Feeding Ecology of Invasive Catfishes in Chesapeake Bay Subestuaries Joseph D. Schmitt Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of: Doctor of Philosophy in Fish and Wildlife Conservation Donald J. Orth Yan Jiao Leandro Castello Brandon K. Peoples Defended on March 23, 2018 Blacksburg, VA Keywords: invasion ecology, food habits, diet, blue catfish, flathead catfish, consumption rates, daily ration, prey selectivity, diet breadth, trophic position, omnivory index Feeding Ecology of Invasive Catfishes in Chesapeake Bay Subestuaries Joseph D. Schmitt Academic Abstract Blue Catfish Ictalurus furcatus are native to tributaries of the Mississippi River but are now invasive in several Atlantic slope drainages. This includes subestuaries of the Chesapeake Bay, where their feeding ecology and potential impact on native species was largely unknown. We collected stomach contents from 16,110 Blue Catfish at 698 sites in three large subestuaries of the Chesapeake Bay (James, York, Rappahannock rivers). Cumulative prey curves revealed that sample size was sufficient for diet description, though 1,000 – 1500 stomachs were needed per river. Blue Catfish are opportunistic generalists that feed on a broad array of plant and animal material. Logistic regression models reveal that Blue Catfish undergo significant ontogenetic diet shifts to piscivory at larger sizes (P<0.01) though the lengths at which these shifts occur varies by river system (500 – 900 mm total length; TL). Over 60% of Blue Catfish stomachs contained other invasive species, primarily Hydrilla verticillata and Asian clams Corbicula fluminea. Canonical Correspondence Analysis (CCA) revealed that salinity and season explained the most variation in Blue Catfish diet, while Generalized Additive Models (GAMs) demonstrated that there is considerable spatiotemporal and length-based variation in predation of species of concern. Species of concern include American Shad, American Eel, and river herring, which are imperiled, and blue crab, which support valuable fisheries in Chesapeake Bay. Predation of American Shad, American Eel, and river herring was rare (max predicted occurrence in Blue Catfish diets = 8%), while blue crab was much more common in the diet (max predicted occurrence =28%). Predation of American Shad and river herring peaks in freshwater areas in April, while predation of blue crab peaks in brackish areas in October. Predation of all species of concern is highest for large catfish (500 – 1000 mm TL). Field and laboratory-based estimates of consumption rate revealed that Blue Catfish feed at similar rates as Channel Catfish Ictalurus punctatus, and daily ration is estimated to be 2-5% bodyweight per day during warm temperatures, while peak feeding (maximum daily ration) can approach 10% bodyweight per day. While consumption of imperiled species is rare, Blue Catfish could still have negative impacts on these species due to dense catfish populations. Feeding Ecology of Invasive Catfishes in Chesapeake Bay Subestuaries Joseph D. Schmitt General Audience Abstract Native to the Mississippi River basin, Blue Catfish Ictalurus furcatus are the largest catfish species in North America; they can exceed 100 lbs and live for over 30 years. They were stocked in Chesapeake Bay as a sportfish 40 years ago and are now considered invasive. We performed a series of experiments to describe how invasive Blue Catfish fit in the food web, including diet and consumption rate analyses. Most of the Blue Catfish population is comprised of opportunistic generalists that feed on a diverse array of plant and animal material. We experimentally demonstrated that Blue Catfish tend to feed on whatever species are locally abundant. The most common prey were other species considered invasive in Chesapeake Bay; primarily Asian clams and Hydrilla verticillata, which is an invasive aquatic plant. Blue Catfish also eat other types of vegetation, clams, snails, crabs, fish, small invertebrates, mammals, birds, reptiles, and amphibians. Consumption of imperiled native fishes (like American Shad, American Eel, and river herring) was uncommon while consumption of the commercially-valuable blue crab was common, especially in brackish areas during the autumn. Blue Catfish consumption rates were similar to a closely related species, the channel catfish, and blue catfish can consume an estimated 2 – 5% of their body weight per day during the summer months, and up to 10% of their body weight per day during times of peak feeding. While consumption of imperiled species is rare, Blue Catfish could still have negative impacts on these species due to dense catfish populations Acknowledgements This project would not have been possible without the assistance and support of many different individuals. First and foremost, I would like to thank my wife, Suzie Schmitt, who suffered through 3+ years of me making the four-hour trip from Blacksburg to eastern Virginia. I was often gone for multiple nights in row, and would return exhausted, disorganized, and covered with catfish vomit. I was definitely not as helpful around the house as I should have been. I appreciate your patience throughout the process. Jason Emmel and Zach Moran were the pillar stones of this project. Their work ethic and “whatever it takes to get the job done” approach fueled our success and contributed to the tremendous amount of stomachs we were able to collect and process in a relatively short period of time. Whether we were rebuilding carburetors in our hotel parking, battling 60 mph squalls on the lower York River, or camping in snow storms at Chickahominy Riverfront Park, we always worked together to “improvise, adapt, and overcome”. Lifelong friendships were formed through the challenges we faced together, and many laughs were shared. Thanks for all your help. I also want to thank my advisor, Donald J. Orth, who provided incredible guidance and support throughout this entire project. Even when things went poorly, he always focused on solutions rather than dwelling on my failures. I realize how fortunate I was to have you as a graduate advisor. Many other individuals were instrumental in the completion of this project. Brandon Peoples, Aaron Bunch, Bob Greenlee, Kaylie Johnson, Hae Kim, Michael Moore, Hunter Hatcher, John Woodward, and Skylar Wolf helped tremendously in the field and in the lab. Brandon Peoples, Bob Greenlee, John Odenkirk, Leandro Castello, Eric Hallerman, Aaron Bunch, and Scott Smith provided guidance and assistance during project scoping, analysis, and writing. The Smithsonian Institution, particularly Rob Aguilar and Matt Ogburn, assisted with the use of advanced molecular techniques to identify digested piscine prey. Trophic position research was assisted by Esra Gokturk, while commercial fishermen B.T. Smith and Wayne Fisher kindly assisted with our field operations and provided invaluable local knowledge. Tom and Florie Spellerburg provided overnight lodging and delicious meals to our smelly catfish crew on many occasions. Aaron Bunch, Robbie Willis, Kirk Dunn, Kaylie Johnson, and Alan Weaver helped with the collection of stomachs during the spring Alosa migration. Zach Moran, Allison Mosley, Hae Kim, Haena Lee, and John Woodward spent hours of effort in the laboratory and sorted through thousands of bags of putrid stomach contents. Finally, I want to thank Virginia Sea Grant folks, Susan Park and Janet Kren, for their help in the field, and also Sam Lake, Sabine Rogers, and Ian Vorster for their guidance and support. This project was funded by the Virginia Department of Game and Inland Fisheries through a Federal Aid in Sport Fish Restoration Grant from the U.S. Fish and Wildlife Service. Additional student support was provided by a graduate student research fellowship from Virginia Sea Grant and the Department of Fish and Wildlife Conservation. During the course of this project I celebrated the birth of my first child, Lily Ava Schmitt. I dedicate this work to her. iv Table of Contents Important Note to Readers ......................................................................................................................... vi Chapter 1: Introduction ................................................................................................................................ 1 1.1 Research Gaps .............................................................................................................. 10 1.2 Objectives and Products ............................................................................................... 11 1.3 Methods Overiew ......................................................................................................... 12 1.4 References .................................................................................................................... 14 Chapter 2: Predation and Prey Selectivity of Nonnative Catfish on Migrating Alosines in an Atlantic Slope Estuary 2.1 Introduction .................................................................................................................. 20 2.2 Methods ....................................................................................................................... 24 2.3 Results .......................................................................................................................... 32 2.4 Discussion ....................................................................................................................