won’t constantly bump into potential aggressors. Natalie Maguire / CC BY-SA 2.0

As the koi you rear alongside bass have to be large, it should follow that your pond would have to be quite big. It should be wide enough to prevent koi from having to constantly bump into potential aggressors. It should also be wide enough to prevent your fish from feeling cornered.

Ponds with resident bass should be no shallower than 4 feet and may be as deep as 10 feet. Your bass is unlikely to spend much time in shallow areas, so an even greater depth would be beneficial. Koi will likely keep closer to the pond’s surface or stay in shallower areas. In this sense, the two species are compatible as they occupy different layers of your water column.

4) Pond water conditions

largemouth bass in clear pond water
Your pond should have filtration systems, pumps, and aerators to keep water conditions in clean and healthy. Oak Ridge National Laboratory Follow / CC BY 2.0

The water quality requirements of each species must be met to avoid stress, which can lead to aggression and disease. Bass are highly sensitive to excess nutrients, pollutants, low oxygen levels, and imbalances in water pH and hardness. Your pond should be equipped with filtration systems, pumps, and aerators to ensure that water conditions are kept in check. Bass thrive best in crystal clear waters with dissolved oxygen concentrations that are higher than the optimal levels for koi.

Your pond should also be surrounded by buffer zones or pond edge plants to ensure that soil nutrients or external elements don’t contaminate the water. Aside from maintaining your pond’s machinery, you must also regularly remove any uneaten bits of fish food, decaying plants, or foreign waste that may decompose in your pond.

5) Live food for bass

Pumpkinseed sunfish underwater
To avoid bass eating your koi, you should feed them common sunfish, such as pumpkinseed sunfish (pictured) or bluegill. Cephas, CC BY-SA 3.0, via Wikimedia Commons

Bass should never be underfed or deprived of live food in a koi pond. To prevent loss of koi, you should have smaller prey items for your bass. Ideal species include common sunfish, such as pumpkinseed sunfish (Lepomis gibbosus) or bluegill (Lepomis macrochirus). These tend to reproduce quite quickly, so your bass will be kept happy with a consistent diet of fry for at least one season each year. At the same time, your bass would be doing you the service of keeping your fish populations at bay. In fact, bass are often used as a means to solve overcrowding problems in fish ponds.

Once fish fry are consumed, you will have to supplement your bass’ diet with feeder fish (e.g. minnows) from a reputable pet shop. It may be tempting to feed your bass with wild-caught live fish from nearby lakes or ponds, but doing so may introduce foreign bacteria and parasites to your pond and cause the entire system to collapse. Rearing fish as food in your pond is the best way of preventing this, while increasing your pond’s biodiversity and promoting a natural food chain as well.

6) Avoid overcrowding

largemouth bass fry
Bass start small as fry but grow massive, so be aware of potential future overcrowding issues when stocking. USFWS Mountain-Prairie. / CC BY 2.0

Overcrowding can cause your fish to become stressed and exhibit aggression. Bass can’t always be relied upon to sort out problems related to overcrowding, especially if most of the fish in your pond are initially mature. Overcrowded ponds can quickly be toxic to your fish, and your bass may be the first to show signs of weakness due to its sensitivity to poor water quality. A sick bass can’t be relied upon to sort out your overcrowding issues.

When stocking your pond, try to compute for an appropriate fish density and consider the lengths that your fish will grow into at maturity. Don’t exclude breeders of prey fish from this computation, as they too will consume oxygen and produce waste. Keep in mind that there should ideally be 3 sexually mature prey fish for each bass in your pond. This ratio increases the likelihood that the prey fish can survive and reproduce without eventually overcrowding your water volume.

7) Introducing wild bass

largemouth bass in pond
Introducing wild-caught bass to an already established koi pond can be very dangerous. You should quarantine bass for at least 40 days so that they don’t bring diseases and parasites into the koi pond.

It can be quite dangerous to introduce wild-caught bass to an already established koi pond. If you intend to do so, it is advisable to quarantine your bass for at least 40 days prior to moving them into your pond. If you skip this step, you run the risk of introducing diseases and parasites, which tend to be present in fairly large numbers in wild fish. Even if they are prevented from coming into direct contact with wild bass, your koi populations could quickly become affected by these parasites.

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