Semester

Spring

Date of Graduation

2026

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Wildlife and Fisheries Resources

Committee Chair

Brent Murry

Committee Member

Kyle Hartman

Committee Member

Garrett Johnson

Abstract

Often accounting for over half of total fish biomass, invasive silver carp have dramatically reshaped the ecosystem of the Mississippi River and its tributaries. These high silver carp densities have been shown to affect native fish body condition, abundance, and community structure, highlighting the need for efficient methods to quantify silver carp’s effect on the native fish assemblage. Community size spectra (CSS) is a metric that quantifies fish assemblage body size structure based on the decrease in fish abundance with increasing size. To estimate silver carp’s effects on CSS in the Ohio River, side looking hydroacoustic monitoring was used alongside electrified dozer trawling and standardized boat electrofishing.

To accurately assess community size spectra using side looking hydroacoustics, in chapter one, we looked to address uncertainty when estimating fish size using side looking hydroacoustics. When observing fish horizontally, fish orientation relative to the hydroacoustic transducer has a significant effect on the fish’s observed size, with fish size decreasing with increasing orientation. As a result, at high orientations fish body size estimates are significantly smaller than the fish’s true size, which reduces accuracy when estimating CSS. To overcome this, we analyzed the effect of fish orientation range on observed fish size distribution. The results of chapter one indicate that by limiting the dataset to include only fish observed at less than 40 degrees of orientation, there is no significant effect of orientation on body size distribution, while sample sizes are maximized.

In chapter two, we then analyzed the effects of silver carp predictor metrics on community size spectra using a Bayesian regression model. The Bayesian regression model was chosen to maintain the error in CSS estimates between sites while estimating their correlation with silver carp biological predictors estimated for each site. The silver carp biological predictors were chosen to best represent silver carp’s measurable effects on CSS: silver carp percent total biomass, CPUE, and average weight. The silver carp predictors were estimated using electrified dozer trawling and boat electrofishing, then applied using the Bayesian regression model to estimate silver carp’s effect on CSS estimated with side looking hydroacoustics, electrified dozer trawling, and boat electrofishing. Results indicate that all three silver carp predictors have a relationship with CSS in the Ohio River. Silver carp percent total biomass and CPUE were both positively correlated with CSS, leading to a flattened CSS slope. Average silver carp weight was negatively correlated with CSS, meaning larger silver carp were correlated with steeper CSS slopes. Finally, estimates of silver carp effect on CSS with hydroacoustics, and boat electrofishing when truncated with the same minimum body size as hydroacoustics, led to minimal effect of silver carp on CSS. These results indicate that the effects of silver carp on CSS are linked to the relative abundance of small-bodied fish, potentially driven by silver carp competition with small-bodied planktivores such as gizzard shad

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