Freshwater Biology (2010) 55, 2138–2152 doi:10.1111/j.1365-2427.2010.02474.x Can filter-feeding Asian carp invade the Laurentian Great Lakes? A bioenergetic modelling exercise SANDRA L. COOKE AND WALTER R. HILL Institute of Natural Resource Sustainability, Illinois Natural History Survey, University of Illinois, Champaign, Illinois, U.S.A. SU M M A R Y 1. There is much concern that filter-feeding Asian carp will invade the Laurentian Great Lakes and deplete crucial plankton resources. We developed bioenergetic models, using parameters from Asian carp and other fish species, to explore the possibility that planktonic food resources are insufficient to support the growth of silver carp (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) in the Great Lakes. 2. The models estimated basic metabolic requirements of silver and bighead carp under various body sizes, swimming speeds and reproductive stages. These requirements were then related to planktonic food resources and environmental temperature to predict when and where silver and bighead carp may survive in the Great Lakes, and how far they may travel. 3. Parameter values for respiration functions were derived experimentally in a coordinated study of silver and bighead carp, while consumption parameters were obtained from the literature on silver carp. Other model parameters lacking for Asian carp, such as those for egestion and excretion, were obtained from the literature on other fish species. 4. We found that full-sized bighead carp required 61.0 kJ d)1 just to maintain their body mass at 20 C, approximately equivalent to feeding in a region with 255 lg L)1 macrozooplankton (dry) or 10.43 lg L)1 chlorophyll a. Silver carp energy requirements were slightly higher. 5. When applied to various habitats in the Great Lakes, our results suggest that silver and bighead carp will be unable to colonise most open-water regions because of limited plankton availability. However, in some circumstances, carp metabolism at lower temperatures may be low enough to permit positive growth even at very low rations. Positive growth is even more likely in productive embayments and wetlands, and the modelled swimming costs in some of these habitats suggest that carp could travel >1 km d)1 without losing biomass. 6. The simulation (and firmly hypothetical) results from this modelling study suggest when and where Asian carp could become established in the Great Lakes. Given the potential consequences to Great Lakes ecosystems if these filter feeders do prove capable of establishing reproducing populations, efforts to keep Asian carp out of the Great Lakes must not be lessened. However, we do encourage the use of bioenergetic modelling in a holistic approach to assessing the risk of Asian carp invasion in the Great Lakes region. Keywords: bighead carp, invasive species, planktivore, risk assessment, silver carp Correspondence: Sandra L. Cooke, Institute of Natural Resource Sustainability, Illinois Natural History Survey, University of Illinois, 1816 South Oak Street, Champaign, Illinois 61820, U.S.A. E-mail: [email protected] Present Address: Biology Department, Duke University, Box 90025, Durham, North Carolina 27708-0025, U.S.A. 2138 2010 Blackwell Publishing Ltd Bioenergetics of invasive Asian carp 2139 Introduction As humans continue to transport species and erode natural barriers to biological invasions, ecosystems throughout the world have born the consequences. The Laurentian Great Lakes have experienced cascading ecological effects from over 180 invasive species (Holeck et al., 2004), and most scientists and resource managers agree that it is critically important to prevent further species introductions to and from this region (Vander Zanden & Olden, 2008). Two aquatic invaders of particular concern in the Great Lakes basin (and in over 30 other countries around the world) are bighead carp (Hypophthalmichthys nobilis Richardson) and silver carp (H. molitrix Valenciennes), collectively known as Asian carp. These carp have invaded the Mississippi River Basin and are now found in waterways connected to the Laurentian Great Lakes (Chick & Pegg, 2001). Bighead and silver carp are fast-growing, high-volume filter feeders with a generalist diet of both phytoplankton and zooplankton (Kolar et al., 2007). Studies on the mobility of silver and bighead carp show they can travel up to 64 km d)1, depending on river flow and other variables (DeGrandchamp, Garvey & Colombo, 2008), and thus they have the potential to enter and invade new regions rapidly. Recent studies suggest that Asian carp may adversely affect native planktivorous fishes including gizzard shad (Dorosoma cepedianum LeSueur) and bigmouth buffalo (Ictiobus cyprinellus Valenciennes) in the Illinois River (Irons et al., 2007). There is concern that Asian carp in the Illinois River will enter Lake Michigan via the Chicago Sanitary and Ship Canal (CSSC) and similarly affect native planktivores, both invertebrate and vertebrate, through competition for limited food resources. Other possible modes of introduction into the Great Lakes include bait-bucket transfer, movement from other U.S. and Canadian catchments and release from live fish markets (Herborg et al., 2007; Keller & Lodge, 2007). Although earlier models of invasion risk predicted the invasion potential of silver carp in the Great Lakes to be low (Kolar & Lodge, 2002), more recent ecological niche models predict that most of the Great Lakes catchments (Chen, Wiley & Mcnyset, 2007), and the Great Lakes themselves (Herborg et al., 2007), offer suitable environments for bighead and silver carp to establish reproducing populations. These models use 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 variables such as precipitation, river discharge, slope and per cent tree cover to predict the potential range of invasive species. While such climatic and hydrological variables are important; biotic variables, including food availability and energetic constraints, can be used to predict a more realistic range of suitable environments. Bioenergetics models are useful tools that allow fisheries ecologists and resource managers to predict growth and food requirements of both native and exotic species in aquatic ecosystems. Developing bioenergetic models of bighead and silver carp would enable a comparison of their energy requirements to the energy available in the plankton of potential invasion sites, such as the Great Lakes, which are oligotrophic in most locations. An understanding of bighead and silver carp bioenergetics should also be valuable for managing their populations in ecosystems where they are already established. Food availability has been suggested as an influence on microhabitat selection of Asian carp in the Illinois River (DeGrandchamp et al., 2008). Despite the abundance of Asian carp in both their native Asian and non-native North American habitats, the bioenergetic requirements of these fishes have only been sparsely reported in the mainstream literature (but see Mukhamedova, 1977). Our purpose was to develop bioenergetics models for bighead and silver carp using empirically determined respiration rates and bioenergetic parameters derived from literature on Asian carp and other fish species. We used these models to assess the theoretical potential of bighead and silver carp to colonise habitats in the Laurentian Great Lakes, based on plankton biomass and surface water temperature data. Our theoretical analysis also takes into account the possible effects of thermal stratification and swimming costs on carp growth and movement estimates. We hypothesised that most open-water habitats of the Great Lakes will be much less likely than littoral habitats to support the establishment of bighead and silver carp, because of between-habitat differences in plankton biomass and temperature. Methods Model development We structured bighead carp and silver carp models following the widely used Wisconsin bioenergetics 2140 S. L. Cooke and W. R. Hill V ¼ ðCTM TÞ=ðCTM CTOÞ model (Hanson et al., 1997). The basic energy balance equation described by the model is: C ¼ ðR þ A þ SDAÞ þ ðF þ UÞ þ ðDB þ GÞ ð1Þ where total consumption of energy (C) is equal to the sum of metabolism (respiration, R; active metabolism, A; and specific dynamic action, SDA), wastes (faecal egestion, F; and urinary excretion, U) and growth (somatic growth, DB; and gonad production, G). Each component of this overall equation has different forms of the temperature dependence and mass dependence functions that can be used, depending on the physiology of each species. We used the form of the consumption function designed for warm-water species (eqn 2 in Fish Bioenergetics 3.0 software; Kitchell, Stewart & Weininger, 1977; Hanson et al., 1997): C ¼ CA W CB p V x eðXð1VÞÞ ð2Þ where C is the specific consumption rate (g g)1 d)1); CA and CB are the intercept and slope of the allometric mass function, respectively; W is fish mass (g); and p is the proportion of maximum consumption. The temperature dependence parameters are defined as follows: Parameter 2 0:5 2 ð2aÞ X ¼ ðZ ð1 þ ð1 þ 40=YÞ Þ Þ=400 ð2bÞ Z ¼ LnðCQÞ ðCTM CTOÞ ð2cÞ Y ¼ LnðCQÞ ðCTM CTO þ 2Þ ð2dÞ where T is temperature (C), CQ is the temperaturedependent coefficient, CTM is the maximum temperature above which consumption ceases, and CTO is the optimum temperature. Bighead and silver carp filtration and consumption rates are highly variable, being dependent not just on body size and temperature, but also on particle size (Kolar et al., 2007). Smith (1989) observed that juvenile silver carp had maximum filtration rates of particles >70 lm compared to other particle sizes. Smith (1989) developed an allometric relationship that explained 99% of the variance for silver carp feeding at 20 C on particles >70 lm, and we use these parameters for our bighead and silver carp consumption equation (Table 1). For the temperature dependence parameters of the consumption equation, we calculated mean CTO and CTM from values reported for Asian carp in the literature (reviewed by Kolar et al., 2007). We used CQ from tilapia (Sarotherodon spp. and Oreochromis spp.; Hanson Description Value Consumption CA Intercept for maximum consumption CB Mass dependence coefficient CQ Temperature dependence coefficient CTO Optimum temperature (C) CTM Maximum lethal temperature (C) Egestion and excretion FA Intercept of the proportion of consumed energy egested FB Temperature dependence coefficient for egestion FG Ration dependence coefficient for egestion UA Intercept of the proportion of consumed energy excreted UB Temperature dependence coefficient for excretion UG Ration dependence coefficient for excretion Metabolism RA Intercept of mass dependence function RB Slope of mass dependence function RQ Approximates Q10 over low temperatures ACT Activity multiplier for a constant swimming speed SDA Proportion of assimilated energy lost to specific dynamic action 1.54* )0.287* 2.5† 29, 26‡ 43, 38‡ Table 1 Bioenergetics parameters for silver (first value) and bighead carp (second value). In some cases, the same parameter was used for both species 0.212§ )0.222§ 0.631§ 0.031§ 0.58§ )0.299§ 0.0028, 0.0053 )0.239, )0.299 0.076, 0.048 1.0 0.1† * Smith, 1989; Nitithamyong (in Hanson et al., 1997) ‡ Kolar et al., 2007; § Elliott, 1976 (in Hanson et al., 1997) † 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 Bioenergetics of invasive Asian carp 2141 et al., 1997) because tilapia are planktivorous filter feeders with thermal tolerances and feeding characteristics very similar to Asian carp (Table 1). We also assumed that Asian carp energy density was the same as adult tilapia (5442 J g)1 wet mass; Hanson et al., 1997). In a review of fish bioenergetics studies, Chipps & Wahl (2008) discuss the importance of not overestimating waste losses when modelling consumption at low rations. Thus, for modelling egestion (F) and excretion (U), we used equation set 2 in Fish Bioenergetics, which addresses this overestimation problem by accounting for fish mass, temperature and ration (Hanson et al., 1997): F ¼ FA TFB eFGp C ð3Þ U ¼ UA T UB eUGp ðC FÞ ð4Þ where FA and UA are intercepts of the proportion of consumed energy egested and excreted, respectively, versus temperature and ration; FB and UB are the coefficients of temperature dependence; FG and UG are the coefficients for ration dependence; p is ration (proportion of maximum consumption); and T is temperature. For these functions, we used parameters modified for brown trout Salmo trutta Linnaeus (Hanson et al., 1997; Table 1). This set of parameters has been used for other species, including yellow perch Perca flavescens Mitchill (Kitchell et al., 1977) and lake trout Salvelinus namaycush Walbaum (Stewart et al., 1983). We used the form of the respiration function that allows an input of swimming speed (eqn 1 in Fish Bioenergetics, Hanson et al., 1997): R ¼ RA W RB eRQT eRTOVEL ð5Þ where R is the specific rate of respiration (g g)1 d)1); RA and RB are the intercept and slope of the allometric mass function, respectively; RQ approximates the Q10; RTO is swimming speed in cm s)1; and VEL is a function that allows swimming speed to vary with body mass and temperature. When using Fish Bioenergetics 3.0 software, if swimming speed is a constant (i.e. not dependent on mass or temperature), then the activity multiplier is set to 1 and RTO is set to the desired velocity. To develop our model, we assumed that average swimming speed was constant because the swimming respirometer allowed mainte 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 nance of a more or less constant swimming velocity for each fish (see below). We also assumed that specific dynamic action (SDA) for both species was equal to 0.10 (Table 1), the value developed for tilapia (Hanson et al., 1997). Respiration rates of adult and juvenile bighead and silver carp were taken from Hogue & Pegg (2009), who measured oxygen consumption with both a static respirometer and a Brett-style swimming respirometer following the methods of Cech (1990). Respiration in the Hogue & Pegg (2009) study was measured over a temperature range of 4.5 C to 26.9 C. Because those authors had difficulty in getting bighead carp to swim in the swimming respirometer, only resting data were used for bighead carp, whereas the silver carp trials were successfully conducted at 0, 20 and 30 cm s)1. To obtain values for RA, RB and RQ for silver carp, we used a Ln-transformed version of eqn. 5, so that the respiration data could be fitted to the equation using multiple linear regression (Stewart et al., 1983). ln R ¼ RA þ RB lnðWÞ þ RQT þ vU ð5aÞ where m is an empirical constant and U is swimming speed in cm s)1. The parameters we obtained were: lnR ¼ 5:880:239lnðWÞþ0:076T þ0:0074U ð5bÞ The model fitted the data reasonably well (adjusted R2 = 0.77), and all parameters were significant (P < 0.0001), with the exception of m (P = 0.12) whose lack of significance may have resulted from relatively few observations at different swimming speeds. Thus, the form of the respiration equation we used for silver carp in Fish Bioenergetics 3.0 was: R ¼ 0:00279 W 0:239 e0:076T ð5cÞ For bighead carp, multiple linear regression of the respiration data was used to fit values for RA, RB and RQ similar to those for silver carp except that a swimming component was not included. The resulting equation was: R ¼ 0:00528 W 0:299 e0:048T ð5dÞ This model fitted the data reasonably well (adjusted R2 = 0.74), and all parameters were significant (P < 0.0002). 2142 S. L. Cooke and W. R. Hill Sensitivity analysis We conducted a sensitivity analysis on the parameters listed in Table 1 following the procedure in Kitchell et al. (1977) and Stewart et al. (1983). Each parameter was separately varied by +10% and )10% from its nominal value, and the resulting output (specific consumption rate required for metabolic maintenance) was compared to the nominal specific consumption rate (from the standard simulation). This sensitivity analysis was carried out for a 2400 g resting adult of each species feeding on zooplankton prey with an energy density of 2512 J g)1 wet mass (Cummins & Wuycheck, 1971) at 20 C. Sensitivities were calculated as follows: sðpÞ ¼ 10DC C ð6Þ where s(p) is sensitivity of parameter p (a value of 1 means that a 10% change in the parameter causes a 10% change in consumption), C is the simulated required daily consumption rate for metabolic maintenance, and DC is the change in C because of the change in p. Parameters that were most sensitive include RA and RB, but no parameter had a sensitivity value larger than 2.58 (Table 2), and all sensitivity parameters were within the range of those observed in other bioenergetics studies (e.g. Kitchell et al., 1977; Stewart et al., 1983). Model application Fish Bioenergetics 3.0 software, which was developed for the Wisconsin model (Hanson et al., 1997), was used to predict daily consumption requirements of bighead and silver carp for basic metabolic maintenance (no growth). We generated simulations for three different body sizes: 10 cm long (10 g) juveniles; 20 cm long (70 g) sub-adults and 60 cm long (2400 g) adults. Consumption requirements were generated for different swimming speeds (0– 4 cm s)1) at 20 C (we kept temperature constant here because our purpose was to assess the effects of body size and swimming speed on consumption). These swimming speeds may seem low, but when calculated as daily distance travelled (e.g. 4 cm s)1 is 3.5 km d)1) they fall within the range of observed mean movement rates for both species (0.21– 10.61 km d)1; DeGrandchamp et al., 2008). Consump- Table 2 Sensitivities of the specific consumption rate required for routine metabolic maintenance (no growth) to deviations of each input parameter. Sensitivities were calculated for 2400 g resting silver carp and bighead carp at 20 C Input error – silver Parameter Consumption CA CB CQ CTO CTM Egestion FA FB FG Excretion UA UB UG Metabolism RA RB RQ SDA Input error – bighead +10% )10% +10% )10% +0.00 )0.00 +0.01 +0.02 )0.00 +0.00 +0.01 )0.00 )0.00 +0.01 )0.01 )0.01 )0.00 )0.01 )0.01 +0.00 )0.00 )0.00 )0.01 +0.00 +0.14 +0.10 +0.02 )0.13 )0.08 )0.02 +0.12 +0.09 +0.00 )0.12 )0.09 +0.00 +0.21 +0.46 +0.02 )0.25 )0.37 )0.02 +0.21 +0.43 )0.03 )0.26 )0.40 )0.03 +1.84 +2.05 +1.65 +0.14 )0.32 )1.70 )1.42 )0.14 +0.96 +2.58 +0.96 +0.11 )1.04 )2.10 )0.94 )0.17 tion requirements were also estimated for a 2400 g female of each species spawning 5% of its body mass, or a gonadosomatic index (IG) of 5. Papoulias, Chapman & Tillitt (2006) report IG values of 0–9.6 for bighead and silver carp in the Missouri River. Simulation outputs were obtained as specific consumption rates (J g)1 d)1) required for metabolic maintenance (no growth) for resting and swimming fish. For each size fish, the consumption rate in kJ d)1 was converted to kJ L)1 by taking into account the filtration rate of the fish. Using the allometric relationship from Smith (1989), we assumed that 10, 70 and 2400 g Asian carp filter 191, 764 and 9502 L d)1, respectively. To express these energetic requirements in terms of environmental prey densities, we converted the kJ L)1 values to chlorophyll a (Chl a) concentrations and zooplankton dry biomass (both in lg L)1). For zooplankton, we assumed that all prey had a mean energy density of 2512 J g)1 wet mass (Cummins & Wuycheck, 1971) and that the ratio of zooplankton wet mass to dry mass was 10:1 (Dumont, Van de Velde & Dumont, 1975). To obtain Chl a concentrations, we assumed that all phytoplankton had a mean energy density of 2460 J g)1 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 Bioenergetics of invasive Asian carp 2143 wet mass (Hambright, Blumenshine & Shapiro, 2002), the ratio of wet mass to dry mass was 2.5 (Reynolds, 1984), and that the ratio of Chl a to dry mass was 100 (Reynolds, 1984). To model the growth of carp feeding in different types of habitats within the five Laurentian Great Lakes, we compiled data on Chl a, phytoplankton biomass, zooplankton densities, zooplankton biomass and water temperature from multiple regions. In cases where surface water temperatures were not provided with the plankton data, we approximated the ‘typical’ temperature for a site and season based on the temperature data available from NOAA’s Great Lakes Environmental Research Laboratory (http:// www.glerl.noaa.gov/data/) and the EPA’s Great Lakes Environmental Database (http://www.epa. gov/greatlakes/monitoring/data_proj/glenda/index. html). Our goal was to include a selection of recently sampled sites representative of offshore pelagic habitats as well as coastal embayments and wetlands with higher plankton biomass. We also included plankton data collected at different times of year across a range of temperatures. This selection of locations and seasons is far from comprehensive, but nevertheless represents a broad range of habitats within the Great Lakes. The data selected include a recent study of a late winter production pulse in southern Lake Michigan (Kerfoot et al., 2008); a survey of Great Lakes wetlands (Lougheed & Chow-Fraser, 2002; open water habitats only); and a comparison of zooplankton biomass and Chl a in embayment, nearshore and offshore regions in Lake Ontario (Hall et al., 2003). We also applied the model to several riverine sites to demonstrate that the model predicts positive growth in habitats where Asian carp have already invaded. We used the bioenergetics models to estimate expected growth (biomass loss or gain) of juvenile (10 g) and adult (2400 g) non-swimming bighead and silver carp in each region over 30 days (a reasonable amount of time to allow a potential new invader to establish a ‘foothold’ in the new habitat). For habitats in which non-swimming Asian carp were predicted to have positive growth, we determined the maximum distance that the carp could travel without losing biomass by setting growth to zero (metabolic maintenance), solving the bioenergetics model for mean velocity (RTO in Fish Bioenergetics 3.0), and then calculating distance travelled over 30 days based on this mean velocity. These distance calculations are 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 merely estimates that assume that water flow is not a factor. In generating estimates of growth and swimming costs and we used the filtration rates and prey energy density stated previously. Most of the zooplankton data available were densities rather than biomass. To convert zooplankton densities to dry mass, we assumed that each taxon exhibited average body lengths and applied length-weight regressions from Culver et al. (1985), which were developed for Great Lakes zooplankton. For rotifers, we used the mean biomasses presented in Dumont et al. (1975). Not all zooplankton data sets included rotifers, and we noted these exceptions. Because most of the phytoplankton data were in Chl a concentrations rather than phytoplankton biomass, we used the conversion factors mentioned previously. To assess the effects of temperature on growth rates, we selected four open-water habitats that would normally be thermally stratified during the summer, for which summertime plankton data are available, and for which modelled growth at surface temperature was negative. These sites were Lake Michigan’s southern basin (September), Collingwood Harbour of Lake Huron, the central basin of Lake Erie, and the nearshore zone of Sandy Pond (Lake Ontario). We simply re-ran the model at different temperatures from 4 to 24 C for 30 days of feeding. We ran separate simulations at a constant temperature for each temperature to compare more effectively the growth of a carp feeding at the chlorophyll maximum and at other depths (with cool water) with carp growth in the warmer epilimnion. Measured respiration rates of any fish species may overestimate resting metabolic requirements because of the difficulty in measuring the respiration of completely inactive fish in a chamber (Cech, 1990; Hogue & Pegg, 2009). To counter the potential issue of overestimating energy requirements and underestimating growth, we made several conservative assumptions. In addition to the assumption of all particle sizes being consumed at the same maximum rate, we assumed that all prey were 100% digestible; all prey were easily captured by passive filter feeding (no escape by swimming zooplankton); and all prey had a constant, relatively high energy density. We also assumed that carp filtration rates did not vary with swimming speed (when in reality a swimming fish would filter a higher volume than a resting fish). 2144 S. L. Cooke and W. R. Hill 10 000 However, because we may have overestimated resting filtration rates and because the simulated swimming speeds were so low, the differences in filtration rates because of swimming speed are probably negligible. The specific consumption rate required to maintain a constant fish mass increased with swimming speed and decreased with fish size (Fig. 1). At 20 C, silver carp specific consumption rates were slightly higher than those of bighead carp: resting silver carp weighing 10 g, 70 g and 2400 g required 1.4, 6.1 and 91 kJ d)1, respectively, while bighead carp of those masses required 1.3, 5.1 and 61 kJ d)1. A 2400 g reproducing silver carp (IG = 5) requires 415 kJ d)1, while a similar bighead required 380 kJ d)1. Consumption requirements of reproducing carp were higher at all swimming speeds, although the differences between reproducing and non-reproducing adults were smaller at higher speeds because of the higher energetic costs of swimming compared to reproducing (Fig. 1). When the daily requirements and Asian carp filtration rates are translated into Chl a currency, resting silver carp weighing 10, 70 and 2400 g require 11.91, 13.08 and 15.50 lg L)1 Chl a to maintain their mass (Fig. 2). When compared as environmental densities of zooplankton, resting silver carp of those masses require 292, 320 and 379 lg L)1 of zooplankton dry mass, and bighead carp require 273, 266 and 255 lg L)1dry mass (Fig. 2). Environmental requirements were similar for non-reproducing Asian carp of different sizes because of the greater filtration rates of larger carp. The model assumed that activity was an exponential function of swimming speed, and thus predicted specific consumption rates of swimming carp were quite high compared to resting carp (e.g. Fig. 1). The projected growth of non-swimming silver and bighead carp feeding on phytoplankton and zooplankton in different regions of the Great Lakes was negative in almost all open-water regions of Lakes Michigan, Superior, Huron and Ontario (Table 3). However, positive growth was predicted in Green Bay, western Lake Erie and all of Lake Erie during the spring, the embayment regions of Sodus Bay and Sandy Pond (Lake Ontario), and some wetlands. Modelled growth was positive in the riverine habitats 1000 Specific consumption rate for metabolic maintenance (J g-1 day-1) Results (a) 100 10 1 0 10 000 1 2 3 4 5 1 2 3 4 5 (b) 1000 100 10 1 0 Swimming speed (cm s-1) Fig. 1 Specific energy consumption rates required for basic metabolic maintenance (no growth) of (a) silver carp and (b) bighead carp. Values were generated for a 10 g juvenile (triangle), 70 g sub-adult (square), 2400 g non-reproducing adult (open circle) and 2400 g female spawning 5% of its body mass (closed circle) at swimming speeds of 0, 1, 2, 3 and 4 cm s)1. where Asian carp have established reproducing populations (Table 3). The maximum distance that carp could travel in different Great Lakes habitats without losing biomass over 30 days ranged from 0.8 to 33.7 km for 10 g bighead and silver carp and 2.4–35.0 km for 2400 g carp (Table 4). The range of maximum movement predicted in riverine habitats ranged from 9.3 to 38.9 km for 10 g carp and 2.6–39.9 km for 2400 g carp (Table 4). 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 Bioenergetics of invasive Asian carp 2145 0.10 150 100 0.06 0.04 50 0.02 0.00 10 g 70 g 2400 g (NR) 2400 g (R) 200 (b) 0.10 150 0.08 0.06 100 0.04 50 0.02 0.00 10 g 70 g 2400 g (NR) 2400 g (R) Body size and reproductive stage Growth simulations at different temperatures suggest that carp would generally have higher growth rates at lower temperatures (Fig. 3). Although carp were projected to lose biomass at surface temperatures, modelled growth was positive at low temperatures (<8 C) in all four habitats. Discussion Our modelling results indicate that the low concentrations of plankton in many open-water regions of the Laurentian Great Lakes cannot support growth of silver and bighead carp. The threat of these filterfeeding fish establishing open-water populations and disrupting the pelagic food web in the oligotrophic regions of the lakes therefore appears to be small. However, our results also indicate that more productive regions, such as Green Bay, the western basin of 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 0 4000 3000 2000 1000 0 5000 4000 3000 2000 1000 0 Required zooplankton biomass (mg L-1) 0.12 0 Required chlorophyll a concentration (mg L-1) Fig. 2 Estimated environmental energy density, Chl a concentration and zooplankton dry mass required for (a) silver carp and (b) bighead carp to meet basic metabolic demands (no growth). Estimates were generated for a 10 g juvenile, 70 g sub-adult, 2400 g non-reproducing (NR) adult and 2400 g female spawning 5% of its body mass (R) at swimming speeds of 0 (black bars), 1 (grey bars) and 2 cm s)1 (white bars). Required energy density to maintain body mass (kJ L-1) 0.08 5000 Required zooplankton biomass (mg L-1) (a) Required chlorophyll a concentration (mg L-1) 200 0.12 Lake Erie and some other embayments and wetlands, may contain enough plankton to meet energetic requirements at certain times of year and at certain temperatures. Furthermore, although modelled energetic costs of swimming were high, the simulations predict that carp in some of these habitats could still travel up to 40 km over a 30-day period while maintaining body mass. Many embayments, wetlands and other coastal zones are important habitats and nurseries for larval fishes such as alewives (Alosa pseudoharengus Wilson; Klumb et al., 2003) and walleye (Sander vitreus Mitchill; Roseman et al., 2005). Thus, although Asian carp would probably not become established in most nearshore or offshore pelagic habitats of Lakes Ontario, Michigan, Superior and Huron, they might indirectly affect those ecosystems if they became established in adjoining embayments and wetlands. The effect of Asian carp on 2146 S. L. Cooke and W. R. Hill Table 3 Projected growth, based on bioenergetics models, of juvenile (10 cm, 10 g) and adult (60 cm, 2400 g) non-swimming bighead carp (BC) and silver carp (SC) foraging on zooplankton (zoop) and phytoplankton (phyto) for 30 days at different times of year and in different regions of Lakes Michigan, Superior, Huron, Erie and Ontario, and riverine habitats currently invaded by Asian carp (Spr = spring; Sum = summer) Location Lake Michigan southern basin, nearshore Time of year, water temperature (C) Phyto. wet mass (mg L)1) Zoop. wet mass (mg L)1) Predicted % biomass gain or loss over 30 days 10 g BC 10 g SC 2400 g BC 2400 g SC References May, 8.8 Jul, 19.4 Sep, 20.2 Apr, 3.4 Apr, 3.6 Apr, 13.4 Jun, 20.3 0.69 0.69 0.69 0.52 0.19 5.5 5.5 0.024 0.15 0.79 0.18 0.045 0.16 4.72 )15 )29 )20 )9 )17 +63 +120 )9 )30 )23 )2 )11 +66 +113 )3 )6 )4 )2 )4 +15 +28 )3 )10 )9 )1 )3 +13 +23 C. E. Cáceres unpubl. data (zoop); Gardner et al., 2004 (Chl a) Kerfoot et al., 2008 May, 3.4 Aug, 9.4 Jul, 22.5 Jul, 22.5 Jul, 22.5 0.31 0.32 1.42 1.13 0.07 0.16 0.59 2.52 2.22 3.19 )13 )12 +13 +3 +2 )6 )6 +5 )4 )5 )3 )2 +4 +2 +1 )2 )3 )3 )5 )5 Brown & Branstrator, 2004 (NR) Lougheed & Chow-Fraser, 2002; pers. comm. Jul, 22.5 1.42 1.83 +2 )5 +1 )5 Jul, 22.5 Jul, 22.5 0.57 0.07 0.056 0.024 )38 )46 )43 )51 )9 )10 )15 )16 Lougheed & Chow-Fraser, 2002; pers. comm. Spr, 6.1 Sum, 21.0 Spr, 6.1 Sum, 21.0 Spr, 6.1 Sum, 21.0 Jun, 22.5 Jul, 22.5 3.31 3.12 0.63 1.21 1.82 1.06 0.14 0.36 0.69 1.74 1.14 0.95 0.29 0.62 0.46 0.40 +46 +31 +8 )12 +13 )20 )38 )36 +50 +25 +14 )16 +20 )23 )43 )41 +10 +8 +2 )2 +3 )2 )9 )8 +9 +2 +2 )7 +3 )9 )15 )14 Conroy et al., 2005 (NR) Lake Ontario Sodus Bay embayment Sodus Bay nearshore Sandy Pond embayment Sandy Pond nearshore Frenchman’s Bay (wet) Bronte Creek (wet) Jul, 20.0 Jul, 20.0 Jul, 20.0 Jul, 20.0 Jun, 22.5 Jun, 22.5 1.87 0.29 1.11 0.64 8.94 1.84 5.91 0.81 6.06 0.76 1.57 0.42 +82 )26 +72 )21 +116 )14 +77 )28 +67 )24 +104 )20 +20 )6 +17 )4 +28 )3 +14 )10 +12 )9 +21 )9 Hall et al., 2003; pers. comm. (NR) Middle Mississippi River Chester Grand Tower Upper Mississippi River Missouri River Aug, 22.0 Oct, 16.0 Sum, 27.0 Sum, 23.0 5.00 10.0 8.77 5.30 0.05 0.01 3.75 1.39 +30 +130 +127 +37 +23 +130 +102 +27 +8 +30 +31 +9 +2 +26 +19 +2 within production pulse outside production pulse Green Bay Lake Superior western arm Chippewa Park (wet) Pine Bay (wet) Hurkett Cove (wet) Lake Huron Collingwood Harbour (Georgian Bay) Oliphant Bay (wet) Baie du Dore (wet) Lake Erie West basin Central basin East basin Rondeau Prov. Park (wet) Long Point Prov. Park (wet) Fulford et al., 2006; pers. comm. (zoop, NR); Qualls et al., 2007 (Chl a) Lougheed & Chow-Fraser, 2002; pers. Comm. Lougheed & Chow-Fraser,2002; pers. comm. Williamson & Garvey, 2005 (NR) A. P. Levchuk unpubl. K. D. M. Dickerson unpubl. (zoop); Bukaveckas pers. comm. (Chl a) Open-water habitats near wetlands are indicated after the site name (wet), zooplankton samples excluding rotifers are noted in the reference column (NR), and negative growth values are highlighted in boldface. 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 Bioenergetics of invasive Asian carp 2147 Table 4. Maximum distance that can be travelled, based on bioenergetics models, by juvenile (10 cm, 10 g) and adult (60 cm, 2400 g) bighead carp (BC) and silver carp (SC) over 30 days in different habitats at different times of year (Spr = spring; Sum = summer). Open-water habitats near wetlands are indicated after the site name (wet) Location Lake Michigan Green Bay Lake Superior Chippewa Park (wet) Pine Bay (wet) Hurkett Cove (wet) Lake Huron Collingwood Harbour Lake Erie West basin 10 g BC 10 g SC 2400 g BC 2400 g SC 27.0 33.4 29.8 31.4 28.8 35.0 22.8 24.4 5.7 1.6 1.0 0.8 2.1 – – – 7.5 2.4 2.9 2.6 – – – – Spr, 6.1 Sum, 22.0 Spr, 6.1 Spr, 6.1 26.7 13.2 6.5 10.9 33.7 10.6 14.5 18.7 28.5 14.8 8.8 13.0 26.7 3.9 8.0 12.2 Jul 1997 Jul 1997 Jun 1998 26.7 24.9 30.8 25.1 23.1 27.2 28.5 26.4 32.7 18.1 16.3 20.5 Aug, 22.0 Oct, 16.0 Sum, 27.0 Sum, 23.0 12.4 38.1 29.3 14.0 9.3 38.9 22.8 10.1 14.3 39.9 31.1 15.8 2.6 32.1 15.6 3.4 Apr 1999 Jun 1999 Jul Jul Jul Jul percent growth Central basin East basin Lake Ontario Sodus Bay embayment Sandy Pond embayment Frenchman’s Bay (wet) Rivers Middle MS River (Chester) Middle MS River (Grand Tower) Upper MS River Missouri River Maximum distance of travel over 30 days (km) Time of year; water temp. (C) 1998 1998 1998 1998 15 10 5 0 –5 –10 –15 –20 –25 –30 –35 Lake Michigan, southern basin, Sep. Lake Huron, Collingwood Harbour, Jul. 40 30 20 10 0 –10 0 10 30 20 30 Lake Erie, central basin, summer 20 10 Fig. 3 Per cent growth of 10 g silver carp (open triangles), 10 g bighead carp (closed triangles), 2400 g silver carp (open circles) and 2400 g bighead carp (closed circles), modelled over 30 days at a constant temperature. The model was run for multiple temperatures in each site and month ⁄ season. The plankton data for these sites and seasons are in Table 3. 50 0 –10 –20 –30 –40 0 10 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 20 30 –20 0 10 15 10 5 0 –5 –10 –15 –20 –25 –30 –35 20 30 Lake Ontario, nearshore Sandy Pond, Jul. 0 Temperature (oC) 10 20 30 2148 S. L. Cooke and W. R. Hill native fishes may be similar to that of zebra mussels, whose invasion in Lake Michigan altered the foraging patterns of alewives and other species despite having only partial spatial and diet overlap (Pothoven & Madenjian, 2008). Bighead and silver carp mostly inhabit warm, shallow waterbodies such as rivers and backwater lakes; and to our knowledge, there is little research indicating at what depth in a thermally stratified pelagic zone Asian carp would prefer to feed. They are not visual feeders, and so temperature and plankton availability, rather than light, are likely to determine depth preference. The temperatures of maximum consumption for bighead and silver carp are 26 and 29 C, respectively, but at low rations maximum growth occurs at lower temperatures, according to the bioenergetic models. This is consistent with bioenergetic simulations by Kitchell et al. (1977), in which the maximum growth rate of 10 g perch occurs around 4 C when only feeding at 3% of their body weight per day, but the CTO is 23 C. Although carp growth was predicted to be negative in the four sites shown in Fig. 3, when modelled at surface water temperatures (20–22.5 C), growth could be positive if the carp were to feed in deeper and cooler water where zooplankton tend to reside during the day. It should be noted that plankton densities and other ecological conditions in the Great Lakes can change because of climate, land use, additional invasive species and other factors. Habitats invaded by filterfeeding zebra mussels might be less conducive to an Asian carp invasion, but it is unknown if the veliger larvae could serve as a substantial food source for carp. Also, selective herbivory by zebra mussels promotes cyanobacteria blooms in some habitats (e.g. Pillsbury et al., 2002), which could benefit carp. Changing temperature and increased nutrient inputs could increase plankton biomass or alter zooplankton and phytoplankton community structure, which could in turn increase the potential for carp growth in the Great Lakes. Our analysis provides some insight into the range of feeding conditions and temperatures that could facilitate Asian carp invasion should conditions change. We emphasise that this study is only a first attempt at a model for these ecologically important invasive species. The model’s conclusions are only as robust as the assumptions and parameters that are part of it. For this exercise, we were forced to borrow some of the bioenergetics parameters from studies on other species. While such sharing of parameters is widespread in the fish bioenergetics literature (e.g. Chipps & Wahl, 2004; Peterson & Paukert, 2005), we acknowledge that further laboratory evaluations of the parameters could improve the performance of the model and the accuracy of its predictions. For example, we assumed that carp had a constant energy density across body sizes, when in reality larger fish would have higher energy densities than smaller fish. Thus, the 5442 J g)1 wet mass value we used from adult tilapia may be too low for large, 2400 g adult Asian carp, and may be too high for sub-adults and juveniles. If this is the case, then adult carp energetic requirements would be even higher than we predicted, and juvenile carp requirements would be lower. Additionally, although we used experimentally derived consumption rates from a study of silver carp (Smith, 1989), we assumed that all particle sizes (phytoplankton and zooplankton) could be filtered at the same maximum rate, despite evidence that carp filter smaller particles less efficiently (Smith, 1989), and we assumed bighead and silver carp have similar consumption rates. The apparent absence of allometry for the energetic requirements of bighead carp (Fig. 2b) may indicate that the filtration rates taken from Smith (1989) were overestimates for sub-adult and adult bighead carp. Caged bigheads ranging in weight from 52 g to 139 g had a mean filtration rate of 4.44 L h)1 g)1 when feeding on rotifers (Opuszynski, Shireman & Cichra, 1991), which is equal to a filtration rate of 311 L d)1 for a 70 g fish. But we assumed a 70 g carp could filter 764 L d)1. Chipps & Wahl (2008) have observed that model-predicted consumption rates are often higher than those observed in the field. The consequence of making such assumptions and overestimating filtration rates is that our conclusions with respect to carp growth are conservative and more likely to predict higher growth rates than might actually occur. A strength of our modelling effort was that we used allometric- and temperature-dependent respiration parameters for both bighead and silver carp that were determined in a coordinated study (Hogue & Pegg, 2009). Although the juvenile and adult life stages of some fish have different allometric curves for metabolism (Post, 1990), we combined the adult and juvenile data into one model because Hogue & Pegg 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 Bioenergetics of invasive Asian carp 2149 (2009) found that life stage did not affect massindependent oxygen consumption rates. The sensitivities of the respiration parameters were higher than those of most other parameters, but they were comparable to other published sensitivity values for metabolism (Stewart et al., 1983). However, the confidence intervals for the respiration parameters were large because of the low sample size. Many more respiration trials were conducted, but we only used those values for which we were confident that the carp were swimming normally (or exhibiting minimal swimming in the resting trials). In modelling the expected growth of Asian carp in different habitats, we assumed these fishes were feeding on zooplankton and phytoplankton only, as most of the published literature on Asian carp feeding has focussed on these two primary food sources (reviewed by Kolar et al., 2007). In some cases where plankton availability is low, both carp species have been reported to consume large quantities of detritus (Opuszynski, 1981). It is not uncommon for particulate organic carbon (POC), rather than algae, to comprise the majority of seston carbon in rivers (Acharya et al., 2006). Thus, it is likely that Asian carp in rivers with low plankton availability, such as portions of the Missouri River, may depend more on POC than in other habitats. The nutritional quality of this food source is often low for zooplankton such as Bosmina (Acharya et al., 2006), so we suspect that POC is likely to be a poor food for Asian carp as well. Also, it is unknown if Asian carp would feed in lake benthic zones where most POC and detritus are found or consume benthic macroinvertebrates. Bighead carp may be able to consume these larger organisms, but it is suggested that a small foregut prevents silver carp from consuming large food items (Kolar et al., 2007). Other potential food sources not considered in these models are protozoans and other microplankton. Carrick (2005) observed that heterotrophic protozoan biomass is equivalent to at least 70% of crustacean biomass in Lake Michigan, and Munawar & Lynn (2002) observed that mean ciliate biomass ranges from <1 to >680 mg m)3 in Lakes Superior, Huron, Erie and Ontario. While even the higher end of this range may be insignificant compared to the biomass requirements of Asian carp, a refined bioenergetic analysis that includes these prey sources may be warranted in certain ecosystems. Another important point is that the plankton densities in Table 3 are regional whole 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2138–2152 water column averages that do not take into account possible zooplankton swarms and patchiness. In the Great Lakes, extreme wind, gyres and other physical phenomena may create patches of zooplankton sufficiently dense to support Asian carp, although such patches are probably temporary and may not be able to sustain lasting populations. Rotifer data were not available for all plankton datasets with which we applied the models (Table 3). A recent study in the Mississippi and Illinois rivers found that the diets of bighead and silver carp were dominated by rotifers rather than crustaceans (Sampson, Chick & Pegg, 2009). This is at least partially because of the fact that rotifers were more abundant than crustacean zooplankton in these habitats, suggesting that the carp modified their diet based on prey availability, as other studies have shown (Kolar et al., 2007). Rotifers generally make up a small percentage (c. 5–20%) of zooplankton biomass in the Great Lakes (Barbiero & Tuchman, 2002), especially compared to some rivers (Sampson et al., 2009), but the absence of rotifer data may slightly underestimate potential growth of carp in some Great Lakes habitats. Potential growth may also be underestimated in the Middle Mississippi River where rotifer data were unavailable, although modelled growth was positive in all riverine habitats that we examined – habitats where Asian carp currently thrive (Table 3). Despite the preceding caveats, some important implications emerge from the bioenergetics modelling. Recent spatially comprehensive studies show that low plankton biomass is prevalent in both nearshore and offshore regions of Lake Michigan (Vanderploeg et al., 2007). If Asian carp were to enter the ‘plankton desert’ of Lake Michigan via the CSSC, it seems unlikely (but not impossible) that they would be able to derive enough energy from the plankton to support the energetic costs of travelling to Green Bay or another ‘plankton oasis’. Our analysis suggests that a greater Asian carp invasion risk may be posed by the inadvertent use of Asian carp as bait or by Canadian live fish markets in close proximity to productive harbours and embayments of Lakes Ontario and Erie (Herborg et al., 2007; Keller & Lodge, 2007). However, we do not advocate removal of the existing electric fish barrier in the CSSC to prevent migration to and from Lake Michigan, as the modelling results suggest that positive growth is indeed 2150 S. L. Cooke and W. R. Hill possible in this region at cooler temperatures despite the low plankton rations (Fig. 3a). Bioenergetics models are only one component of what should be a more extensive approach to assessing the invasion risk of Asian carp. We advocate testing model predictions empirically, under as realistic conditions as possible. A recent laboratory experiment showed negative growth of bighead carp feeding at mesotrophic plankton densities and positive growth at eutrophic values (Cooke, Hill & Meyer, 2009); however, the plankton composition in this experiment (primarily Daphnia magna Straus and Microcystis spp.) did not resemble that of most Great Lakes habitats. Lakeside growth studies, in which Lake Michigan water is fed to Asian carp enclosed in mesocosms, would be preferable in testing our hypotheses that plankton resources in the Great Lakes are too sparse to support the growth of filter-feeding carp. Hitherto, government agencies have been reluctant to approve lakeside studies because of concern over the potential escape of Asian carp from the mesocosms. Appropriate design features could be implemented to eliminate the potential for escape, however, and given the current public concern about the invasion threat posed by the fish, additional information gained through empirical testing should be welcomed by both environmental managers and politicians. Further, while bioenergetic predictions may provide some information on the potential for Asian carp to grow and reproduce in new habitats, recent research highlights the importance of river discharge and temperature in influencing the spawning success and larval recruitment of bighead and silver carp (DeGrandchamp, Garvey & Csoboth, 2007; Lohmeyer & Garvey, 2009). Data from the Upper Mississippi River System suggest that the reproductive potential of both species is reduced in slow-flowing water (DeGrandchamp et al., 2007; Lohmeyer & Garvey, 2009). However, spawning in low-flow conditions has been observed in isolated cases (Kolar et al., 2007), and more research is needed on Asian carp spawning and recruitment. In conclusion, our results suggest that silver and bighead carp will be unable to colonise many openwater regions because of limited plankton availability. However, our results also suggest that in some habitats plankton resources are sufficient to support positive growth of bighead and silver carp, even when taking into account high swimming costs. Furthermore, carp metabolism in cool water is low enough to support positive growth at very low rations in some habitats. Because the probability of a successful invasion increases as more Asian carp individuals are introduced into the Great Lakes, current efforts to prevent introductions should at least be maintained if not expanded. More broadly, we recommend that aquatic resource managers in other locations threatened by bighead and silver carp incorporate a bioenergetics approach into more holistic invasion risk assessments. Acknowledgments We thank the following people who provided unpublished data or further information on their published datasets: Paul Bukaveckas; Carla Cáceres; John Chick and Alexander Levchuk; Kelli Dickerson and John Havel; Richard Fulford; Spencer Hall; and Vanessa Lougheed and Patricia Chow-Fraser. 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