Effects of bigheaded carp on zooplankton and larval fish along the Upper Mississippi River invasion front

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Tillotson, Nathan Andrew
Major Professor
Weber, Michael J
Pierce, Clay L
Dixon, Phillip M
Committee Member
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Natural Resource Ecology and Management
The successful establishment and spread of invasive Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis (collectively bigheaded carp) throughout the Mississippi River Basin has been a great concern to resource managers since their introduction in the 1970s. However, the ecological ramifications of these invasive planktivores are still not fully understood. While competition between bigheaded carp and native planktivorous fishes is intuitive and has been documented, little is known about the effects of bigheaded carp on native omnivorous and piscivorous fishes despite diet overlap when these native fishes are at the larval stage. Here, we contributed to filling this knowledge gap by evaluating zooplankton communities and larval fish (Freshwater Drum Aplodinotus grunniens) foraging patterns and growth across a gradient of bigheaded carp relative abundance at the invasion front in the Upper Mississippi River (UMR). We sampled zooplankton (2016 – 2018), larval fish (2017 – 2018), and adult bigheaded carp (2016 – 2018) from five pools of the UMR (Pools 14, 16, 18, 19, and 20). In chapter two, we found UMR zooplankton communities were invariably dominated by rotifers and that crustacean zooplankton densities were well-below threshold levels for native fish recruitment. Zooplankton communities differed relative to bigheaded carp presence at sampling locations: Cladocerans and copepods were both reduced where bigheaded carp were present but copepods increased with bigheaded carp relative abundance. Ostracod biomass increased in the presence of bigheaded carp, while rotifers declined with bigheaded carp relative abundance. In chapter three, we found that environmental abundances of crustacean zooplankton were the most important factor related to larval Freshwater Drum foraging success. Copepods were the dominant prey taxa, but larvae also consumed large proportions of unexpected prey (i.e., rotifer eggs and benthic insect larvae), especially where bigheaded carp were present. Larval diets differed based on the presence of bigheaded carp at sites and bigheaded carp CPUE was positively associated with increased consumption of atypical prey taxa. In chapter four, we found bigheaded carp relative abundance and water temperature were the most important factors associated with larval Freshwater Drum growth and length-at-age in the UMR. While increasing water temperature was positively associated with larval length-at-age, increasing bigheaded carp relative abundance was negatively related to larval growth. Simulations based on estimates from our top model revealed that the effects of bigheaded carp relative abundance were significant at the 25th and 50th percentiles of our water temperature range, and estimated larval total length at 17 days of age could be up to 42% (10.33mm) smaller where bigheaded carp were abundant relative to their absence. Collectively, results from these studies suggest bigheaded carp are altering zooplankton communities in the UMR, causing larval fish dietary niche shifts that may be influencing their reduced growth where bigheaded carp are abundant. These results offer insight into an area of bigheaded carp invasion ecology that has, until now, lacked appropriate study. Furthermore, these results suggest a threshold level of bigheaded carp abundance exists that constrains dynamics of native taxa (>0 CPUE via boat electrofishing), and managers should strive to reduce their populations below catchable levels.