Quantifying removal success and reproductive biology of Grass Carp in the Lake Erie basin
Authors:   , Wilson Tammy Michelle 
Published: September 2024

Grass Carp (Ctenopharyngodon idella) are invasive, herbivorous freshwater fish that are considered an ecological threat in North America. Originating from eastern Asia, they can remove large amounts of aquatic vegetation which can be detrimental to other species that depend on the habitat and the resources they provide and cause significant changes in aquatic ecosystems. There is particular concern for Grass Carp establishment within Lake Erie and its tributaries as this is the gateway for Grass Carp to invade the rest of the Great Lakes. Since 2017, there have been multi-jurisdictional Grass Carp removal efforts to reduce Lake Erie's population and the chance of dispersal to the rest of the Great Lakes. Considerable research has been conducted on this population's ecology and population structure, but reproductive biology specific to Lake Erie has yet to be studied. Data is limited on Grass Carp reproductive strategies in Lake Erie, and more information on Grass Carp reproductive traits such as spawning strategies and age-at-maturity specific to the Lake Erie population could be useful to determine reproductive potential and inform future population modeling efforts. Monitoring shifts in ageat- maturity through time can provide information on population health. Currently gonad histology is the only reliable method to determine Grass Carp maturity. But as histology is an expensive and time intensive method, establishing an inexpensive method to determine maturity of removed Grass Carp that can be accomplished in the field will allow for easy age-at-mature monitoring. In addition, quantifying the effectiveness of current removal efforts is required to assess control and determine if objectives are being met to ensure efficient use of resources. My objectives were: 1) Determine Grass Carp fecundity type, developmental timing, spawning strategy and minimum age-at-maturity for Lake Erie and Missouri populations. 2) Calculate a Gonadosomatic index (GSI) threshold; the GSI value representative of maturity for at least 50 percent of Grass Carp and quantify its accuracy as a proxy for maturity for Grass Carp in North America. 3) Quantify the effectiveness of Gras Carp removal efforts in Lake Erie. In this study, I used gonad histology to identify Grass Carp fecundity type, ovarian developmental timing, spawning strategy, and age-at-maturity. Because young fish are rarely captured in Lake Erie, I used Grass Carp capture data from the Missouri, Mississippi, and Lamine Rivers within the state of Missouri as a proxy population to help identify Grass Carp reproductive traits. I used logistic regression to establish a Gonadosomatic index (GSI) threshold as a proxy for maturity. I used analysis of variance (ANOVA) to determine differences in GSI and body condition between Lake Erie and Missouri populations. I used a length-based Bayesian biomass (LBB) estimation model to quantify the Lake Erie Grass Carp removal efforts from 2019-2023. Based on gonad histological samples (n = 274), North American Grass Carp are batch spawners with indeterminate fecundity and asynchronous ovarian developmental timing. A spawning strategy that maximizes flexibility in response to environmental conditions, exemplified by the observed protracted spawning season in Lake Erie that ranged from April to November. Grass Carp minimum observed age-at-maturity for both males and females in Lake Erie and females in the Mississippi River was age-3. While males in the Mississippi River minimum observed age-at-maturity was a year earlier at age-2. GSI thresholds were calculated at 1.22 and 0.42 for females and males, with a classification accuracy rate of 98.7 percent and 87.9 percent, respectively. Spawning-ready females in Lake Erie have greater body condition values and GSI than in the Mississippi River. The LBB model predictions of fishing pressure and unexploited biomass indicated that the Grass Carp population in Lake Erie is not being overfished, nor has overfishing occurred. This suggests that current removal efforts are insufficient to meaningfully reduce population size, and investigating other management techniques might be beneficial for controlling the invasive Grass Carp population in Lake Erie. Although current removal efforts in Lake Erie are not enough to meaningfully reduce the population, densities appear to be low. Considering that reproductively capable Grass Carp have been in the system for at least four decades, the availability of quality habitat, and the reproductive ability of Grass Carp, perhaps other unknown factors may have operated to keep the population numbers low. However, since Grass Carp spawning strategy exhibited maximum flexibility in response to novel environmental conditions, if Grass Carp are allowed to disperse into the adjoining Great Lakes, probability of establishment is high.