Should I stock grass carp in my pond?

Many people are interested in stocking grass carp because they are generally cheaper and last longer than chemical and manual control methods. Although they can be cheaper, they are often still a significant financial investment with each fish costing anywhere between $12 and $20. There are many things to consider when stocking grass carp that should guide your decision and reduce cost, but the primary questions that should be asked and answered are will grass carp control the problem plant and will the carp stay in my pond?

Grass carp will only consume certain species of plants (see question below for more information) and can be quite picky when a variety of plants are available for food. Knowing the type of plants you want to control and the other plants you have in your pond is very important in determining if grass carp will work. You can get aquatic plants identified by contacting your county agriculture extension agent or your district fisheries biologist.

Finally, grass carp will migrate out of ponds with flowing water conditions such as water draining over spillways or through pond outlet pipes/towers. If your pond routinely has flowing water leaving the pond after rain events, you should reconsider stocking grass carp or consider some type of barrier/containment device on your pond outlets (http://dgifwebtest.gooutdoorsvirginia.com/forms-download/PERM/PERM-001.pdf). In this situation, could lose your investment due to grass carp escapement and you could potentially endanger the vegetative habitat in the streams and rivers downstream of the pond.

How do I find the physical address for my pond/lake?

The physical address or location of your pond is the 911 address with street number and name, county/city, and zip code. This is a location that a 911 operator would ask from you in an emergency. In most cases, pond/lakes have a physical address that is associated with the surrounding property. If the address is unknown by the landowner, the address can easily be located in most internet mapping programs (i.e. Google Maps, Mapquest). Find the pond in the mapping program by zooming in on the County, narrow in on the pond by finding closer landmarks (i.e. roads and  intersection, then right or left click with the mouse to drop a marker that will display the physical address (street number and name, city/county, and zip code).

How many carp do I need to stock for vegetation control?

Stocking guidelines for grass carp have been established through many years of trial and error, but stocking grass carp is not an exact science. Some trial and error may still be warranted on your pond to find the right number of carp needed to meet your objectives. Below are the recommended grass carp stocking rates for Virginia:

• 2 fish per acre for slight plant infestations (< 30% plant coverage of pond during the peak growing season-primarily August through September)
• 5 fish per acre for moderate plant infestations (30 – 60% plant coverage of pond during the peak growing season-primarily August through September)
• 10 fish per acre for heavy plant infestations (> 60% plant coverage of pond during the peak growing season-primarily August through September)

Calculations:

• Pond acreage with plants during peak growing season / total pond acreage x 100 = Percent plant coverage (this is number used to determine the stocking rate needed)
• Acreage x Stocking Rate (based on percent plant coverage) = Number of fish needed for stocking

Again, these are recommended stocking rates and some deviation from the recommendations may be necessary. We advise individuals to stock the minimal number of carp needed to prevent overgrazing, starvation, and fish migration. On rare occasions, high stocking rates (up to 15 fish per acre are permitted in Virginia) are needed to control trouble vegetation (i.e. for filamentous algae or duckweed control).

How do I determine my pond acreage?

Unless your pond has been surveyed or you have the original construction plans, getting the exact surface acreage is difficult. Many people will just visually estimate the size, but this can be problematic without much experience. If your acreage estimate is far from true, your stocking estimate will be off and your desired result will likely not be achieved. Below are some field methods for roughly calculating pond acreage.

• Circular shaped pond
  • Measure distance around pond in feet
  • Use the equation: Distance in feet² / 547,390 = surface area of pond in acres
• Rectangular shaped pond
  • Measure the length and width of the pond in feet
  • Use the equation: Length in feet x Width in feet / 43,560 = surface area of pond in acres
• Irregular shaped pond
  • Measure the length and width of the pond at multiple locations, making certain you get the longest and shortest values
  • Calculate the average width and length by dividing each by the number of measurements collected for that value
  • Use the equation: Average length x Average width / 43,560 = surface area of pond in acres

If you are more tech savvy, there are multiple websites that will let you locate your pond on a google based map and drop pins to calculate the acreage, just google search “acreage calculator map”.

Will grass carp eat the plants in my pond?

Grass carp are opportunistic herbivores that will consume a variety of aquatic plants. Unfortunately, like some young children, grass carp decide what they want to eat rather than us deciding what we want them to eat. Their preference is based on vegetation taste and texture and not necessarily on plant availability. Grass carp generally only consume submerged vegetation that has soft/tender, non-fibrous stems and leaves. Some common plants they will readily consume are hydrilla, elodea, bladderwort, coontail, najas, milfoil, potomegton spp. (pondweeds), chara, and nitella. They dislike and will not eat woody or hardy-stemmed plants such as cattails, lillypads, sedges, primrose, and many more.

The use of grass carp to control filamentous algae, watermeal, and duckweed has shown mixed results and often the effectiveness cannot be predicted. When they work to control these species, they are usually stocked at high rates in waterbodies with little to no other vegetation. If they can control these species, it is often cheaper and less time consuming than control with other methods (chemical and mechanical) but be advised that stocking grass carp to control these species may not work.

What time of year should I stock grass carp?

Studies have shown that one of the main causes of stocking mortality of grass carp is hauling and stocking water temperatures. Mortality can be up to 4 times higher when grass carp are stocked in the summer months, when water temperatures are high, compared to spring and fall stockings. Grass carp stockings should take place when water temperatures are routinely below 80°F and never when water temperatures are at or above 90°F. Grass carp suppliers will often quit sell grass crap in summer months due to the problems with hauling the fish in high temperatures. In addition, grass carp should not be stocked when water temperatures drop below 50°F, because most vegetation growth has stopped and the plant has started dying back. During this time, grass carp have a hard time adjusting to their new environment due to lack of food and they can experience increased mortality rates and are also more likely to migrate with high flows to find a more suitable environment.

In the Virginia, grass carp should ideally be stocked from March through May or September through October when water temperatures are between 50 and 70° F. This time frame may vary depending on the location of your pond and on yearly variations in temperature.

How do I find approved triploid grass carp suppliers?

In Virginia, you can only purchase grass carp from a certified triploid grass carp producer that is authorized to sell carp by The Virginia Department of Game and Inland Fisheries. These vendors go through rigorous testing procedures to ensure that all the carp they sell are triploid (have an extra set of chromosomes) and cannot reproduce. The list of approved vendors may change annually, so please make sure your vendor is authorized to sell grass carp in Virginia at the time of application submission. The list of approved vendors can be found on this website or attached to the permit application.

I just stocked grass carp this year and they are not working, do I need to stock more?

Grass carp take time to acclimate to their environment and may not be productive at controlling vegetation initially. In most cases, carp are stocked when vegetation is actively growing in the pond and can be quite thick. The newly stocked carp have a hard time catching up with the vegetation growth that first year after stocking, but usually are able to keep up with the growth the next spring. The carp will be most effective 2 to 5 years after stocking, when they are experiencing their fastest growth (sort of like teenagers). After 5 years in the pond, restocking at a reduced rate (usually 1⁄2 the original stocking rate) is often required to continue vegetation control. At the older ages and larger sizes, grass carp consume less vegetation and offer little control. If the vegetation is a type that is easily controlled and does not produce long lasting seeds or tubers, then only one stocking may be needed for complete control. When stocking grass carp for hydrilla, maintenance stockings for 10 years are often needed to control new plant growth from tubers and seeds that may lay dormant for years in the pond substrate.