Transactions of the American Fisheries Society 137:262–277, 2008 [Article] Ó Copyright by the American Fisheries Society 2008 DOI: 10.1577/T05-146.1

A Bioenergetics Approach to Modeling and Threadfin Shad Predator–Prey Dynamics in Lake Powell, Utah–Arizona

SHANE VATLAND,PHAEDRA BUDY,* AND GARY P. THIEDE U.S. Geological Survey, Utah Cooperative and Wildlife Research Unit, Department of Watershed Sciences, 5210 Old Main Hill, Utah State University, Logan, Utah 84322-5210, USA

Abstract.—An understanding of the mechanisms driving the cyclic relationship between piscivorous sport fish and their food base is needed in systems where predator and prey are tightly coupled. To understand the dynamics between striped bass Morone saxatilis and threadfin shad petenense in Lake Powell, Utah–Arizona, we (1) synthesized and evaluated 20 years of historical data on temperature, diet, growth, and abundance of these fish, (2) collected similar data on a finer scale in 2003–2004, (3) used components of this data set to develop specific conversions between coarser historic data and present data, and (4) modeled striped bass and threadfin shad dynamics within a bioenergetics framework. We estimated the consumption of threadfin shad by age-0, subadult, and adult striped bass in Lake Powell from 1985 to 2003. During this period, threadfin shad abundance peaked at approximately 5- to 7-year intervals and striped bass growth, condition, and abundance corresponded closely to peaks in threadfin shad foraging. Individual consumption of threadfin shad by striped bass was highest for adults followed by subadult and age-0 fish; scaling individual consumption up to the population level resulted in highly variable subadult consumption that fluctuated from highs of about 1.2 million kg (18.4 kg/ha) to lows of about 0.3 million kg (4.6 kg/ha) on a 1- to 2-year cycle. Despite these fluctuations, the consumption of threadfin shad was dominated by subadult striped bass, which appeared to control threadfin shad numbers in all but the highest peak years of shad abundance. Based on bioenergetics output, striped bass demonstrate a type II functional feeding response; consumption rates reach an asymptote when the threadfin shad biomass index exceeds 10,000 kg/year. We demonstrate a modeling approach that allowed us to evaluate the large fluctuations in predator and prey populations, which often become evident only over long time periods. Our results increase our understanding of the Lake Powell ecosystem and highlight areas for future study.

The abundance and growth of top piscivores in lentic Maintaining stable predator and prey populations systems is often tightly coupled to the availability of often challenges lake and reservoir fisheries managers key prey species (Ney 1990). Striped bass Morone (Ney 1990). Accordingly, understanding the mecha- saxatilis, a popular sport fish, are established in several nisms driving dominant predator–prey relationships in freshwater reservoirs throughout the United States these fluctuating pelagic fisheries is critical for making (Axon and Whitehurst 1985). As dominant top informed management decisions (e.g., Johnson and predators, striped bass can manipulate the population Goettle 1999). Although evaluating these often com- dynamics and structure of prey species (Filipek and plex and highly dynamic interactions can present a Tommey 1987; Olson 1996; Ludsin and DeVries 1997; difficult task (Beauchamp et al. 2007), bioenergetics- Schramm et al. 2001). Accordingly, striped bass growth based modeling provides a quantitative and predictive and abundance were closely related to the consumption tool to address some of these challenges (Brandt and of shad Dorosoma spp. and herring Alosa spp. Hartman 1993). Bioenergetics modeling has been populations in Santee–Cooper Reservoir, South Caro- applied to several fisheries management and research lina (Fuller 1984). Similarly, striped bass introduced issues and has been particularly useful in assessing a into Lake E. V. Spence, Texas, demonstrated decreased whole suite of predatory impacts (Hansen et al. 1993). growth rates and stomach fullness that corresponded Commonly, predator demand on prey fish populations directly to declines in prey-size-shad abundance is modeled using a bioenergetics approach, and these (Morris and Follis 1979). The dynamics of striped bass results are then used to better understand food web and their primary prey are often tightly coupled, dynamics and assess the efficacy of stocking or harvest especially in relatively simple pelagic systems. regulations (Baldwin et al. 2000; Beauchamp and Van Tassell 2001; Irwin et al. 2003). Because the * Corresponding author: [email protected] bioenergetics approach is based on physiological and Received June 3, 2005; accepted March 28, 2006 allometric relationships driven by food and tempera- Published online February 14, 2008 ture, it allows users to explicitly consider the

262 STRIPED BASS AND THREADFIN SHAD DYNAMICS 263 mechanisms that operate to structure populations and 1999; Vatland 2006), and midsummer surface temper- food webs and to thus link predator and prey dynamics atures are typically around 258C throughout the (Hanson et al. 1997). As such, the bioenergetics reservoir (Sollberger et al. 1989). Since the reservoir approach is an effective tool that has been used to reached full pool in 1980, surface elevation has assess the relationship between striped bass predators fluctuated between 1,123 m and 1,091 m, with the and their piscine prey (Cyterski et al. 2003; Hartman lowest elevations occurring in 1993 and 2003. Most of 2003; Uphoff 2003). the shoreline consists of steep sandstone cliffs and talus Lake Powell, which is located in southern Utah on slopes. the Utah–Arizona border, provides one of the most Lake Powell is an oligotrophic reservoir (Paulson popular sport fisheries in the United States. For this and Baker 1983; Potter and Drake 1989) with relatively reason, the Utah Division of Wildlife Resources low nutrient levels; main-channel chlorophyll a (UDWR) has monitored the top predator, striped bass, concentrations are generally less than 2 lg/L during and its primary prey, threadfin shad Dorosoma summer and fall, but concentrations are slightly higher petenense, in Lake Powell for over 25 years (Vatland in areas nearer the Colorado River inflow (Sollberger et 2006). The abundance of threadfin shad has fluctuated al. 1989) and side canyons (Wurtsbaugh and Gallo widely since the introduction of striped bass in 1974, 1997). The zooplankton community in Lake Powell is creating an apparent boom–bust cycle that complicates similar in structure to that in other lower Colorado fisheries management and frustrates anglers. In an River reservoirs (Sollberger et al. 1989). Cladocerans attempt to reduce predation pressure on shad and generally dominate lakewide zooplankton biomass in stabilize cycles, catch limits on striped bass have been the spring and summer, whereas copepods usually removed and a progressive public education program dominate biomass in the fall. introduced to promote angler harvest of striped bass. The pelagic fish community of Lake Powell is Despite these measures, fishery managers have iden- dominated by threadfin shad and striped bass. Striped tified a need to more explicitly understand the bass growth and survival is highly variable in Lake mechanisms driving this predator–prey cycle. Powell, but these fish are capable of attaining lengths The goal of our study was to link the predator and of nearly 1 m after 10 years of growth (Gustaveson prey dynamics of Lake Powell using a mechanistic, 1999; this study). Conversely, threadfin shad rarely bioenergetics-driven approach, such that we could survive to age 2, attaining a maximum size of 200 mm better understand the factors driving the cycle between in total length (Blommer and Gustaveson 2002; this predator and prey and ultimately predict their dynam- study). Other piscivorous fish species found in the ics. To achieve that goal, we (1) compiled and reservoir include Micropterus dolo- synthesized 20 years of data on fish growth, abun- mieu, M. salmoides, walleye Sander dance, and diet and on the abiotic factors that affect vitreus, and channel Ictalurus punctatus. fish (e.g., temperature), (2) collected similar but more Potential forage species other than threadfin shad detailed data in 2003 and 2004, (3) used this refined include bluegill Lepomis macrochirus, green sunfish L. data set to develop specific conversions between cyanellus, black crappie Pomoxis nigromaculatus, historic and present data, and (4) used a bioenergetics- white crappie P. annularis, red shiner Cyprinella based modeling approach to estimate individual and lutrensis, and common carp Cyprinus carpio. Gizzard population-level consumption and the annual abun- shad D. cepedianum recently invaded the reservoir and dance of predator and prey species from 1985 to the were first found in June 2000 near the San Juan River present. inflow. Linking past and present population monitoring.— Methods From 1980 to the present, the UDWR has monitored Study site.—Glen Canyon Dam impounds the the striped bass predator population and the threadfin Colorado River about 10 km south of the Utah– shad prey population on an annual basis. To make use Arizona border and forms Lake Powell (Figure 1), one of this long-term annual data set, we intensively of the largest reservoirs in the United States (653 km2). monitored the striped bass and threadfin shad popula- Lake Powell is a warm, meromictic reservoir, and tions in 2003 and 2004. We selected monitoring thermal stratification of the mixolimnion generally locations and methods to both maintain consistency begins in April and persists through November. A well- with past methodology and provide additional infor- defined thermocline forms around a depth of 8 m in the mation on within-year predator–prey dynamics. We early summer and will reach depths of 12–17 m in late collected data in May–June, July–August, and Octo- summer (Sollberger et al. 1989). Average surface water ber–November in six areas of the reservoir in 2003 temperatures range from 6.78C to 29.48C (Gustaveson (Wahweap Bay, Padre Bay, San Juan Arm, Bullfrog 264 VATLAND ET AL.

FIGURE 1.—Map of the area in which the predator–prey dynamics of striped bass and threadfin shad were studied. The sites that were intensively sampled in the spring, summer, and autumn of 2003 and 2004 were Wahweap Bay, Padre Bay (2003 only), San Juan Arm, Bullfrog Bay, Good Hope Bay, and Trachyte Canyon. Historical data were collected by the Utah Division of Wildlife at Wahweap Bay, San Juan Arm, Rincon, Bullfrog Bay, and Good Hope Bay.

Bay, Good Hope Bay, and Trachyte Canyon) and five described above (Wahweap Bay, San Juan Arm, areas (same as previous excluding Padre Bay) in 2004 Bullfrog Bay, Good Hope Bay; Figure 1). The trawl (Figure 1). All sampling, gill netting, trawling, and net was 3.05-m square at the mouth and 15.24 m in limnology occurred at these sites. We used components length, with bar mesh tapering from 20.32 cm at the of this data set to develop specific conversions between mouth to 3.17 mm at the cod end. Each oblique tow coarser historic data and present data analyzed or sampled the top 10 m of the water column (epilimnion) collected on a finer scale (e.g., diet data calculated as and filtered 7,150 m3 of water (Gustaveson et al. 1980). percent occurrence versus proportion by weight). Using Trawling occurred at night just before, during, or our 2003 and 2004 seasonal data, we also evaluated shortly after new moon events to minimize avoidance some key assumptions, which were necessary when (Rieman 1992) and patchy distributions of shad modeling seasonal consumption of threadfin shad by (Vatland 2006). striped bass in years before 2003, years for which intra- We used a combination of trawl catch abundance, annual data were less detailed. trawl filter volume, and reservoir volume to estimate Relative abundance and biomass of young-of-year the relative abundance of pelagic threadfin shad in the threadfin shad.—Since 1981, midwater trawling has top 20 m of the reservoir (;40% of the total volume). been used to monitor the relative abundance and First, we divided trawl catches by filter volume to distribution of pelagic threadfin shad in Lake Powell. estimate the density of shad sampled in the top 10 m of

In July and August of each year, UDWR conducted the reservoir (density0–10m ¼ number caught/trawl midwater tows at three or four of the same sample areas volume). We then averaged the July and August STRIPED BASS AND THREADFIN SHAD DYNAMICS 265 density estimates across sites within each year. To annual index of striped bass population abundance. We estimate abundance in the 0–10-m depth range, we modeled the top 20 m of the reservoir because striped multiplied average densities by the year-specific bass were rarely limited by the thermal structure and volume for this depth range (abundance0–10m ¼ dissolved oxygen levels in this depth range, and most density0–10m 3 volume0–10m). Based on UDWR hydro- fish were found within this depth range during gill acoustic surveys, summer threadfin shad densities are netting (i.e., late afternoon and evening; Mueller and approximately 10 times lower at depths below 10 m Horn 1999). and virtually absent below 20 m (G. Blommer, UDWR, We structured the population indices for striped bass personal communication). Accordingly, we estimated into three stages (age 0, subadult, and adult) using the the abundance of shad in the 10–20-m depth range by proportion of each stage caught during fall gill netting. dividing catch densities by 10 and multiplying this by We grouped age-0 fish by size, based on an analysis of the year-specific volume of the 10–20-m depth range scale-aging data and length-frequency modes collected

(abundance0–20m ¼ [density0–10m/10] 3 volume0–20m). from 1985 through 2003. Based on average size to We then summed estimates of abundance for the two maturity (Gustaveson 1999) and an ontogenetic shift in depth ranges, 0–10 and 10–20 m, to provide an annual diet composition (e.g., adults rarely consume zoo- index of threadfin shad abundance. To estimate an plankton; Budy et al. 2004, 2005; this study), we index of summer biomass, we multiplied abundance considered fish age 1 and older to be adults if they were indices by the average (across years) weight of a trawl- greater than 500 mm in total length (TL) and subadults caught threadfin shad. We averaged shad weight across if they were less than 500 mm TL. years to standardize sampling bias because trawl- Individual- and population-level striped bass con- caught shad were not measured in all years of the time sumption.—We used the Wisconsin bioenergetics series. approach (Hanson et al. 1997) to model the consump- Relative abundance of striped bass.—Since 1980, tion of threadfin shad by age-0 (100-mm), subadult annual gill netting has been used to monitor the relative (,500-mm), and adult (500-mm) striped bass in abundance, size, and age structure of striped bass in Lake Powell from 1985 through 2003. Physiological Lake Powell. In November of each year, personnel of parameters used in the bioenergetics model were based the UDWR set 10 horizontal gill nets for two on laboratory-derived data for juvenile and adult subsequent nights in each of four of the designated striped bass (Hartman and Brandt 1995). The bioener- sites of Lake Powell (Wahweap Bay, San Juan Arm, getics model performs best when constrained by Rincon, Good Hope Bay; Figure 1). The experimental seasonal estimates of growth (Beauchamp et al. 1989; gill nets (1.8 m deep 3 30 m long; four 7.6-m panels of Brodeur et al. 1992; Ruggerone and Rogers 1992), so 19-, 25-, 38-, 50-mm square mesh) were set offshore in we estimated consumption by average individual the late afternoon and pulled in the morning. Within striped bass from May to November in each year of each year, catches from all gill nets were summed and the time series, hereafter referred to as seasonal divided by total effort to estimate reservoirwide catch consumption. Since age-0 striped bass do not consume per unit effort (CPUE). Gill-net CPUE data were shad until they reach approximately 100 mm TL adjusted for mesh-size selectivity (Helser et al. 1998), (Harper et al. 1969; Gatlin 1996; Sutton and Ney and encounter probability (Rudstam et al. 1984; Porch 2001), we interpolated the day of the year that they et al. 2002). reached this size and started age-0 simulations on that To further assess striped bass population dynamics, day based on observed growth rates (Table 1). We we converted gill-net CPUE data to an estimate of scaled individual consumption estimates up to the population density, and ultimately an index of population level using a stage-structured population population abundance, using hydroacoustic informa- index for striped bass. This modeling approach tion. Mueller and Horn (1999) used hydroacoustics and required seasonal estimates of stage-specific striped vertical gill-net surveys to estimate pelagic striped bass bass growth, thermal history, diet composition, and densities in Lake Powell in August 1996 and 1997. We prey energy content (Table 2). regressed these hydroacoustics-based density estimates Striped bass growth.—We analyzed the size and age against UDWR fall gill-net CPUE data for correspond- structure of striped bass caught annually during autumn ing locations and years. We used the resulting linear gill-net surveys to back-calculate annual growth rates. 5 relationship (density ¼ 0.0281 CPUE 2.00 3 10 ; To estimate mean size at age, we used a combination of R2 ¼ 0.66, df ¼ 6, P ¼ 0.03) to adjust year-specific gill- scale-aging data, analysis of length-frequency modes, net CPUE data to an index of striped bass density. We and year-specific length–weight regressions (Table 3). then multiplied density estimates by the year-specific Slope coefficients (b) from length–weight regressions volume of the top 20 m of the reservoir to provide an were also used as an index of striped bass condition. 266 VATLAND ET AL.

TABLE 1.—Simulation days on which age-0 striped bass adult growth for the year before and after that particular reach 100 mm in total length. Simulation days range from 1 year. The default model energy density was used for (May 15) to 185 (November 15). striped bass (Hanson et al. 1997). Year Day Striped bass diet composition.—Since 1985, the

1985 109 percent occurrence of prey items in stomachs has been 1986 115 used to characterize striped bass diets in Lake Powell. 1987 97 However, the bioenergetics model requires diet data as 1988 114 1989 102 proportion by wet weight. To address this issue, we 1990 87 independently calculated diet data as both proportion 1991 87 1992 91 by wet weight and proportion by occurrence in the 1993 91 spring, summer, and autumn of 2003 and 2004. Using 1994 85 the 2003 data, we averaged the weight of each prey 1995 77 1996 86 item by occurrence (e.g., when present in a stomach, 1997 72 threadfin shad weighed an average of 5.1 g; Table 4). 1998 108 1999 85 We adjusted past diet occurrence data to an index of 2000 95 proportion by wet weight for individual fish using the 2001 84 following equation: 2002 87 2003 71 oiwi Wi ¼ Xn ; i ¼ 1; 2; ...; n ðoiwiÞ We then used mean weight-at-age data for age-classes i¼1 with sufficient sample size (n 5 aged fish) to back- where W is an index of proportion by wet weight of diet calculate annual growth: item i, n is the number of diet items i, o is the occurrence of diet item i (1 ¼ present, 0 ¼ absent), and w is the Gx;t ¼ Mx;t Mx1;t1; average weight of each prey item per occurrence (Table where G is the annual growth (g), x is the age-class, t is 4). To test for potential bias due to inter-annual the year, and M is mass (mean weight at age [g]). In variability in the mean weight of prey items, we directly each year of the time series, age-classes were placed compared Wi values to observed wet-weight proportions into stage classes (i.e., age 0, subadult, adult as of striped bass in 2004 (Table 5); the diet proportions that previously defined) based on average autumn length results from the two types of data were not significantly at age. We then averaged annual growth estimates different in 2004 (v2 ¼ , 0.01, df ¼ 5, P ¼ 1.00). Within across age-classes within each stage class, weighted by each year of the time series, we calculated Wi for all the relative abundance of each age-class. We assumed individual striped bass with stomach contents and that the growth from May to November comprised averaged this diet data for two size-classes of fish 80% of back-calculated annual growth (e.g., seasonal (100–500 mm and 500 mm) by season, defined here as size-at-age data; Budy et al. 2005), and we used the spring (May–June), summer (July–August), and autumn resulting May–November growth estimates for stage- (October–November). The frequency and location of and year-specific growth (g) inputs in bioenergetics historical (UDWR-collected) spring gill netting and diet simulations. The number of adult striped bass was sampling varied considerably through the historical time limited in some years (n , 5 in 7 of 19 years), such that series, so we averaged spring diet data across all years to size-at-age data were not available for adults; in those estimate spring diet composition for subadult and adult years, we estimated adult growth as the average of striped bass. Similarly, we averaged all summer diet data

TABLE 2.—Energy densities used for prey in bioenergetics modeling simulations.

Prey item Energy density (J/g) Source Comments

Thread fin shad 5,536 Eggleton and Schramm (2002) Other fish 5,025 Eggleton and Schramm (2002), Average of threadfin shad, bluegill, centrarchid, and Hewett and Johnson (1992), red shiner energy densities (5536, 5,384, 4,185, Bryan et al. (1996) and 4,995 J/g, respectively) Zooplankton 2,302 Snow (1972) Based on cladocerans (low end of reported range) Chironomids 2,511 Cummins and Wuycheck (1971) Based on dipteran larvae (high end of reported range) Crayfish Orconectes spp. 6,160 Stein and Murphy (1976) Based on the same genus found in Lake Powell Terrestrial insects 2,741 Cummins and Wuycheck (1971) Based on a variety of terrestrial insects STRIPED BASS AND THREADFIN SHAD DYNAMICS 267

TABLE 3.—Year-specific length–weight regressions for TABLE 4.—Mean weights of diet items per occurrence in striped bass captured by gill netting, angling, and via creel striped bass stomachs collected in the spring, summer, and fall surveys. The regressions took the general form weight (g) ¼ a of 2003. The number of stomachs containing a particular diet b TL (mm) . We also used the parameter b as an index of striped item is represented by n. bass condition. Diet item Mean weight SE n Year a (3 105) br2 n Threadfin shad 5.33 1.72 19 1984 9.47 2.66 0.96 926 Zooplankton 0.75 0.14 29 1985 12.3 2.58 0.93 584 Other fish 0.74 0.24 9 1986 8.74 2.62 0.94 822 Chironomids 0.29 0.13 39 1987 22.2 2.46 0.91 416 Crayfish 8.33 2.15 16 1988 14.3 2.54 0.92 383 Terrestrial insects 0.29 0.01 2 1989 16.9 2.51 0.93 132 1990 33.2 2.43 0.87 597 1991 2.26 2.88 0.95 677 1992 5.78 2.70 0.94 440 shad consumption (average annual) to evaluate the 1993 50.4 2.35 0.91 237 1994 49.3 2.34 0.86 221 functional feeding response of striped bass in Lake 1995 39.3 2.39 0.87 466 Powell using the approach of Beauchamp et al. (2007). 1996 1.47 2.95 0.98 456 We did not include age-0 bass in our functional feeding 1997 9.97 2.60 0.95 568 1998 65.9 2.27 0.87 225 responses because zooplankton is often a large 1999 89.3 2.26 0.89 312 component of their diet. Thus, for subadult and adult 2000 20.6 2.48 0.89 271 2001 24.4 2.44 0.87 348 bass we plotted average individual consumption rates 2002 32.0 2.42 0.88 302 (g/d) as a function of our annual index of shad 2003 7.05 2.69 0.96 394 abundance (kg/year) for the available time series and considered both linear (type I) and hyperbolic (type II) functional feeding response curves (SAS Institute for juvenile striped bass (,500 mm) to estimate the 2002; Haefner 1996). initial diet composition of 100-mm age-0 fish. We applied these seasonal diet data to the appropriate stage- Results class bioenergetics simulations and used the bioenerget- Relative Abundance of Threadfin Shad and Striped Bass ics model to interpolate changes in diet composition Indices of abundance for striped bass and age-0 between static spring or summer inputs and the year- threadfin shad fluctuated throughout the time series specific autumn inputs. Fine-scaled diet data collected in from relatively low levels to peaks in 1991, 1996, and 2003 and 2004 indicated that consumption of threadfin 2003 (Figure 2). Shad abundance changed between shad increased dramatically in the late summer and fall, these dramatic peaks to low levels within 1 to 2 years. and that fall diet was in fact driving annual differences in Peaks in striped bass abundance generally paralleled shad consumption (Budy et al. 2004). shad abundance, but additional peaks occurred in 1988 Striped bass thermal history.—To estimate the and 1999. Striped bass abundance also varied less in thermal history of striped bass, we reviewed ambient magnitude and appeared to attenuate from peaks more temperature and dissolved oxygen (DO) profiles for slowly than threadfin shad abundance. Proportionally, 1985–2003 (Grand Canyon Monitoring and Research the striped bass population was dominated by subadults Center, unpublished data) for the availability of (71% average) in most years, with a relatively large theoretical optimum temperatures and DO values potentially limiting respiration. In the bioenergetics model, we set optimum temperatures for age-0 (100- TABLE 5.—Striped bass diet composition in 2004 calculated mm), subadult, and adult striped bass to 19, 18, and as a proportion by wet weight (based on laboratory weight measurements) and an index of proportion by wet weight 158C, respectively (Hartman and Brandt 1995). We (based on 2003 mean size per occurrence data). These two assumed striped bass did not occupy habitats with DO data types were compared to test for biases in adjusting past concentrations less than 4 mg/L (Coutant 1985; Zale et diet occurrence data to an index of proportion by wet weight al. 1990; Bettoli 2005). Of the remaining available for bioenergetics simulations; see text for details. habitat with DO concentrations of at least 4 mg/L, we Diet item Proportion Index of proportion assumed striped bass occupied waters at their theoret- ical optimum temperature or the ambient temperature Threadfin shad 0.778 0.780 Other fish 0.031 0.025 nearest optimum. Chironomids 0.084 0.086 Finally, to contribute to our understanding of the Zooplankton 0.080 0.078 mechanisms that drive the predator–prey cycle, we Crayfish 0.027 0.031 Terrestrial insects 0.000 0.000 used the resulting estimates of individual threadfin 268 VATLAND ET AL.

FIGURE 2.—The upper panel presents an index of threadfin shad summer abundance based on summer trawling, the middle panel an index of striped bass total population abundance based on adjusted catch per unit effort. The lower panel presents an index of the striped bass population disaggregated by age (adult [500 mm], subadult [,500 mm], and age 0 [variable]). These data were used to scale individual-level consumption estimates up to the population level. portion of age-0 fish in some years (Figure 2); the and remained relatively high and stable until 2001 for proportion of adults remained low in most years (19% juveniles (,500 mm) and throughout the rest of the average). time series for adults (500 mm; Figure 3). The diet composition of juvenile striped bass paralleled that of Striped Bass Diet Composition, Growth, Condition, adults in most years, except in 2001 when the and Thermal History proportion of shad dropped considerably in juvenile The proportion of threadfin shad in striped bass but not adult diets. Overall, adult striped bass diets autumn diets increased consistently from 1988 to 1991 contained a greater proportion of crayfish Orconectes STRIPED BASS AND THREADFIN SHAD DYNAMICS 269

FIGURE 3.—Diet composition of juvenile (age-0 and subadult) and adult striped bass in autumn, by year. See Figure 2 for age classifications.

spp. and red swamp crayfish Procambarus clarkii than did subadults. The zero or negative growth rates that those of juveniles, which contained a greater propor- occurred for adults in some years likely resulted from a tion of zooplankton. The proportions of other fish, combination of realistic negative effects of depressed chironomids, and terrestrial insects in diets were low in food availability and artificial effects of limited size-at- most years for both adult and juvenile striped bass. age data for some years. Age-0 striped bass growth was relatively variable Dissolved oxygen concentrations of at least 4 mg/L and generally increased across years in the time series, with peaks in 1995, 1997, and 2003 (Figure 4; Table and optimum or near-optimum temperatures were 6). Subadult growth was most variable, alternating available to age-0, subadult, and adult striped bass between relatively high and low levels every year or throughout most of the time series. Accordingly, two. Adult growth peaked in 1987, 1991, 1995, and thermal history inputs rarely deviated from optimum 2003, and adults had longer periods of low growth than values (Table 7). 270 VATLAND ET AL.

decline in 1992, increased to high levels again in 1995. Subadult striped bass also demonstrated a large decline in individual consumption in 2001, a pattern that was not observed in the adults. Individual consumption of threadfin shad by age-0 striped bass was relatively low and invariable throughout the time series. Scaling individual consumption estimates up to the population level resulted in highly variable subadult consumption that fluctuated from highs of approxi- mately 1.2 million kg (18.4 kg/ha) and lows of approximately 0.3 million kg (4.6 kg/ha) on a 1- to 2-year cycle (Figure 5). Despite these fluctuations, consumption of shad at the population level was dominated by subadult striped bass (72% average) in FIGURE 4.—Estimated seasonal (May–November) growth of age-0, subadult, and adult striped bass. most years. The patterns of population-level consump- tion by subadult striped bass differed greatly from the trends in age-0 striped bass and adult striped bass Shad Biomass and Striped Bass Consumption consumption and deviated from the trends in the index Bioenergetics model predictions of individual con- of summer threadfin shad biomass. In contrast, age-0 sumption of threadfin shad by striped bass captured the and adult population shad consumption corresponded variability in diet, growth, and thermal history. directly, or with a 1-year lag, to trends in the index of Individual consumption rates were highest for adult shad biomass. In addition, both the subadult and adult life stages of striped bass, followed by subadults and then age-0 fish, striped bass appeared to generally demonstrate a type II and corresponded closely to the biomass index of functional feeding response to threadfin shad in Lake summer threadfin shad (Figures 5, 6). Individual Powell (Figure 6). Individual consumption versus shad consumption of threadfin shad by adult striped bass abundance data were best described by a hyperbolic, peaked in 1991, 1996, and 2003. Individual consump- two-parameter curve emulating a convex, type II tion by subadults averaged 61% of that demonstrated functional feeding response (y ¼ ax/b þ x, where y by adult striped bass and exhibited much greater inter- equals individual consumption rates and x equals annual variability. Individual subadult consumption summer shad biomass [Haefner 1996]). For subadults generally increased from 1985 to 1991 and following a a ¼ 10.0382, b ¼ 1475.8, df ¼ 19, and P , 0.001; for

TABLE 6.—Initial and ending weights (g) used in striped bass bioenergetics simulations. Simulations for subadult and adult striped bass were run from day 1 to day 185, simulating growth from May to November in each year of the time series. See Table 1 for further information.

Initial simulation weight Ending simulation weight

Simulation Year Age-0 Subadult Adult Age-0 Subadult Adult

1985 18.0 454.3 1,658.8 71.0 376.0 1,340.5 1986 15.1 286.8 1,303.8 51.6 275.7 1,189.0 1987 18.8 185.2 607.2 75.7 599.0 992.0 1988 16.8 472.2 765.2 57.9 551.5 992.0 1989 17.7 397.6 765.2 77.3 557.3 992.0 1990 23.7 380.2 835.9 116.3 694.7 1,249.0 1991 12.7 621.5 906.6 108.6 1,197.0 1,506.0 1992 14.2 450.9 1,427.3 92.9 641.0 1,664.0 1993 25.3 345.2 956.6 82.6 748.0 1,313.0 1994 24.1 584.1 940.5 132.5 704.0 1,403.8 1995 23.5 399.8 877.8 190.2 817.5 1,448.0 1996 11.5 290.7 955.3 100.2 691.0 1,494.5 1997 16.0 439.8 1,348.2 163.4 695.0 1,286.0 1998 22.6 346.2 947.3 75.4 487.5 970.0 1999 29.0 448.3 947.6 114.0 726.7 970.0 2000 19.1 617.2 947.6 87.0 813.8 970.0 2001 18.1 603.5 913.6 110.0 655.4 1,069.0 2002 22.1 499.4 879.7 109.9 862.3 1,168.0 2003 17.0 232.8 1,044.1 201.3 725.0 1,524.3 STRIPED BASS AND THREADFIN SHAD DYNAMICS 271

TABLE 7.—Thermal history inputs (8C) used in bioenergetics simulations for striped bass in three age-classes.

Age-0 Subadult Adult Time period Average Range Average Range Average Range

1985–1990 19.0 19 18.5 18.5 15.0 15 1991 19.5 18.5–20.5 19.2 18.5–20.5 15.6 15–16.8 1992–1995 19.0 19 18.5 18.5 15.0 15 1996 19.3 19–19.8 18.9 18.5–19.8 15.0 15 1997 18.9 19–19.8 18.6 18.5–18.7 15.0 15 1998 18.8 18.3–19 18.4 18.3–18.5 16.0 16 1999 19.0 19 18.5 18.5 15.0 15 2000 18.7 18–18.5 18.3 18–18.5 15.0 15 2001 18.6 17.8–19 18.3 17.8–18.5 15.0 15 2002 19.6 19–20.7 19.2 18.5–20.7 16.1 15–18.3 2003 19.1 19–19.3 18.8 18.5–19.3 15.0 15 adults a ¼ 14.8444, b ¼ 1284.3, df ¼ 19, and P , 0.001 the time series for the different life stages of striped (SAS PROC NLIN; SAS Institute 2002). Discrepancies bass. Individual consumption estimates of threadfin in model fit, particularly for the adult response, were shad were consistently greatest for adult striped bass, driven by points representing low (near-zero) abun- followed by those for subadults and age-0 fish. dance of threadfin shad. Nevertheless, individual Population-level consumption of threadfin shad was consumption rates increased rapidly as a function of dominated by subadults (72% average) in most years. increasing shad abundance but then hit an asymptote Similarly, Cyterski (1999) estimated age-specific (maximum feeding rate) when our index of shad striped bass consumption at the individual and abundance exceeded 10,000 kg/year (0.15 kg/ha). population levels in Smith Mountain Lake, Virginia, Similarly, the annual index of striped bass condition and found that adult striped bass experienced greater was positively and significantly related to the threadfin individual consumption rates and subadults demon- shad summer biomass index (Figure 6; linear regres- strated greater population-level consumption rates. sion; df ¼ 18, adjusted R2 ¼ 0.53, P ¼ 0.00). While age-0 and adult striped bass consumption rates in Lake Powell were generally stable and responded Discussion directly, or with a short time lag, to the annual Based on our analyses of the available time series abundance of threadfin shad, subadult consumption and the bioenergetic simulations, striped bass and fluctuated dramatically and did not always appear to threadfin shad appear to be tightly coupled and respond to indices of threadfin shad abundance. The demonstrate a relatively distinct predator–prey cycle decoupling of subadults and threadfin shad in some in Lake Powell. Summer threadfin shad abundance years may be partly due to the ability of juvenile striped peaks on an approximate 5- to 7-year interval, and bass to successfully switch to zooplankton or other striped bass growth, consumption, condition, and prey resources in Lake Powell, as demonstrated by our abundance responds directly to peaks in threadfin shad diet data. In addition, our reliance on population-level forage. Accordingly, our index of threadfin shad indices of abundance, versus true estimates of biomass appears to explain 53% of the variation in abundance, limits our ability to assess traditional condition of striped bass. However, as expected based standing stock proportions (e.g., predator demand on the inherent delays common to many predator–prey versus prey supply; Cyterski et al. 2003). Nevertheless, cycles (May 1980), striped bass abundance varied less based on the proportion of shad consumed by in magnitude and exhibited slower attenuation from subadults, the type II functional feeding response we peaks than did threadfin shad abundance. The close observed in Lake Powell, and the linear relationship relationship between the dynamics of striped bass and between striped bass condition and shad abundance, it threadfin shad populations in Lake Powell corresponds appears that striped bass may control shad numbers in well to trends observed in other freshwater lakes and all but the highest peak years of shad abundance. reservoirs, where striped bass populations and their As observed in other systems where estimates of piscine prey populations are frequently coupled striped bass consumption met or exceeded the supply (Morris and Follis 1979; Fuller 1984; Cyterski et al. of clupeid prey (Cyterski et al. 2003; Hartman 2003; 2003). Uphoff 2003), the ability of striped bass to respond Bioenergetic estimates of individual and population- quickly, at both the individual and population levels, to level consumption of threadfin shad varied throughout changes in threadfin shad foraging supports the 272 VATLAND ET AL.

FIGURE 5.—The upper panel presents an index of threadfin shad biomass in summer, the middle panel estimates of the seasonal consumption of threadfin shad by average individual striped bass, and the lower panel population-level striped bass consumption. Note the differences in the scale of the y-axis. The saturation of individual striped bass consumption (the dotted line in the upper panel) is based on functional feeding relationships (see Figure 6). argument that striped bass may control threadfin shad 2005). In Lake Powell, both temperature and oxygen abundance in Lake Powell. Further, Lake Powell levels generally remained within the ranges considered differs environmentally from some striped bass systems acceptable or optimal for striped bass growth and in which striped bass are spatially and energetically survival in some portion of the reservoir depth strata, at restricted due to high temperatures or low dissolved all index sites, and in most years of this time series oxygen values (Coutant 1985; Zale et al. 1990; Bettoli (Vatland 2006; this study). Bettoli (2005) suggested STRIPED BASS AND THREADFIN SHAD DYNAMICS 273 that the fundamental thermal niche of striped bass should be expanded to include cooler temperatures, such as those simulated in our model. We note, however, that the assumed optimum thermal history used in our simulations is likely lower than the realized thermal history of striped bass. During the May to November growing season in Lake Powell, striped bass must occupy waters warmer than their assumed optimum for at least short time periods in order to use their highly favored prey, threadfin shad. Overall, however, the stable and favorable environmental conditions in Lake Powell support the high growth potential of striped bass in this system and contribute to the ability of these predators, especially subadults, to respond quickly and likely control threadfin shad populations in most years. In general, predator abundance changes at a slower rate than prey abundance (Hall and Van Den Avyle 1986), thus the slower attenuation of striped bass abundance from peaks in shad abundance is not surprising. The slower decline in striped bass abun- dance is at least in part due to their ability to exhibit a more generalist feeding behavior during some life stages or seasons (Gardinier and Hoff 1982; Hartman and Brandt 1995; Setzler-Hamilton and Hall 1991); striped bass in Lake Powell are able to switch from threadfin shad to alternative prey, such as zooplankton FIGURE 6.—The upper panel shows the relationships between the consumption rates of threadfin shad by individual for juveniles and crayfish for adults. We observed this subadult and adult striped bass and the biomass of threadfin apparent ‘‘decoupling’’ of threadfin shad and striped shad in summer, the lower panel the relationship between bass in 1985–1989. However, when striped bass striped bass condition and threadfin shad biomass, both over consumed alternative prey in Lake Powell, growth the period 1985–2003. and condition generally decreased, especially for adults, and fewer striped bass achieved sufficient size this type of feeding relationship would indicate that at and condition to reproduce (Gustaveson 1999). Subse- high prey abundance, handling times of prey exceed quently, recruitment could be limited, and the striped the other components of predator consumption (e.g., bass population would likely slowly decrease in encounter rates) and digestion. In those years of abundance until sufficient shad forage was again extraordinarily high threadfin shad abundance, the available. Although somewhat simplified here, the striped bass and threadfin shad populations in Lake growth rate and thus the consumption of the predator Powell appear to demonstrate a classic predator–prey may not have been sufficient to control threadfin shad cycle over most of the time series (Lotka 1925; abundance above a certain threshold (Michaletz 1997). Volterra 1926; Turchin 2003). Cyterski (1999) similarly modeled clupeid consump- A functional response provides a simple mathemat- tion by striped bass in a freshwater reservoir as a ical expression of the relationship between predator convex relationship between prey density and predator feeding behavior and prey density and can thus aid in growth. Assuming that food resources or other bottom- our understanding of the factors driving the predator– up influences are not limiting for shad, those years of prey cycle (Haefner 1996; Beauchamp et al. 2007). extremely high shad abundance may serve to invigorate Based on our bioenergetic estimates of individual the forage base through greater recruitment rates of consumption, striped bass (especially subadult striped age-0 threadfin shad to the pelagic zone, as compared bass) appear to demonstrate a type II functional feeding with the years in which striped bass retain top-down response to shad in Lake Powell, such that consump- control over their threadfin shad prey base (Filipek and tion rates increase to a certain point, then reach a Tommey 1987; Schramm et al. 2001). Alternatively, plateau when our index of shad abundance exceeds high threadfin shad abundances could result in reduced approximately 10,000 kg/year. On an individual level, recruitment rates due to intraspecific competition. We 274 VATLAND ET AL. note that this type of functional feeding response is Ney 1996; Vatland and Budy 2007). As such, annual limited, however, in that it does not account for fluctuations in natural discharge, reservoir elevation, or differences in consumption rates across different life other top-down or middle-out factors that can affect stages or sizes of threadfin shad prey or prey switching, secondary productivity may exert additional constraints as may occur when threadfin shad abundance is low on the threadfin shad population, and thus predator (Griffin and Margraf 2003; Hartman 2003). Nonethe- response, in Lake Powell (DeVries and Stein 1992; less, based on our functional response and bioenergetic Sammons et al. 1998). Nonetheless, the results from simulations, and in the absence of other factors (e.g., our study in combination with similar work by others, changes in reservoir productivity), we predict that at confirm the utility of a bioenergetics-based approach in threadfin shad index values less than approximately understanding, predicting, and potentially managing 10,000 kg/year predator consumption will exceed prey predator demand and prey supply. Further, we supply, striped bass will retain control over threadfin demonstrated how a long time series of coarse data shad abundance, and no extreme peak or drop in can be refined and synthesized to provide a compre- striped bass abundance will occur. When threadfin hensive understanding of the mechanisms driving a shad values exceed this threshold, we predict a surplus predator–prey cycle. in threadfin shad for striped bass consumption and a The results presented in this study support current subsequent peak in abundance 1 to 2 years afterward. attempts by fisheries managers to reduce the predatory Although the appropriate scale at which to analyze demand of striped bass on threadfin shad, as striped predator–prey relationships largely depends on the bass consumption appears to control threadfin shad research objectives and available resources (Mason and forage in most years. These data suggest that if striped Brandt 1999), bioenergetics modeling has proved bass abundance were left unchecked, demand would useful for assessing the relationship between striped likely greatly exceed supply with potentially negative bass and their piscine prey at many different spatial and effects on striped bass health, growth, and ultimately temporal scales. For example, Hartman (2003) esti- abundance (e.g., Uphoff 2003). We showed that when mated striped bass consumption at two relatively the threadfin shad biomass index peaks (every 5–7 coarse spatial scales, Atlantic Coast–wide and within years), managers can expect a striped bass response the Hudson River, whereas Brandt and Kirsch (1993) that is relatively rapid and predictable for adults, but used bioenergetics to estimate striped bass growth and less predictable for subadult bass. The apparent foraging in 0.5-m deep 3 30-m-long ‘‘cells’’ in decrease in the magnitude of abundance fluctuations Chesapeake Bay. Bioenergetic analysis at such a fine in recent years, however, may signify that this system spatial scale requires a considerable effort and ample is working toward a more balanced predator–prey resources (though remote sensing data are making relationship with less fluctuation. We advocate contin- these approaches more feasible), and these data are ued monitoring of the two fish populations as well as infrequently available in annual agency data. In this that of phytoplankton and zooplankton, and finer study, we used relatively coarse historic data, in spatial and temporal scales should be used in future combination with more refined current data, to assess analyses, where possible. In addition, recently intro- the predator–prey dynamics between striped bass and duced gizzard shad have demonstrated a great potential threadfin shad from 1985 to 2003. For future analyses, for altering reservoir food webs (Dettmers and Stein we recommend examining these population dynamics 1992; DeVries and Stein 1992); the implications of this at a finer spatial scale, as Lake Powell demonstrates a introduction may alter the established striped bass and spatial gradient in productivity (Budy et al. 2005) threadfin shad populations and the future of the fishery typical of many reservoirs (Hall and Van Den Avyle overall (but see Vatland and Budy 2007). The 1986). Thus, in some years the relationship between bioenergetics framework we have developed for Lake striped bass and threadfin shad may differ in various Powell provides a template within which we can now parts of this large and spatially heterogeneous reservoir explore the future effects of gizzard shad on the fishery (e.g., inflow versus lacustrine areas; Haddix and Budy and provides a predictive tool for evaluating harvest 2005). scenarios. In addition to examining predator–prey dynamics at a finer spatial scale, we recommend exploring the Acknowledgments effects of other physical–chemical factors on the We gratefully acknowledge Wayne Gustaveson, threadfin shad population and food web of Lake George Blommer, and Don Archer of Utah Division Powell. For example, density and biomass of threadfin of Wildlife Resources for provision of data, assistance shad generally increase with increased primary pro- and guidance. Kate Behn, Lora Tennant, Erin Van- ductivity (Bachmann et al. 1996; DiCenzo et al. 1996; Dyke, Andy Dean, Travis Sanderson, Wes Pearce, STRIPED BASS AND THREADFIN SHAD DYNAMICS 275

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