Creature Feature: Grass carp

Known to many landowners as a lawnmower for aquatic plants, Ctenopharyngodon idella, or grass carp, can eat up to 100% of their body weight in aquatic vegetation every day [1]. Their passion for eating plants makes these fish useful to humans, and they are often placed in ponds and lakes to keep the water clear of weeds. Grass carp are native to Eastern Asia, but they have been introduced to waterways around the world by humans.

Grass carp can be identified by their three dorsal fins, torpedo-shaped body, firm lips, olive or green back, and silvery yellow belly. But don’t wait too long before you come back and see them again; young grass carp can grow 20–45 cm in just a few months, while adults can reach lengths of two meters over their lifespan of about nine years [2].

In their native eastern Asian habitats, grass carp gravitate toward lakes and ponds that are connected to a fast-moving river, since they need flowing water in order to spawn (spawning is when females release eggs and males release sperm so that fertilization takes place in the water). The fertilized eggs drift downstream with the current and are believed to die if they sink to the bottom [3, 4], so a strong enough current is necessary for reproduction.

**Grass carp have spread across continents, facilitated by humans, from Asia to North America and Europe. Photo by Rainer Lück [Source]**

However, most people in the United States don’t think of these fish in their native Asian habitats; they know about them as pond and lake weed killers. In 1963, the first grass carp were brought to the United States to help control aquatic plants in aquaculture facilities in Alabama and Arkansas. An accidental release from one facility allowed fish to escape to the wild in 1984, however, these fighters had been caught in the Mississippi and Missouri Rivers before that throughout the 1970s, suggesting some fish had escaped even earlier [5].

Today, the grass carp purchased to reduce pond and lake vegetation are a triploid fish, which means each individual has three sets of chromosomes (as opposed to the typical two sets). Because of their odd chromosome number, they cannot reproduce. Before the fish are sent to be stocked in area lakes, each fish undergoes mandatory blood tests by the U.S. Fish and Wildlife Service and the diploid fish are removed to prevent unwanted spread of the introduced fish [6].

Despite these precautions, grass carp have already established and are now an invasive species in the United States. By eating all the vegetation and even roots of vegetation, they hurt the habitats of bass, bluegill, turtles, snakes, aquatic insects, and other animals. Their feeding behavior may cause transitions in the ecosystems they live in. By eating so much vegetation, they can eliminate places for smaller fish to hide before they grow up. To add to their impact, a single grass carp can only digest about half of what it eats each day. The other half is released to the water where it acts as fertilizer, promoting algae blooms and decreasing oxygen levels [5]. Grass carp invasion in the Midwest could result in billions of dollars of damage to Great Lakes fisheries, and researchers are working to develop ways to prevent this by containing the fishes’ spread [3].

Grass carp are one of four species of carp from Asia that have invaded North American waters. The fish were intentionally introduced by people to help control aquatic vegetation, but the four Asian carp are now considered invasive species, meaning they have negative impacts on the environment, economy, or human health. However, this does not mean they aren’t great species to fish and eat! Photos by Ryan Hagerty/U.S. Fish and Wildlife Service [Source].

Partly to help our ecosystems, and partly because grass carp are a fun fish to catch, many anglers have started fishing for and tagging grass carp to help biologists track their whereabouts in the wild. If you are looking for a new fish to try and catch, grass carp are a fly fisher’s dream. They fight like a bull, are extremely nervous, and often bring their tails out of the water when eating at the shoreline to make for exciting sight fishing.

Like many non-native species, the introduction of grass carp has brought unexpected consequences. Yet, despite the grass carp’s negative environmental impacts, their ability to adapt and survive makes them much more than an aquatic lawn mower—it makes them a strong and beautiful fish.

**Anglers can contribute to conservation and science by fishing for invasive species, like the grass carp, and helping to track their spread. Photo by Wesley Anderson taken in North Carolina [Source]**

Wesley Anderson is a life-long angler and kayaker who grew up fishing the waters of Florida. In his free time, he runs the fishing website BassGrab.com with his brother. He has fished all over the United States and in Central America mainly using kayaks as his primary method of travel.

References

O’Keefe, D. (2002). “The other Asian carp: Why grass carp deserve more attention.” Michigan State University Extension. https://www.canr.msu.edu/news/the_other_asian_carp_why_grass_carp_deserve_more_attention
Pink, J. P. & Socha, R. C. (2003). Longevity and persistence of triploid grass carp stocked into the Santee Cooper reservoirs of South Carolina. Journal of Aquatic Plant Management, 41: 90–92. http://www.apms.org/japm/vol41/v41p90.pdf
Krykhtin, M. L. & Gorbach, E. I. (1981). Reproductive ecology of the grass carp, Ctenopharyngodon idella, and the silver carp, Hypophthalmichthys molitrix, in the Amur Basin. Journal of Ichthyology, 21(2):109–123.
George, A. E. & Chapman D. C. (2015). Embryonic and larval development and early behavior in grass carp, Ctenopharyngodon idella: implications for recruitment in rivers. PLoS ONE, 10(3): e0119023. https://doi.org/10.1371/journal.pone.0119023
Nico, L. G., Fuller, P. L., Schofield, P. J., Neilson, M. E., Benson, A. J., & Li, J. (2020). Ctenopharyngodon idella (Valenciennes in Cuvier and Valenciennes, 1844). U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL. https://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=514
Conover, G., Simmonds, R, & Whalen, M., Eds. (2007). Management and control plan for bighead, black, grass, and silver carps in the United States. Asian Carp Working Group, Aquatic Nuisance Species Task Force, Washington, D.C. 223 pp. https://www.anstaskforce.gov/Documents/Carps_Management_Plan.pdf