Research Document 2016/117

Ecological Consequences of Grass Carp, Ctenopharyngodon idella, in the Great Lakes Basin: vegetation, fishes and birds

By Gertzen, E.L., Midwood, J.D, Wiemann, N., and Koops, M.A.


Grass Carp (Ctenopharyngodon idella), a large, herbivorous fish, was first introduced to North America in 1963 for aquatic macrophyte control. Grass Carp populations have since escaped from impoundments where they were stocked, entered rivers in central United States and continued making their way up the Mississippi River basin towards the Great Lakes. Grass Carp have also been spread by commerce in the United States, where sale of live diploid and certified triploid (reproductively sterile) Grass Carp is legal in several states. Between 2007 and 2012, 45 Grass Carp were caught in the Great Lakes basin and captures within the basin are continuing to occur with increasing frequency. The potential for Grass Carp to invade the Great Lakes is of increasing concern and Great Lakes managers need to understand the potential ecological consequences of Grass Carp to the Great Lakes basin. Using different approaches, the potential ecological impacts of Grass Carp on aquatic vegetation, native fishes, and bird communities in the Great Lakes basin were assessed. Under the evaluated scenarios, Grass Carp invasion typically predicted there would be a decline in low marsh biomass of less than 5% given the high amount of estimated submerged aquatic vegetation biomass across the Great Lakes. However, at the site-level, a greater range of variability was observed, with a large proportion of sites seeing a 50% decline in biomass, particularly at higher densities of large Grass Carp. The potential negative effect of Grass Carp on Great Lakes fishes was evaluated using spawning characteristics and habitat preferences for 136 fish species occurring in the Great Lakes Basin. The potential negative impact of Grass Carp, as predicted by overlap in spawning and habitat needs, was high for 33 fish species, moderate for 33 fish species and low, nil or unknown for 70 fish species. A total of 47 bird species were identified that use Great Lakes coastal wetlands in Canada as breeding habitat and that may experience consequences following the introduction of Grass Carp into the Great Lakes. Based on use of wetlands for feeding needs and nesting habitat, 18 species were predicted to experience high potential negative ecological consequences following Grass Carp introduction and the remaining 29 were predicted to experience moderate potential negative ecological consequences following Grass Carp introduction. In general, the predicted negative impacts of Grass Carp on aquatic vegetation, fishes and waterbirds are variable; however, the impacts may be extreme for certain sites and species in the Great Lakes.

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