Using Grass Carp to Remove Watermeal/Duckweed – Part 3 of FISH

In Fish by Alastair OngLeave a Comment

After sending photos of my fish grate which you can read about here, Mindy from the Connecticut Department of Energy and Environmental Protection (DEEP) sent me an approval for getting my grass carp!  Simultaneously, she sent it to a fish hatchery, RowledgePond Aquaculture, that I put on my application where I would buy my grass carp.

Todd Bobowick (the third-generation owner) was an extremely nice guy.  I decided to use Rowledge, because prior to filing the application, I was hunting around for grass carp hatcheries to confirm the green plant growing on my lake.  I took detailed photos and sent them to half a dozen hatcheries.  Todd at RowledgePond Aquaculture, was the only one to respond – and his response was very detailed.  Wheras I thought I had a growth of duckweed, Todd corrected me and told me that I had watermeal.  He went on to detail the reproduction cycle of watermeal (invasive) and then detailed the three methods of removing them (mechanical, i.e. filtration; chemical, i.e. herbacide; or biological, i.e. grass carp).  I later learned that Rowledge is the oldest private hatchery in the state of Connecticut

Reproduction of watermeal is truly amazing.  Watermeal reproduces asexually, and produce one daughter bud per day.  So in about two weeks you can go from a single bud (which is 1/8 the size of an eye of a needle) to a 1/2 acre lake completely covered!  I swear that last spring when we bought the lake, it was clear and then overnight, bamm, the entire lake was covered.


There is a good article here about the differences between Duckweed and Watermeal.  If your growth has three leaves and has a root that hangs from the leaves, chances are you have duckweed.  If your growth are tiny leaves without any root, then you have watermeal.

Differences between Duckweed and Watermeal

In this photo you can see duckweed vs. watermeal. Watermeal is much smaller than duckweed and are approximately 1/16″ wide and do not have roots that duckweed do

watermeal migration

This is how the term “duckweed” was coined. Here on the breast of this duck, you can see watermeal stuck to the feathers. When the duck migrates to another lake, that lake will soon be infested with it.



Todd detailed that watermeal has an interesting biological mechanism whereby if it has completely covered your lake, reproduction will slow down.  The moment it detects that there is room to grow, reproduction will kick start.

So by using a mechanical filter, it will be expensive losing game. The more you take out of your lake, the faster it will grow.  I had done some searches to see different mechanical filters that I could use/buy, but Todd seemed right, I would have to place these filters in my lake and run them 24/7 during the warm months, and there was no guarantee that it would work.  By expensive, I mean first you have to buy a pump and hose and then you must use electricity to power it all spring, summer and fall.


As far as I know, there are at least half a dozen or so different companies that produce chemical herbicides to remove watermeal.  The cost range from anywhear around $150 to $700 per quart.  The primary difference in these chemicals is the amount of application.  The cheaper ones usually require frequent applications while the more expensive ones less so because they have stronger chemicals.  As most of you know from this post, I have stocked my lake with bass and catfish.  Although the labels on these chemicals say safe for fish I did not think that the chemicals would be good for them either.  Plus I did not relish in the thought of funding chemical companies with frequent purchases of herbacide.  I note that my area has a lot of lakes, ponds and rivers.  Many of the owners of these use herbicides.  You can tell if a lake owner is using herbicides by looking at the grass that grows along the edge of a waterbody.  If it is yellow or dead, then they are using herbicide.


Grass carp is the best and most natural solution.  Come this spring, when watermeal or duckweed begins to grow, the grass carp should consume them prior to them being able to reproduce.  These fish are pretty amazing. Todd told me that they can grow up to 4 feet in length!  Of course the older they get, the less they tend to eat, so I plan on buying more around 2020.  Grass carp need to be restocked every 5-7 years (when they get larger than 30″, their consumption goes down).

Mindy from the DEEP granted me 20 grass carp, although I thought that she and Todd both agreed that a dozen would probably be sufficient.  I later learned that the number of carp for the pond is  function to the weed type and density.  There is a mathematical formula for calculating the number of grass carp that we ( and the state) use.  The math holds up well until you have a pond with about 50% coverage with duckweed or watermeal, then the math sort of falls apart….meaning you have to increase the number to get effective control.  That is why my initial stocking was for 20 fish.  Any future stocking will likely be for less fish (closer to 12 fish).  Grass carp is the most economical solution to control your watermeal/duckweed problem but still they are not cheap.  Each grass carp costs about $18 for a fingerling about 10″-12″ long. Rowledge Pond Aquaculture only delivers, unlike the bass and catfish, where I got to pick them up thereby saving the transportation costs.  All in all, $500 for 20 grass carp delivered.  I like that there are no further maintenance costs.

Below are photos of Todd and him stocking my  lake with grass carp.


Rowledge Pond Aquaculture delivery truck. Inside the white box were hundreds of jumping grass carp



Here’s a closeup of a fingerling grass carp. Each one measures around 10″ – 12″.


Here is Todd stocking my lake with grass carp. No acclimatization necessary. Just toss them in and I’m good to go.

Only time will tell if the Grass Carp are able to control the watermeal in my lake.  Stay tuned next spring/summer and I will let you know if it works.

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