Predictors of bigheaded carp drifting egg density and spawning activity in an invaded, free-flowing river

Successful reproduction of non-native species is a critical step in the invasion process, influencing establishment and spread. Predicting invasive species' reproduction is often based on information collected from species' native ranges; however, environmental variables within the invaded range may provoke different responses. Bigheaded carp (Hypophthalmichthys spp.) have successfully invaded many water bodies worldwide and are currently threatening the Laurentian Great Lakes. We examined bigheaded carp spawning in a mostly unregulated, invaded ecosystem (Wabash River, IN, USA) which has similarities to multiple Great Lakes tributaries; it can therefore provide insights into the potential spawning of bigheaded carp in the Great Lakes. Spawning was protracted, and while initiation of spawning has been reported to require changes in discharge in their native ranges, this association was not observed in the Wabash River. However, changing discharges did appear to influence egg density, with rises in late spring producing high drifting egg densities. Cumulative growing degree-day (sum of days where [(max temperature − min temperature / 2) − 10 °C base temperature] is > 0) was the most influential factor predicting spawning initiation. Overall, it appears that bigheaded carp exhibit the ability to spawn under a variety of environmental conditions, with changing discharges targeting spawning to periods when reproductive success may be more likely. The successful establishment of invasive species may hinge on their ability to reproduce across wide-ranging environmental conditions. The reproductive plasticity displayed by bigheaded carp could therefore contribute to their successful reproduction in multiple invaded ecosystems, including Great Lakes tributaries.