Effects of Invasive Bigheaded Carp and Environmental Factors on Larval Fish Growth

Silver carp Hypophthalmichthys molitrix and bighead carp H. nobilis (collectively bigheaded carp) are abundant planktivorous invasive species in North America that negatively affect adult native fishes. It is hypothesized bigheaded carp may also compete with native fish larvae but these potential interactions have not been investigated. We investigated larval freshwater drum Aplodinotus grunniens length-at-age (i.e., growth) across a gradient of adult bigheaded carp relative abundance in the Upper Mississippi River. We sampled fish larvae and zooplankton every 10 days (May–August, 2017 and 2018) from UMR Pools 14-20, collected environmental data from nearby gaging stations, and indexed adult bigheaded carp relative abundance during fall with boat electrofishing. We removed larval freshwater drum otoliths and two independent readers estimated daily ages. We then evaluated the effects of environmental covariates on larval freshwater drum length-at-age using linear mixed-effects models. Larval freshwater drum length-at-age was negatively related to adult bigheaded carp relative abundance and positively associated with water temperature. Negative effects of adult bigheaded carp relative abundance were observed at the 25th and 50th water temperature percentiles but not at the 75th percentile, indicating warmer temperatures may outweigh the effect of bigheaded carp. We did not detect effects of river discharge, larval hatch date, taxa-specific zooplankton abundance, or larval fish abundance on larval freshwater drum length-at-age. Our results indicate adult bigheaded carp abundance and larval freshwater drum length-at-age co-vary, suggesting bigheaded carp abundance can reduce freshwater drum growth during early life stages and may have similar effects on larvae of other fishes with implications for larval survival and recruitment.