Forecasting the Impacts of Silver and Bighead Carp on the Lake Erie Food Web

Nonindigenous bigheaded carps (Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix; hereafter, “Asian carps” [AC]) threaten to invade and disrupt food webs and fisheries in the Laurentian Great Lakes through their high consumption of plankton. To quantify the potential effects of AC on the food web in Lake *Corresponding author: [email protected] Received December 6, 2014; accepted July 15, 2015 136 Transactions of the American Fisheries Society 145:136–162, 2016 American Fisheries Society 2016 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2015.1069211 D ow nl oa de d by [ N oa a G le rl L ib ra ry ] at 0 8: 57 0 7 Ja nu ar y 20 16 Erie, we developed an Ecopath with Ecosim (EwE) food web model and simulated four AC diet composition scenarios (high, low, and no detritus and low detritus with Walleye Sander vitreus and Yellow Perch Perca flavescens larvae) and two nutrient load scenarios (the 1999 baseline load and 2£ the baseline [HP]). We quantified the uncertainty of the potential AC effects by coupling the EwE model with estimates of parameter uncertainty in AC production, consumption, and predator diets obtained using structured expert judgment. Our model projected mean § SD AC equilibrium biomass ranging from 52 § 34 to 104 § 75 kg/ha under the different scenarios. Relative to baseline simulations without AC, AC invasion under all detrital diet scenarios decreased the biomass of most fish and zooplankton groups. The effects of AC in the HP scenario were similar to those in the detrital diet scenarios except that the biomasses of most Walleye and Yellow Perch groups were greater under HP because these fishes were buffered from competition with AC by increased productivity at lower trophic levels. Asian carp predation on Walleye and Yellow Perch larvae caused biomass declines among all Walleye and Yellow Perch groups. Large food web impacts of AC occurred in only 2% of the simulations, where AC biomass exceeded 200 kg/ha, resulting in biomass declines of zooplankton and planktivorous fish near the levels observed in the Illinois River. Our findings suggest that AC would affect Lake Erie’s food web by competing with other planktivorous fishes and by providing additional prey for piscivores. Our methods provide a novel approach for including uncertainty into forecasts of invasive species’ impacts on aquatic food webs. Invasive species are a major stressor on biodiversity, energy flow, and productivity in terrestrial and aquatic ecosystems. Currently, we lack an effective way to predict their impacts on food webs prior to establishment (Ricciardi et al. 2013). Such information and its associated uncertainty is instrumental for decision makers because not all nonindigenous species pose a threat. Knowledge of those species that may damage an ecosystem must be quantified to effectively allocate resources for prevention and control. Past efforts to forecast the ecological impacts of nonindigenous species in new environments have been based on the invasion history of target species (Ricciardi 2003; Kulhanek et al. 2011) or comparisons of the functional traits of the nonindigenous species with those of the native species residing in the recipient community (Kolar and Lodge 2002; Dick et al. 2013). Introduced species can also have impacts that vary across space and time, which can present substantial challenges to accurately forecasting the impacts of nonindigenous species in novel environments (Branch and Steffani 2004; Ricciardi and Kipp 2008). Furthermore, when combined with other stressors (e.g., excess nutrient loading), the impacts of nonindigenous species can lead to complex, profound, and long-term changes (or regime shifts) to the recipient ecosystems (Madenjian et al. 2013). To assess the potential ecosystem impacts of these synergistic and complex interactions, ecosystem models have increasingly been adopted to predict food web responses to the introduction of nonindigenous species (Pine et al. 2007; Langseth et al. 2012; Pinnegar et al. 2014). In the Laurentian Great Lakes there are at least 184 established nonindigenous species (Ricciardi 2006), with many other species having a high potential to become established (Snyder et al. 2014). Two species of concern to the Great Lakes watershed include the Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis (Kelly et al. 2011). Silver and Bighead carp, collectively known as bigheaded carps (henceforth referred to as “Asian carps” [AC]), are filter feeders that consume phytoplankton, small zooplankton, detritus, and bacteria (Chen 1982; Sampson et al. 2009) and that have high rates of consumption, growth, and fecundity (DeGrandchamp et al. 2007). Asian carps have created unwanted impacts in systems where they have invaded (Cudmore et al. 2012). For example, in the Illinois River AC populations have increased exponentially (Chick and Pegg 2001) and have outcompeted native planktivores, leading to declines in native fishes (Schrank et al. 2003; Williamson and Garvey 2005; Sampson et al. 2009). Asian carps are currently established in watersheds adjacent to the Great Lakes. Furthermore, AC have been detected in the Great Lakes; three Bighead Carp have been collected in western Lake Erie since the 1990s (Morrison et al. 2004), and AC environmental DNA (eDNA) has been detected in tributaries to Lake Michigan as well as in Lake Erie (Jerde et al. 2011, 2013). Thus, the potential is very real for AC populations to become established in the Great Lakes and to impact food webs there. Of all the Great Lakes, Lake Erie may be the most susceptible to AC. Lake Erie is at high risk owing to its connectivity to watersheds where AC occur (Kocovsky et al. 2012; Murphy and Jackson 2013), its high productivity (Cooke and Hill 2010; Cudmore et al. 2012), and the availability of suitable spawning habitats (Kocovsky et al. 2012) and adequate food (Anderson et al. 2015). Asian carps may directly disrupt the Lake Erie food web by competing with native planktivores and have a negative effect on fish recruitment by reducing prey for larval fishes. However, these negative effects may be countered by the potential of young AC to become an alternative prey for native predators (e.g., Walleye Sander vitreus, IMPACTS OF BIGHEADED CARPS ON LAKE ERIE FOODWEB 137 D ow nl oa de d by [ N oa a G le rl L ib ra ry ] at 0 8: 57 0 7 Ja nu ar y 20 16 Yellow Perch Perca flavescens, and Smallmouth Bass Micropterus dolomieu). Food web models can capture the complexities of large aquatic ecosystems and track the direct and indirect response of the food web to perturbations to the system (e.g., a new invader). However, as model complexity increases, the ability to estimate model parameters and their uncertainty is reduced. In this study, we combined a food web model, Ecopath with Ecosim (EwE) (Christensen and Walters 2004), with a novel approach to estimating parameter values with uncertainty called structured expert judgment (SEJ; Wittmann et al. 2015) to forecast with uncertainty the potential changes in the Lake Erie food web following the establishment of AC. We hypothesized that if AC were to become established in Lake Erie they would deplete the plankton in the water column and reduce the biomass of planktivores through competition for prey. We also hypothesized that the impacts of AC would differ depending on the productivity (nutrient loading) of the Lake Erie ecosystem and the availability of AC to piscivores. Specifically, we expected that higher nutrient loads would support higher biomass of AC and greater productivity of lower trophic levels, thus reducing the negative effects on native fishes. High availability of young AC to native piscivores would lower AC population growth and equilibrium biomass. In contrast, we hypothesized that if AC were to consume the larvae of other fishes, their impacts on fish recruitment would be greater.