How To Use Grass Carp For Natural Weed Control In Ponds

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Written By Mark Washburn

Mark has 20 years of experience as a professional pond management consultant.

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Grass Carp can be a viable and effective management tool for dealing with a variety of pond and aquatic weeds and vegetation.  They offer a non-chemical and eco-friendly way to keep many unwanted plants at bay.  Their cost is pretty affordable, particularly in relation to how long (usually years) they may provide valuable support for a pond.

That said, they may not be the complete solution to every invasive plant, and using them does come with a few potential downsides.  In this article, we’ll cover some tips on how to use them most effectively, while avoiding any unintended, or undesirable side effects that people might experience when adding them to a pond environment.

Identify The Plant

If you have a weed related problem in your pond, the first step is to clearly identify what plants are actually in the pond.  There are a couple of important reasons for doing so.  As far as carp are concerned, they can have a selective palette…and they prefer the more fleshy plants over anything else.  Secondly, as much as grass carp may help with a weed issue, there is a chance you may still need to use a herbicide at some point, and some treatments work better on specific plants…so identification is just a good place to start.

You can use a site like Aqua Plant to help with the ID, and while there, you can also confirm if carp are a useful management option to consider.  Normally they will mention if carp will readily consume the plant…or not.

Aquatic Plants That Grass Carp Will Eat

Grass carp rely almost entirely on aquatic plants for their diet and they will help with many weeds but some stand out more than others.  Like you and I, they will consume what they like the most…first.  For example, they can work well on S. Naiad, Hydrilla, and Chara (early stage growth).  Marginal plants that they might consume if nothing else is present include Milfoil, Coontail, or a variety of Pondweeds (Potamogeton species).

There are a few species of plants that Grass Carp won’t help with very much.  This includes late season Chara, which becomes quite gritty as it ages, Filamentous Algae, Duckweed, Watermeal, Large Leaf Pondweeds, or any emergent plant like Cattails.

If you’re unsure of whether carp will consume what you have in your pond, again, be sure to visit the Aqua Plant website, or any number of Extension Service websites that will provide a bit more local or regional information for your area.  It’s also important to check with your state fisheries division or natural resources department to make sure Grass Carp are legal to use in your home state.  Most states allow their use in a genetically sterilized form.

Stocking Rates For Grass Carp

Traditionally, it’s been common to run across the suggested stocking rate for Grass Carp to be from 5 to 10 fish per acre.  However I think there’s a better way to go about this.  The whole point of using carp in a pond is to control or manage vegetation. So my suggestion is to use about 5 fish per acre of the vegetation itself. 

Overstocking can create its own set of problems.  If the carp consume all of the vegetation in the pond, and keep in mind some vegetation is actually a good thing…for fish habitat, nutrient balancing, water clarity, and other things…so complete eradication may not be desirable.  Many natural ponds are used as waterfowl habitat, and carp eat the same plants as many waterfowl do, so they may be counter productive for this kind of pond setting.  As well,  if the carp have nothing left to eat as far as vegetation, they are far more likely to move in on any fish feeders that may be in use, and could inhibit other fish from feeding as regularly from the feeders.

For those reasons, it’s best to avoid overstocking, but instead, to stock at a minimal level and then slowly work up to a satisfactory density for the actual plant problem at hand.  Carp will typically not consume very much until they are about six months of age or older, but after that point, you should get a good idea of just how well they will help manage any vegetation issues.  If you find they are not quite keeping up with the growth, you can always add a few more as you go along.  Their peak consumption ages range from about six months to five or six years, after which restocking may be necessary.  It should be noted however that carp can live a long time, and may provide productive plant management for several decades.

Contact Your State Fisheries Department For More Information

If you’re considering using Grass Carp for pond weed management, it’s important to note again, that while most states allow them to be stocked, some do not as they are considered an invasive species.  In the states that allow their use, you may need to get a permit to stock them, and acquire the fish from approved dealers.  Triploid Grass Carp are genetically sterile to ensure that they won’t breed successfully.  However carp are strongly drawn toward moving water and prefer river environments over most pond settings.  Some states will require barriers or other protective measures in water bodies that have the possibility of outflows, to ensure the fish don’t escape the stocked pond or lake, and of course, the carp should never be released in a river environment.

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