1. It sometimes takes years to see changes in aquatic vegetation abundance following grass carp stockings. Monitoring changes in aquatic vegetation following stocking should be done each year.
2. If 20% (as described in no. 3) or less of your pond is covered by aquatic vegetation, stocking grass carp is not recommended. Use chemical or mechanical spot treatments as necessary. This level of aquatic vegetation is generally accepted as optimal for sport fish population and the ecology of the pond.
3. All grass carp stocking recommendations are based on an accurate determination of the acres of aquatic vegetation covering the ponds surface plus the areas covered by submerged vegetation as far as you can see into the water. The best way to do this is to physically measure and calculate the square feet of vegetation covering your pond. Divide the number of square feet by 43,560 to determine acres of aquatic vegetation. An alternative method is to determine the entire surface area of the pond in acres then estimate the percent of aquatic plant coverage from a vantage point where you can see the entire pond. Take the surface area of your pond times the percent of plant coverage to determine acres of aquatic vegetation.
4. Stocking rates are based on the use of 8-11 inch long triploid grass carp. When using fish less than 8 inches long, increase the stocking rate by 40%, except in ponds that do not contain largemouth bass. In these ponds, stock at standard rates. For fish larger than 12 inches, decrease the stocking rate by 30%.
5. If your objective is to totally eliminate aquatic vegetation, stock 30 grass carp per acre of vegetation. If significant vegetation reduction has not occurred by the end of the third summer, stock and additional 15 fish per acre of vegetation. Mechanical and chemical methods can be used for initial control, however, be careful not to kill the grass carp. Keep in mind that total elimination of aquatic vegetation will seriously affect sport fish populations and normal pond ecology.
6. The recommended aquatic vegetation objective is to maintain some vegetation in your pond. Reducing the grass carp stocking rate to 15 per acre of vegetation generally offers the best chance of meeting this objective. If no improvement in vegetation coverage has occurred after four summers, stock and additional 7 grass carp per acre of vegetation. These stocking rates were developed in the absence of any supplemental chemical of mechanical vegetation control measure. Any interim measures taken should be very selective and cover only small areas, otherwise elimination of all vegetation may occur. Results of using this approach for vegetation control will be highly variable. No two ponds will react the same way. You will have to refine your vegetation management strategies over the years on an individual basis.
7. Grass carp will not control cattails, spatterdock (yellow water lily) or filamentous algae (pond moss) without first eliminating most other plants in the pond. If these plants are your problem and elimination of all plants in acceptable, use the method described in number 5, otherwise, spot treatments by chemical or mechanical methods are recommended.
8. Stocking grass carp in a lake that normally has high discharges is not recommended. In this situation, grass carp will leave the lake. If grass carp are stocked, a barrier should be erected at the spillway to prevent the fish from escaping. The recommended barrier uses 3/4 inch smooth, round metal bars as vertical supports on five-foot centers and 3/8 inch smooth, round bars horizontally on 1-3/8 inch centers.
Source: GRASS CARP GUIDELINES FOR USE IN INDIANA, Department of Natural Resources, Division of Fish and Wildlife 402 W. Washington Street, Room W273 Indianapolis, Indiana 46204
