Environ Biol Fish https://doi.org/10.1007/s10641-018-0783-6 Feeding ecology of generalist consumers: a case study of invasive blue catfish Ictalurus furcatus in Chesapeake Bay, Virginia, USA Joseph D. Schmitt & Brandon K. Peoples & Leandro Castello & Donald J. Orth Received: 5 October 2017 /Accepted: 15 June 2018 # Springer Nature B.V. 2018 Abstract Globally, invasive species cause extensive considered with caution. Blue catfish feed primarily on economic damage and are a major threat to biodiversity. invasive aquatic vegetation and Asian clams, though the Generalist species are particularly dangerous invaders, economically-valuable blue crab Callinectes sapidus is as they can thrive in degraded habitats and endure also consumed regularly. While the per capita impact of environmental stochasticity, often outcompeting more blue catfish on imperiled native species appears to be specialized native taxa. Blue catfish Ictalurus furcatus low, this impact could still be substantial due to high were first introduced into the Chesapeake Bay during population densities. the 1970s, and now form dense populations in several tidal rivers. Despite being labeled as a dangerous inva- Keywords Invasive species . Food habits . DNA sive, the feeding ecology of this species is largely un- barcoding . Diet . Generalist invaders . Biotic known. We used a stratified random design to collect homogenization . Feeding ecology. Blue catfish . Diet stomachs from 16,110 blue catfish in tidal freshwater, breadth . Trophic level . Omnivory index . Ontogenetic oligohaline, and mesohaline segments of the James, shifts . Predator-prey interactions Pamunkey, Mattaponi, and Rappahannock Rivers. Indi- ces of diet breadth and omnivory reveal that blue catfish are generalist omnivores with some of the highest diet Introduction breadths ever observed in an estuarine fish species, while trophic level calculations demonstrate that blue Invasive species can cause population declines and ex- catfish are a mesopredator occupying lower trophic tinction of native species (Mills et al. 2004), and are a levels than previously claimed. Cumulative prey curves major threat to biodiversity (Lockwood et al. 2013). revealed that large numbers of stomachs are necessary to Globally, specialist taxa are being replaced by invasive adequately characterize the diet of blue catfish, thus generalists with broad ecological niches, a trend known previous diet descriptions of this species should be as biotic homogenization (McKinney and Lockwood 1999;Claveletal.2010). Many generalist species have : : J. D. Schmitt (*) L. Castello D. J. Orth a competitive advantage due to their ability to thrive in Department of Fish and Wildlife Conservation, Virginia degraded habitats, which has sparked a renewed interest Polytechnic Institute and State University, 106 Cheatham Hall, in the characterization of generalist consumers world- Blacksburg, VA 24061, USA e-mail: [email protected] wide (McKinney and Lockwood 1999; Layman and Allgeier 2012). Some of the most successful invasives B. K. Peoples consume a broad array of food items, with diets com- Department of Forestry and Environmental Conservation, Clemson University, 256 Lehotsky Hall, Clemson, SC 29634, prised of both plant and animal material (Twardochleb USA et al. 2013; Jackson et al. 2017). These omnivorous food Environ Biol Fish habits place them in the middle of the food web, with have been captured in salinities as high as 21.5 ppt direct consumptive links to multiple trophic levels; this (Fabrizio et al. 2017). Blue catfish dominate the fish helps them endure adverse conditions and changes in biomass in some locales, which has caused concern prey availability (Layman and Allgeier 2012;Jackson about their potential interactions with native species et al. 2017). Despite the considerable attention that (Greenlee and Lim 2011; Schloesser et al. 2011), and invasive species receive, little is known about the life prompted the Chesapeake Bay Program (CBP) to de- history and feeding ecology of many of these organisms, velop an Binvasive catfish policy^, which calls for more and more observational and experimental studies are research on the life history and ecological role of this urgently needed (García-Berthou 2007; Layman and species (CBP 2012). Allgeier 2012;Brandneretal.2013). Previous diet studies of blue catfish in the Chesa- Once established, novel generalist consumers can peake Bay are limited by sample size, spatiotemporal drastically alter invaded ecosystems. Food webs are scope, or include only small individuals (Schmitt et al. restructured, which can lead to changes in ecosystem 2017). This is problematic because prey assemblages function, productivity, and the deterioration of ecosys- vary seasonally and spatially in the Chesapeake Bay tem goods and services (Mack et al. 2000; Clavel et al. (Jung and Houde 2003) and blue catfish regularly ex- 2010). Populations of invasive generalists can reach ceed 40 kg in Virginia’s tidal rivers (Greenlee and Lim densities that are orders of magnitude greater than sim- 2011). Only one study has assessed sample size suffi- ilar native species, and, because they are linked to ciency for blue catfish, and found that large numbers of multiple trophic levels, result in widespread impacts stomachs (≈ 1500) were needed for diet description due on invaded communities (Snyder and Evans 2006). to the diversity of resources consumed (Schmitt et al. Feeding ecology studies are of particular importance, 2017). Considering this, the authors concluded that most as diet is a primary determinant for predicting how of the previous diet work in Chesapeake Bay is unlikely invasive species will affect food webs of receiving sys- to provide a realistic picture of the full dietary breadth of tems (Brandner et al. 2013; Garvey and Whiles 2017). this species. To date, no studies have assessed trophic Moreover, the classification of a novel species along position, individual diet specialization, or diet breadth the generalist-specialist feeding continuum has sig- for blue catfish, all of which relate to potential impacts nificant implications for their long-term success af- in novel environments (Layman and Allgeier 2012; ter establishment (Moyle and Light 1996), and the Garvey and Whiles 2017). precise ecological impact of an introduced species The current study will provide several valuable pieces depends largely on its trophic position within the of information. First, it provides another example of an food web (McKnight et al. 2016). opportunistic generalist species taking over a degraded Native to tributaries of the Mississippi River, the blue ecosystem, a trend that continues to gain attention glob- catfish Ictalurus furcatus is a large catfish species that ally and has serious ecological consequences (McKinney has been widely introduced into Atlantic and Pacific and Lockwood 1999; Layman and Allgeier 2012). Sec- drainages in the U.S. (Graham 1999;Eggletonand ond, the current study will fully characterize spatiotem- Schramm Jr 2004). This species can weigh in excess poral variability in blue catfish diet for three large of 50 kg, reaches high population densities, and may be subestuaries of the Chesapeake Bay, which will help a dangerous invader (Graham 1999;GreenleeandLim fisheries managers make decisions moving forward. 2011; Howeth et al. 2016). Even so, there is a general Management of blue catfish will be complicated, as blue paucity of information on this species (Graham 1999) catfish support recreational fisheries and expanding com- and little is known about the feeding ecology of this mercial fisheries, yet potentially threaten important native species outside of its native range (Schmitt et al. 2017). resources including blue crab Callinectes sapidus,Amer- Blue catfish were stocked in tidal freshwater portions of ican shad Alosa sapidissima,alewifeA. psuedoharengus, the Chesapeake Bay from 1973 to 1985 to create new blueback herring A. aestivalis, and American eel Anguilla recreational fisheries (Greenlee and Lim 2011). Blue rostrata (Schmitt et al. 2017). catfish populations have since expanded to occupy all Considering this, our specific research objectives major tributaries of the Chesapeake Bay, and are now were to: 1) characterize the feeding ecology of blue considered invasive (Fabrizio et al. 2017). Blue catfish catfish by determining individual diet specialization, have expanded to brackish portions of the estuary, and trophic position, and generalist versus specialist feeding Environ Biol Fish strategies; 2) explore spatiotemporal patterns in prey (Shiah and Ducklow 1994). Each river was stratified consumption, size-based variation in diet, and assess into freshwater sections (0–0.5 ppt), oligohaline sections sample size sufficiency to ensure a robust diet charac- (>0.5–5 ppt), and mesohaline sections (>5–18 ppt). Fur- terization; 3) collect blue catfish stomachs across broad thermore, each stratum was divided into 2-km reaches spatiotemporal scales in three large subestuaries of the which were then randomly sampled. Individuals were Chesapeake Bay, so that inference can be drawn for this collected monthly within each stratum of each river at a region as a whole. minimum of two randomly selected reaches, with a minimum of five sites sampled within each reach. We sampled both nearshore and main channel habitats at Methods each site, and recorded time of day, water temperature, salinity, geographic coordinates, and tide phase at each Study area sample location. When possible, we attempted to collect