If I told you that a creature nearly 5 feet long, up to 80 pounds, and capable of eating nearly 150% of it’s body weight A DAY was lurking in Lake Gaston (and now Kerr Lake), you would likely be concerned…. Well, in actuality there IS a creature such as this that you share the water with in many reservoirs, lake, and ponds across the United States. Don’t rush out of the water just yet however….
Known to scientists as Ctenopharyngodon idella, the white amur or grass carp is a herbivorous (only eats plants!) fish introduced to control nuisance aquatic plant species. The grass carp is actually native to large river systems of Eastern Asia and has been distributed worldwide for use as a biological control and as a food fish. Despite what one might think, the grass carp is very different from the well-known common carp, which is also nonnative having been introduced from Europe. One noticeable difference in the location of the grass carp’s mouth which is located on top of the head for feeding on plants. The mouth on the common carp is positioned low on the head to aid in bottom feeding in shallow water. Grass carp can live nearly 25 years and can grow as much as ten pounds per year. As eluded to earlier, grass carp consumption is obviously dependent on the size of the fish but these fish have amazing appetites. Carp over 15 pounds consume up to 30% of their body weight daily, whereas smaller fish (less than 10 pounds) consume as much as 150% of their body weight a day. Feeding does slow down in colder water (below 55 degrees) or when oxygen levels in the water drop.
Grass carp are generalists feeders and will eat almost any plant material however they do prefer the more tender, soft material of submersed species over the waxy, sometimes hard material of floating and emergent plants. Grass carp have a sweet (or green rather) tooth for southern naiad, hydrilla, and duckweed. There is one exception to the grass carp’s diet as they do NOT care for Eurasian water milfoil. Grass carp are also poor controllers of filamentous algae.
Grass carp stocked in North Carolina reservoirs are triploid, which means that they are infertile and unable to reproduce. This keeps grass carp populations in a water body “in check” and allows the NC Wildlife Resource Commission to track and model grass carp populations in a Lake. This is important because stocking too few grass carp annually will have little to no affect on controlling problem species like hydrilla. Overstocking can also lead to rapid devegetation of a lake which can negatively affect water quality. Grass carp are also only stocked into water bodies in which they can be contained to prevent their escape downstream. While grass carp are extremely effective in controlling invasive submersed plant species, they will also feed on native species and if allowed to be introduced into open system, could devastate native plant populations.
Hopefully, you have learned a little more about grass carp. Next time you see large fin snaking through the shallows, remember that this is likely a grass carp innocently feeding on vegetation and aiding in plant management in the Lake. Although large and menacing looking, these fish are merely “cattle with fins” who are an integral part of plant management on the Lake. For more information about grass carp, see the “web links for more information”. Pictured is a grass carp caught during research on Lake Gaston in 2013.
Web Links For Additional Information:
AERF BMP – Chapter 10 Grass Carp
UF Grass Carp Information