Effects of Lotic Environmental Conditions on Larval Fish Growth and Prey Consumption

Rivers throughout the United States have undergone extensive modifications, altering their original dynamic state. Knowledge of how flow and temperature regime alterations may affect foraging and growth of larval fishes is limited. We investigated the effects of discharge, growing degree days (GDD) and zooplankton densities on larval Sciaenidae and Catostomidae feeding success (probability of prey encountered in the stomach), prey consumption (number of prey consumed) and growth (length at age). We sampled ichthyoplankton and zooplankton from the Des Moines and Iowa rivers, Iowa, USA from April–June 2021 and 2022. Catostomidae feeding success on cladocerans and rotifers was negatively associated with prey density, while feeding success on copepods was positively associated with prey density; consumption of cladocerans was negatively associated with prey density, and consumption of copepods and rotifers was positively associated with prey density. Discharge was positively associated with Sciaenidae feeding success and consumption of cladocerans and Catostomidae consumption of cladocerans but negatively associated with Sciaenidae feeding success on copepods, Catostomidae feeding success on cladocerans, and Catostomidae feeding success and consumption of copepods. Water temperature was negatively associated with Sciaenidae and Catostomidae feeding success and consumption of cladocerans and positively associated with Catostomidae feeding success and consumption of copepods. Larval Sciaenidae growth was positively associated with copepod density, while Catostomidae growth was positively associated with cladoceran and rotifer densities. Catostomidae growth was negatively associated with copepod density and GDD. We did not find an effect of discharge for growth of either taxa. Our results suggest zooplankton density is an important limiting factor for larval fish growth, while environmental factors such as discharge and water temperature influence larval fish foraging in lotic systems.