Plasticity in Reproductive Potential of Bigheaded Carp along an Invasion Front

Invasive species can have varying and unpredictable reproductive potential among native and invaded populations or along invasion fronts, potentially contributing to invasion success. Understanding the reproductive potential (fecundity and egg size) of low-density, leading-edge populations is important to inform the management of Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis populations in the upper Mississippi River. Therefore, we compared batch fecundity and eggs/g of ovary subsample (an egg-size surrogate) of Silver Carp (n = 145) and Bighead Carp (n = 75) along a decreasing invasion gradient from Pool 20 (higher abundance) to Pool 17 (lower abundance) in the Mississippi River. Bighead Carp in Pools 17 and 18 (461 +/- 22 eggs/g [mean +/- SE]) had significantly fewer eggs/g than those in Pools 19 and 20 (higher abundance; 483 +/- 13 eggs/g). Log-transformed batch fecundity for Bighead Carp was positively correlated to log-transformed fish length, and batch fecundity was higher in Pools 17, 18, and 19 than in Pool 20. The eggs/g values for Silver Carp, which have been previously shown to be independent of fish length, were similar among all pools (94 +/- 11 eggs/g). Batch fecundity for Silver Carp was significantly higher in Pools 17-19 than in Pool 20, where densities are higher. Our results suggest that at lower densities, Bighead Carp produce more, larger eggs and Silver Carp produce more eggs, which collectively could increase their reproductive success along invasion fronts.