Grass Carp in Lake Erie

Science Center Objects

Grass Carp, commonly used in aquaculture to control plant growth, escaped captivity in the Mississippi River and have been in the Great Lakes since 1975. Spawning surveys have documented spawning since 2015 in the Sandusky River leading to expanded surveys and an effort to confine reproduction to the western part of Lake Erie.

Nicole King Holding A Grass Carp

Nichole King (USGS) holding a Grass Carp captured during a Lake Erie survey.  Grass Carp are an invasive species.

(Public domain.)

Grass Carp is a plant-eating fish native to Asia, originally brought to the US to control plant growth in aquaculture ponds. Soon after their arrival, several Grass Carp escaped into the Mississippi River basin during flooding and have since spread throughout the US. Grass Carp are sold through fish farms, sometimes under the name white amur, for vegetation control in private ponds. Most of the Grass Carp sold are treated to be sterile and incapable of reproducing, especially in states that surround the Great Lakes. However, some states in the Mississippi River basin allow the sale of fertile Grass Carp. When Grass Carp escape captivity into the broader environment, their feeding habits can damage wetlands and submerged vegetation beds, which provide important ecosystem services and habitat to many native animals.  

Although Grass Carp have been captured in the Great Lakes since 1975, it was assumed that most were sterile and hence incapable of spawning. Grass Carp spawn in rivers because their eggs need to stay suspended in the current to develop and hatch. USGS research determined that some tributaries to Lake Erie (1) are highly suitable for reproduction of Grass Carp. In 2012, spawning was suspected when several juvenile grass carp (2) were captured in the Sandusky River, a Lake Erie tributary in Ohio. More research was needed to determine whether this suspected risk was real.

Spawning surveys of the Sandusky River began in 2014 but yielded no evidence of Grass Carp reproduction. However, spawning was documented in 2015 (3) with the collection of eight fertilized eggs in the Sandusky River. This was the first documentation of reproduction by Grass Carp or any of the four major Chinese Carp species in the Great Lakes. Spawning was documented again in 2017, 2018, and 2019, all correlated with high flow events.

Hydrograph Taken in the Sandusky River

Hydrograph of the Sandusky River at Fremont Ohio (USGS gauge 04198000; waterwatch.usgs.gov) 15 May-15 September 2017. The upper dashed line indicates the 90th percentile flow. The lower dashed line represents the minimum flow for grass carp eggs to remain suspended in the water column while drifting. Green circles indicate dates on which fertilized eggs were collected. Red circles indicate sampling was conducted but no eggs were collected. This graph was used in an update to the Great Lakes Panel on Aquatic Nuisance Species in May 2018.

(Public domain.)

In 2017, spawning surveys were expanded to a second Lake Erie tributary, the Maumee River, and spawning was again documented that year and during 2018 and 2019, with early-stage larvae (4) collected in 2018 (USGS 2019). Other Lake Erie tributaries in Ohio were added to surveys in following years, including the Portage River (2018), Huron River (2018, 2020), and Cuyahoga River (2019, 2020), but no evidence of spawning has yet been detected.

Evidence of Grass Carp reproduction has increased interest in population control and the need to identify spawning grounds. The State of Ohio’s response strategy (5) aims to “prevent Grass Carp expansion beyond western Lake Erie, the Maumee and Sandusky rivers.” Part of that strategy includes closing knowledge gaps and gaining a better understanding the life history of Grass Carp. Potential spawning locations (6) in the Sandusky River were identified with an egg drift simulator. Downtown Fremont, Ohio, approximately 3 km downstream from an impassable barrier, the Ballville Dam—which was removed in 2018—was identified as the most likely location. The actual spawning location was confirmed (6) by field observations of spawning behavior and the presence of early stage eggs immediately downstream of the estimated spawning location. Efforts to identify the spawning location in the Maumee River are underway.  

This research, which is led by USGS in collaboration with the University of Toledo, the Ohio Department of Natural Resources, and other universities and state, federal, and provincial agencies, has informed management actions to reduce Grass Carp numbers in Lake Erie. Strike teams have removed hundreds of adult Grass Carp from the Sandusky and Maumee rivers. Removing adults reduces reproduction and the risk that Grass Carp will establish a large population in western Lake Erie or spread to other Great Lakes.