Hi Tom,

1. Are Koi an invasive species in Michigan?

Koi are not native. They are the domesticated form of Amur carp, which is a very close relative of common carp. Common carp are an invasive species that spread through Michigan beginning in the 1880s. Koi are so closely related to common carp that they can hybridize and produce fertile offspring. 

Like common carp, koi can root around in bottom sediments, disturb vegetation, and lead to lower visibility water. This is more of a problem in waters that are already very rich in nutrients and experiencing excessive weed growth. In rocky, well-maintained koi ponds you can certainly achieve clear, muck-free water. 

If koi escape into the wild they may survive, but their bright coloration makes them a target for predators. Another threat is is their potential to spread diseases and parasites to native fish.

For a species to be considered invasive it needs to be both non-native and capable of establishing breeding populations and expanding. Koi is in a bit of a grey area. There are a few established populations in other areas of the U.S., but not in Michigan. Koi have been released and captured in public waters of Michigan, but there is no official record of breeding and establishment of self-sustaining populations. However, it certainly seems possible.

Interesting side note: goldfish do regularly form breeding populations in Michigan after being released into the wild and are an invasive species.

2. Was it illegal to put Koi in a retention pond?

It is best to contact DNR if you want to be certain, but to my understanding it is not illegal so long as there is no inlet or outlet that connects the retention pond to public waters of Michigan. However, just because it is not illegal does not mean it was a good idea.

3. Are their fish we can put in this pond?

Yes. Bluegill and largemouth bass would probably be the best choice. Due to the shallow water, there is a good chance that winterkill could occur during harsh winters, though.

4. Are their plants we should add to the pond?

This depends on the goals for pond management. Adding emergent vegetation can help improve scenic value and wildlife habitat. Submerged plants can lead to clearer water and provide habitat for fish and food for waterfowl. However, a shallow nutrient-rich pond like yours can also be "overrun" with aquatic plants. I have that in quotes since it is a matter of opinion what constitutes "overrun." 

5. Do we need to aerate and why?

Aeration can help to prevent winterkill that occurs when thick snow on top of the ice prevents light penetrating into the water. In nutrient-rich ponds like yours, decomposition continues through the winter and uses up oxygen. With no light for algae and plants to photosynthesize during the winter, oxygen levels can drop below the lethal limit for many fish. Aeration can also prevent stratification and stagnation of water during summer months. 

My two cents:

It wasn't necessarily helpful to put koi into the pond, but they probably are not the cause of the siltation and algae you are seeing. Retention ponds are designed to catch stormwater and are expected to have high turbidity (muddiness), high nutrient levels, and abundant plant growth. In systems like this, excessive plant growth can either be in the form of algae or rooted plants. Pond managers can sometimes push a pond to either 1) overabundant algae and muddy water, or 2) overabundant rooted plants and clear(er) water. 

Koi (and also common carp) can tend to push a system toward situation 1, but they are not necessarily the root cause. Getting rid of the koi is not a bad idea, but it won't address the issue of abundant nutrients that will either result in algae or rooted plant growth.

Combining koi removal with aeration and planting of native vegetation could help push the pond toward situation 2, but if nutrients are too abundant and water quality is too poor it may still be difficult to maintain populations of native fish. 

In fact, the shallow depth of your pond will make it difficult to maintain quality fish populations no matter what you do. When designing a pond for sport fishing in Michigan, the recommendation is to include a maximum depth of at least 15 feet. This is not to say it is impossible to maintain fish in a shallower pond, but it is certainly more difficult. 

This bulletin has more details on pond design and management:

Hope this helps,

Dan O'Keefe

MSU Extension

Michigan Sea Grant

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