INTRODUCTION

Reservoirs present challenging problems for fishery managers because of their variable ecological conditions (Summerfelt 1999). Most are built primarily for the purposes of hydroelectric generation, flood control, and/or water storage rather than for a fishery. Because of their design, reservoirs generally have limited littoral areas, steep depth contours, and water fluctuations that limit productivity in littoral zones (Hayes et al. 1999). These systems are prone to invasions of non-native species because resident riverine fishes may have a difficult time establishing viable populations in systems with limited littoral zones. Consequently, fishery managers often stock apparently suitable fish species to fill perceived vacant niches and meet management objectives (Kohler et al. 1986). Non-native introductions can have a dramatic effect on resident fish populations, creating instable and unpredictable environments (Magnuson 1976). In recent years, non-native species have been defined as “invasive” if their introduction has the potential to cause environmental or economic harm (Clinton 1999). Invasions by non-native species occur by many means other than agency stockings. Canal and river systems, unauthorized angler stockings, and bait-bucket introductions are all channels for invasion of non-natives (Li and Moyle 1999).

The white perch (Morone americana) is an invasive species that has steadily increased its distribution throughout inland waters of North America. White perch are native to estuaries in the Atlantic slope but beginning in the late 1880s, white perch have invaded inland waters in north-Atlantic states (Zeurlein 1981), the Great Lakes (Boileau 1985), Nebraska (Hergenrader and Bliss 1971), and mid-Atlantic states (Wong et al. 1999). White perch were introduced to non-native waters by many means including voluntary and incidental agency stockings, migration, and possible angler introductions (Jenkins and Burkhead 1993; Carlander 1997; Fuller et al. 1999). White perch have become the most abundant species in many of these waters as a result of their opportunistic feeding, broadcast spawning with no preference for substrate type, and high fecundity (Wong et al. 1999).

Kerr Reservoir (Buggs Island Lake) is a 19,790-ha hydroelectric and flood control impoundment in south-central Virginia that has a reputation for its largemouth bass (Micropterus salmoides) and trophy crappie (Pomoxis spp.) fisheries. Kerr also features one of the few self-sustaining landlocked striped bass (Morone saxatilis) populations. White perch populations have been established in Kerr Reservoir since 1988 (Wilson 1997), probably through angler introductions. In 2000, the white perch was the most abundant sportfish species collected in Virginia Department of Game and Inland Fisheries’s (VDGIF) fall gillnet samples (N =711, catch rate 15.0/net; VDGIF, unpublished). A small panfish fishery has even become established for white perch in Kerr Reservoir (Wilson 1997; DiCenzo 2001), but there is concern that an overabundance of white perch in Kerr Reservoir may produce a stunted population. Many instances of stunting in white perch populations have been observed in other inland waters (Hergenrader and Bliss 1971; Marcy and Richards 1974; Zeurlein 1981; Boileau 1985; Wong et al. 1999).

White perch invasions can have dramatic negative effect on resident fish populations. Drops in abundance of resident fishes have often followed white perch invasions (Hergenrader and Bliss 1971; Zeurlein 1981; Boileau 1985; Gopalan et al. 1998; Wong et al. 1999; Madenjian et al. 2000). Madenjian et al. (2000) found that the most plausible explanations for the reduction of native white bass (Morone chrysops) populations in Lake Erie and Oneida Lake, New York were that white perch interfered with the survival of white bass eggs, larvae, and/or juveniles. It is likely that the mechanisms causing the declines in resident fish populations are white perch competition and/or predation.

White perch are opportunistic feeders (Wong et al. 1999). Small white perch (<150 mm TL) primarily feed on zooplankton and benthic invertebrates, the important foods of larval and early stages of most age-0 fishes. Significant trophic overlap (> 0.6 on a 0-1.0 scale) has been reported between age-0 striped bass and age-0 white perch in the lower James River and Potomac Estuary, Virginia (Setzler-Hamilton et al. 1982; Rudershausen and Loesch 2000), age-0 yellow perch (Perca flavescens) and age-0 white perch in Oneida Lake, New York (Prout et al. 1990), and age-0 white bass, alewife (Alosa pseudoharengus), and yellow perch versus age-0 white perch in western Lake Erie (Gopalan et al. 1998). Larger white perch (>200 mm) have been known to eat a variety of foods, including other fish species (Warner 1974; Elrod et al. 1981; Boileau 1985; Weisberg and Janicki 1990).

White perch are also known to be ovivores (Elrod et al. 1981; Bath and O’Connor 1985). White perch have been documented eating the eggs of white perch and striped bass in the Pamunkey River, Virginia (McGovern and Olney 1988); white perch, white bass, and walleye (Stzostedion vitreum) eggs in the Sandusky River tributary of Lake Erie (Schaeffer and Margraf 1987); alewife eggs in Lake Ontario (Danehy et al. 1991); and walleye eggs in the western basin of Lake Erie (Roseman et al. 1996). In addition, white perch have been found to feed on striped bass eggs and larvae even in instances of high turbidity (McGovern and Olney 1988; Monteleone and Houde 1992).

Although white perch can be detrimental to native fish populations, they can also provide a valuable forage base for sportfishes. In their native, estuarine environment, white perch have been important forage for striped bass over 200 mm TL (Gardinier and Hoff 1982), and age-0 bluefish (Pomatomus saltatrix) (Juanes et al. 1993). Hartman and Margraf (1992) found that walleye in Lake Erie fed on Morone species (primarily white perch) when gizzard shad (Dorosoma cepedianum) were not abundant. White perch constituted a seasonally important food source for the white bass in Lake Erie throughout the summer (Hartman 1998). In Lake Ontario, northern pike (Esox lucius) had substantial proportions of white perch in their diets two years after white perch had become established (Boileau 1985). Piscivores that eat spiny-rayed prey (such as largemouth bass) have also been shown to target white perch, when white perch was the most abundant prey fish (Ward and Neumann 1998).

State biologists have speculated that white perch may be having a significant effect on resident sport fishes in Kerr Reservoir. Indices of striped bass year-class strength were reduced in the early 1990’s directly after the first white perch were found in Kerr Reservoir gillnet samples (Wilson 1997). Since the establishment of white perch in Kerr Reservoir, anglers have suggested that striped bass fishing is not as good as it used to be and requested supplemental stocking of fingerings. Additionally, the adult white bass population has declined dramatically since the establishment of white perch (V. DiCenzo, Virginia Department of Game and Inland Fisheries, personal communication). The trophic significance of white perch on sportfish communities in Kerr Reservoir is unknown, but the white perch’s reputation and performance record leads to some concern. To determine the impact of white perch to the overall fishery of Kerr Reservoir, it is imperative to gain an understanding of the trophic relationships between white perch and resident sportfishes.

To assess the impacts of the white perch introduction into Kerr Reservoir, I conducted an analysis of trophic benefits (utilization) and detriments (predation and competition) to resident sportfishes. My research focused on the trophic interactions between white perch and resident sportfishes, in particular striped bass.

White Perch Biology and Ecology

Distribution - The white perch is native to estuaries in the Atlantic Slope of North America as far south as South Carolina (Jenkins and Burkhead 1993), and as far north as the Miramichi River Estuary in New Brunswick (Johnson and Evans 1991). During the period between 1880 and 1950, federal and state hatcheries established inland populations of white perch in Maine, Maryland, Connecticut, New York, and Massachusetts (Zeurlein 1981).

After initial introductions, the white perch extended its western range by means of canals and river systems. In the late 1940s, white perch became established in Lake Ontario from a population in Oneida Lake, New York and in Lake Erie via the Erie Barge Canal (Scott and Christie 1963). By 1983, populations were established in the Detroit River, Lake St. Clair, the St. Clair River, and Lake Huron (Boileau 1985). By 1988, white perch populations had been established in Lake Michigan, the Illinois River (Cochran and Hesse 1994), and more recently have been found in the Upper Mississippi River System (Irons et al. 2002).

Before 1964, the western range of white perch distribution was the Great Lakes. In 1964, white perch were stocked in Nebraska lakes by the Game and Parks Commission to establish suitable fisheries in highly alkaline lakes (Hergenrader and Bliss 1971). From initial stockings, white perch have established populations in the Platte and Missouri River systems (Hergenrader and Bliss 1971). From Nebraska, the white perch has moved south into Oklahoma through the Arkansas River system, and white perch have been recently found in Oklahoma reservoirs (K. Kuklinski, Oklahoma Department of Wildlife Conservation, personal communication).

In Virginia, the white perch is indigenous to all of the coastal drainages below the Fall Line (Jenkins and Burkhead 1993). By means of canal systems and possible stockings, the white perch has colonized waters above the Fall Line in the Potomac, Roanoke, and York drainages (Jenkins and Burkhead 1993). Inland waters of North Carolina have also experienced establishment of white perch populations (Wong et al. 1999).

Food Habits - The white perch has a terminal mouth and a tongue with two narrow tooth patches on the anterolateral margin for grasping prey items (Jenkins and Burkhead 1993). Larval white perch (3-23 mm TL) first feed on micro-zooplankton (rotifers, cladocerns, and copepods) (Setzler-Hamilton et al 1982). Hjorth (1988) found that white perch larvae are efficient predators at a relatively small size, because they shift diets to smaller zooplankton prey when abundances of larger, preferred zooplankton are low.

The diets of young-of-year and older white perch are very diverse, varying with spatial and temporal availability of suitable prey. Young-of-year white perch feed on zooplankton early in life, but soon shift to benthic invertebrates (Gopalan et al. 1998). In the James River, Virginia, the diets of age-0 white perch up to 40 mm TL consisted primarily of copepods and leptodorids, but diets shifted to mysids, insect larvae and pupae in white perch above 40 mm TL (Rudershausen and Loesch 200). A similar diet shift at 40 mm TL (from copepods and daphnids to amphipods and chironomids) was documented in the diets of age-0 white perch in Oneida Lake, New York (Prout et al. 1990). Diets of white perch in the Richibucto Estuary, New Brunswick shifted at 50 mm TL from copepods and amphipods to sand shrimp (Crangon septenspinosa) (St-Hilaire et al. 2002).

Age-1 and older white perch are opportunistic, benthic feeders. Aquatic insects (chironomids, trichopterans, and ephemeropterans), amphipods, isopods, and zooplankton (cladocerns and copepods) all constitute seasonally important prey items for age-1 and older white perch (Alsop and Forney 1962; Moore et al. 1975; Elrod et al. 1981; Zuerlein 1981; Schaeffer and Margraf 1986; Weisberg and Janicki 1990; Danehy et al. 1991; Hurley 1992). White perch also feed on the eggs of fish. White perch have been documented eating the eggs of white perch (McGovern and Olney 1988), white bass (Schaeffer and Margraf 1987), alewife (Danehy et al. 1991), freshwater drum (Aplodinotus grunniens), and walleye (Roseman et al. 1996). Schaeffer and Margraf (1987) observed that in the Sandusky River tributary of Lake Erie, fish eggs constituted 100% (by volume) of white perch diets from May-June. In larger white perch (>200 mm TL), fish constitute an important prey item. When the prey was abundant, white perch have eaten gizzard shad (Schaeffer and Margraf 1986), other white perch, yellow perch (Madenjian et al. 2000), shiners (Notropis spp.) (Schaeffer and Margraf 1987), and bluegill (Lepomis macrochirus) (Zuerlein 1981).

Age and Growth - White perch prolarvae are approximately 3 mm TL, and postlarvae (yolk and oil globule absorbed) are approximately 4 mm TL (Mansueti 1964). Scales develop on white perch between 20 and 25 mm TL, after which most of the adult meristics are attained (Mansueti 1964). White perch growth is greatest during the first two growing seasons and decreases after age 3 (Alsop and Forney 1962; Marcy and Richards 1974; Zuerlein 1981).

Growth of white perch varies dependent on water body (Zuerlein 1981). In Nebraska reservoirs (Zuerlein 1981) and in Oneida Lake, New York (Alsop and Forney 1962), white perch reached 230 mm TL by age 3. Comparatively, white perch did not reach 200 mm TL until age 4 in the Lower Connecticut River (Marcy and Richards 1974), age 5 in the Fox River, Wisconsin (Cochran and Hesse 1994), and age 7 in the Delaware River (Wallace 1971). In the tidal James River, Virginia, mean length-at-age for white perch observed was 76 mm TL, age 1; 120 mm TL, age 2; 149 mm TL, age 3; 172 mm TL, age 4; 189 mm TL, age 5; 204 mm TL, age 6; 220 mm TL, age 7; 236 mm TL, age 8; 253 mm TL, age 9; 262 mm TL, age 10 (St. Pierre and Davis 1972). The maximum age of 17 and total length of 483 mm of a white perch was found in Maine (Jenkins and Burkhead 1993).

Growth of white perch can also be density dependent. Stunting of white perch can occur when population densities are high (Boileau 1985; Wong et al. 1999). When abundances increased, mean length-at-age of age-1 white perch declined from 130 mm TL in 1966 to 112 mm TL in 1967 in Wagon Trail Reservoir, Nebraska (Hergenrader and Bliss 1971), and from 96 mm TL in 1959 to 69 in 1965 in the lower Connecticut River (Marcy and Richards 1974).

Spawning and Reproduction - In its native estuarine environment, the white perch is semianadromous and spawns in the spring when water temperatures are between 10 and 16º C (Mansueti 1961; Jenkins and Burkhead 1993). In Kerr Reservoir, this temperature range occurs in May, the same month when the majority of striped bass spawn (Neal 1969). In landlocked waters, white perch spawn in both river and reservoir areas, and migrate from deep to shallow waters to spawn when temperature are between 15 and 20º C (Zuerlein 1981). White perch have no preference for habitat types during spawning and egg deposition (Zuerlein 1981). Spawning occurs in a 1- to 2-week period, with individual females expelling eggs on more than one occasion (Mansueti 1961). Female white perch are oviparous, broadcasting demersal, adhesive eggs to be fertilized externally (Mansueti 1961). White perch fecundity ranges between 20,000 and 150,000 eggs per individual female (Jenkins and Burkhead 1993).

White perch maturation is size-specific with males maturing at smaller sizes than females (Mansueti 1961). In the Patuxent Estuary, Maryland, male white perch began maturing at 80 mm SL (50% mature by 100 mm SL), while females began maturing at 90 mm SL (50% mature by 105 mm SL) (Mansueti 1961). In this same system, all males were mature by age 2 and all females by age 4 (Mansueti 1961). Zuerlein (1981) found that male and female white perch matured earlier (by age 1 with total lengths between 102-127 mm) in Nebraska reservoirs where they exhibit rapid growth.

Goal and Objectives

Scientific evidence reveals that white perch can be detrimental to sportfish populations when introduced to inland waters. Reported negative effects of the white perch are mostly due to some form of predation and/or competition. However, it has also been reported that white perch can add to the forage base for piscivorous fishes in these systems when less than ideal forage is unavailable. My research focused on the trophic impacts, both beneficial and detrimental, of white perch on pre-existing sportfishes (striped bass, largemouth bass, and crappie) in Kerr Reservoir, Virginia. The study particularly focused on the impacts to striped bass. The Kerr Reservoir self-sustaining striped bass population supports a very popular sport fishery, but there is growing concern among striped bass anglers that the fishing success has decline. Anglers have requested supplemental stockings of striped bass to improve the fishery. Specific biological data to support a need for supplemental stockings have not as yet been developed, but the history of the white perch provides many reasons for concern.

Findings from this study may also reveal important information on the effects of white perch introductions to the community of fishery mangers and professionals. The range of the white perch is extending rapidly, and climate warming may accelerate range expansion (Johnson and Evans 1990). Fisheries personnel must recognize the effects on and changes to sportfish communities after white perch invasions to minimize impacts. This study was one of the most comprehensive investigations undertaken to date on the trophic interactions between white perch and other sportfishes. The goal of this two-year study was to describe the trophic relationships and assess impacts of white perch on the Kerr Reservoir sport fish community, especially striped bass: Specific objectives were to:

1. Describe the intensity and extent of white perch predation on the eggs and larvae of co-occurring sportfishes;

2. Evaluate the potential for trophic competition between white perch and age-0 sportfishes;

3. Quantify the contribution of white perch to the diets of piscivorous sport fishes; and

4. Compare growth and abundance of sportfishes before and after white perch establishment.

STUDY SITE

Kerr Reservoir is a 19,790-ha impoundment on the Roanoke (Staunton) and Dan Rivers (Figure 1). The reservoir borders Virginia and North Carolina between Clarksville, Virginia and Henderson, North Carolina. Kerr Dam was constructed in 1952 and is operated by the U.S. Army Corps of Engineers for the purposes of flood control, hydroelectric generation, and water supply. The eutrophic reservoir is highly dendritic with 1,287 km of shoreline, and it has an annual water level fluctuation of approximately 5 m (Wilson 1997). The high water level fluctuations limit the establishment of significant aquatic vegetation (DiCenzo 2001) and result in severe shoreline erosion (Wilson 1997). The reservoir has a maximum depth of 32 m, a mean depth of 10 m, a retention time of approximately 98 days, and a drainage area of 2,020,184 ha (DiCenzo 2001).

Kerr Reservoir supports sport fisheries for blue catfish (Ictalurus furcatus), channel catfish (Ictalurus punctatus), crappie, flathead catfish (Pylodicyis olivaris), largemouth bass, striped bass, and walleye (Dicenzo 2001). The Kerr Reservoir crappie fishery is sustained by populations of both black crappie (Pomoxis nigromaculatus) and white crappie (Pomoxis annularis), but is dominated by black crappie (approximately 95% black crappie; V. DiCenzo, Virginia Department of Game and Inland Fisheries, personal communication). For my study black and white crappie were pooled for analysis, because anglers seldom differentiate between the two species (Mitzner 1981; Maceina and Stimpert 1998; Isermann et al. 2002; Sammons et al. 2002). The reservoir’s sportfish assemblage is supported by an abundance of available forage fish populations including threadfin shad (Dorosoma petenense), gizzard shad, alewife, and blueback herring (Alosa aestivalis). The reservoir is renowned for its largemouth bass and trophy crappie fisheries. It also has one of the few self-sustaining landlocked striped bass stocks

DISCUSSION

The focus of this study was to evaluate the trophic relationships between white perch and sportfishes (striped bass, crappie, and largemouth bass) of Kerr Reservoir to identify interactions that may affect the recruitment and performance of the latter. Specific trophic interactions examined were predation on sportfish eggs, larvae, and juveniles by white perch, competition for food resources between white perch and sportfish species, and contribution of white perch to diets of adult sportfish. Additionally, trends in sportfish performance (abundance and growth) before versus after white perch establishment were evaluated and related to trophic interactions with white perch.

White Perch Age and Growth

In Kerr Reservoir, white perch reached 100 mm TL by age 1, 200 mm TL by age 4, 250 mm TL by age 6, and 300 mm TL by age 9. Similar patterns in white perch growth have been reported in North Carolina reservoirs (Tatum 1961; Carlander 1997) and in Lake Ontario (Sheri and Power 1969). Higher growth of white perch has been documented in Nebraska reservoirs, with perch attaining 230 mm TL by age 3. In their native estuaries, white perch growth is typically slower than exhibited in Kerr Reservoir, with perch not achieving 100 mm TL until age 2, and 200 mm until age 6 (Mansueti 1961; Wallace 1971; St. Pierre and Davis 1972; Marcy and Richards 1974; Cochran and Hesse 1994).

Indications that density-dependent factors are affecting white perch growth were apparent in Kerr Reservoir due to the observance of declining growth of white perch in years after they became abundant. Mean length of age-1 white perch declined from 124 mm TL in 1995 to 101 mm TL in 2005, and mean length of age-2 perch was reduced to 142 mm TL in 2004 compared to 159 mm in 1995. Density-dependent growth of white perch is common in waters where white perch are abundant (Hergenrader and Bliss 1971; Marcy and Richards 1974; Boileau 1985; Wong et al. 1999).

Density-dependent reduction in growth of white perch can often lead to severe stunting of white perch populations (Grice 1958). In Kerr Reservoir, the high abundance of white perch does not appear to dramatically affect growth to the point that the population has become severely stunted. In contrast, stunted populations of white perch in Indian Lake, Massachusetts did not approach 150 mm TL until age 8 and no white perch 200 mm TL or greater were collected (Grice 1958). In this lake, white perch grew less than 10 mm per year after age 2. Kerr Reservoir white perch were approaching 150 mm TL by age 2 and reached 200 mm by age 4. In Kerr Reservoir, anglers start to harvest white perch when they reach sizes of 150 mm and regularly harvest fish that are greater 200 mm (DiCenzo 2001). Kerr Reservoir white perch reached harvestable lengths by age 2 and preferred lengths by age 4.

Predation

Egg and Larval Predation by White Perch. - White perch collected in the Roanoke River were ovivorous in May of both years, consuming the eggs of striped bass and white perch. Predation on eggs of moronid species by white perch has also been observed in the Sandusky River tributary of Lake Erie (Schaeffer and Margraf 1987), and in the Pamunkey River, Virginia (McGovern and Olney 1988). Throughout the Roanoke River, striped bass eggs were low proportions of the diets of the white perch I collected during May of 2004 (< 1% by weight) and 2005 (12% by weight), and were eaten in low frequency (FOO < 11% in both years). In contrast, Schaeffer and Margraf (1987) reported that nearly 100% of white perch stomach contents by weight were moronid and walleye eggs from May to June in the Sandusky River.

Although egg predation by white perch was observed in both 2004 and 2005, consumption of striped bass eggs by white perch that I collected was much greater in 2005. Difference in intensity and distribution of sampling effort and a longer peak spawning period in 2005 were the likely causes for the observed increased egg predation. In 2004, white perch were only sampled during weekdays (Monday-Friday) and the majority of striped bass spawning occurred during a weekend period (May 7-9). To avoid missing the peak spawning period in 2005, white perch were collected on both weekday and weekend periods during May of 2005. Additionally, the peak spawning period of striped bass in 2005 was two days longer than in 2004, and the river had eggs in the water column for a longer period of time.

White perch consumption of striped bass eggs varied temporally and spatially during May of both years. Based on sampling results, white perch primarily fed on striped bass eggs during the peak period of striped bass spawning, and eggs were considerable proportions of the diet during this period especially in 2005 (31% by weight for all sites combined). Eggs were also eaten by a large percentage (FOO > 42% for all sites combined) of white perch collected during this period in 2005. During this peak striped bass spawning period, greater than 80% of the Roanoke River striped bass eggs produced are spawned (Neal 1969). It appears that Roanoke River white perch are not specifically targeting striped bass eggs, but are only consuming eggs at times when large quantities are present in the water column. Similarly, Roseman et al. (1996) observed that a large proportion of white perch (approximately 86%) collected in western Lake Erie contained walleye eggs when walleye incubation periods were prolonged, because these eggs were then abundant when the reproductive periods of the two species overlapped. Further evidence of opportunistic consumption of eggs by white perch is the egg cannibalism evidenced by white perch collected from the Roanoke River.

Striped bass eggs were primarily consumed at only two of the five locations that I sampled, Ridgeway Farms and Watkins Bridge. In 2004, striped bass eggs were only found in the stomachs of white perch collected at these two sampling sites. In 2005, 41% (Ridgeway Farms) and 52% (Watkins Bridge) of non-empty white perch stomachs collected during May contained striped bass eggs. At all other locations, frequency of occurrence of eggs in white perch diets never exceeded 6%. Striped bass eggs also constituted larger proportions of white perch diets at Ridgeway Farms and Watkins Bridge (> 30% by weight at both sites in 2005) versus all other locations where perch were collected (< 3% by weight at all locations in 2005). Egg predation by white perch was especially high on specific days during the peak spawn with up to 100% of the white perch collected on these days contained striped bass eggs. The increased instances of predation observed at these locations likely were due to the abundance of striped bass spawning within and in close proximity to these locations. Both Ridgeway Farms and Watkins Bridge sites are located within the mean spawning grounds of striped bass determined by Neal (1969) and Whitehurst (1982). I observed large numbers of striped bass spawning directly upstream and within this section of river in both 2004 and 2005, but not in other sections of the river. For this reason, large quantities of eggs were only available for white perch to consume in this section of river and directly downstream. By the time eggs reached the 360 Bridge site (14 km upriver from Kerr Reservoir), the majority of eggs appeared to have hatched because eggs were no longer observed in the water column.

The impact of egg predation on the recruitment of striped bass in 2005 is unknown, but it is possible that predation by white perch had some effect. Projected population sizes of white perch in the Roanoke River needed to consume between 50% and 90% of striped bass eggs produced during the peak spawn ranged from 138,680 to 531,619, depending on inputs of consumption and egg production. These estimates would only require recruitment rates of white perch to be between 8 and 27 perch/ha from the 19,790-ha Kerr Reservoir. White perch densities in Kerr Reservoir are likely higher than this, as indicated by VDGIF gillnet CPUE data with average catch rates of 15 perch/net in 2000 samples. Although a sufficient reservoir population of white perch was probably present, it is unlikely that they ascended the Roanoke River in high densities. At least 5,715 white perch/river km would have been needed in the 44-km section of river (between Clarkston and Watkins Bridges) to consume at least 50% of striped bass eggs spawned during the peak. Population estimates could not be calculated for white perch in the Roanoke River, but high densities were never observed during electrofishing collections in May of 2005. On the most efficient collection days, I was only able to collect between 20 and 40 perch/km from both sides of the river. Although capture efficiency was probably lowered due to turbidity and high flows, it is unlikely that population densities of white perch were more than one hundred times greater than catch rates. In Tennessee tailwaters, capture efficiency with boat electrofishers ranged from 5% to 54% dependent on the fish species collected (Ruhr 1957; Jacobs and Swink 1982). If capture efficiency of white perch was as low as 5% in the Roanoke River, collections numbers of 40 white perch per kilometer would only translate to 800 white perch in that river kilometer which is still well below projected estimates of white perch needed to have a substantial effect on striped bass recruitment.

In Kerr Reservoir, it appears that the occurrence of a peak striped bass spawning period is an evolutionary advantage because potential predators (e.g. white perch) are swamped with a large abundance of eggs during this peak spawning period. During the peak spawn, predators likely become satiated on eggs allowing for a large percentage of eggs to pass downriver and hatch. If eggs were spawned evenly over the entire spawning period of striped bass as opposed to a one-week period, white perch would have the able to consume much greater proportions of striped bass eggs produced annually. Reductions in percentage of predation mortality have been documented for salmon (Oncorhynchus spp.) fry when fry numbers increased and predators were swamped with the abundance of fry (Neave 1953; Hunter 1959; Peterman and Gatto 1978). Predator swamping has been observed to be an important ecological strategy for prey survival in many other organisms including mayflies (ephemeropterans), cichlids, Japanese flounder (Paralichthys olivaceus), greater snow geese (Chen caerulescens atlantica), common tern (Sterna hirundo), mule deer (Odocoileus hemionus), and bison (Bison bison) (Green and Rothstein 1993; Becker 1995; Whittaker and Lindzey 1999; Kellison et al. 2002; Gauthier et al. 2004; Matter and Mannan 2005)

Without estimates of current striped bass egg production and white perch population size in the Roanoke River, the true impact of egg predation is unknown. Projected number of white perch needed to consume given percentages of eggs were calculated with particular assumptions and uncertainties. Egg production estimates were gathered from historical studies on striped bass in the Roanoke River, and these estimates could be substantially different from current production numbers. Secondly, I assumed that these eggs were spawned evenly throughout the peak spawning period. If larger percentages of eggs were spawned on specific days, then greater densities of white perch would be need to consume eggs on that day. Additionally, sparse information on white perch daily feeding rates was available, so data for striped bass were substituted. Finally, the contribution of eggs to the diets of white perch could be higher than what was observed in this study, if the digestion rate for eggs was faster than for other foods (principally aquatic invertebrates). I could find no published study that compared digestion rates of fish eggs versus other food items for any species of fish. However, if striped bass eggs were underrepresented in the stomach contents that I examined, then the true number of white perch required to eat a set percentage of eggs would be lowered. The Roanoke River would require only 3,895 perch/km to consume 50% of striped bass eggs produced annually if white perch diets were completely (100%) composed of eggs. Even this lower white perch density was unlikely, given my much lower electrofishing catch rates. Additionally, the overall impact of egg predation on Kerr Reservoir striped bass recruitment is unknown, because the intensity of egg predation in the Dan River (the other important tributary of Kerr Reservoir for striped bass spawning; Whitehurst 1982) was not examined.

Predation by white perch on striped bass larvae was observed in 2005, but not 2004. Striped bass larvae were found in the stomachs of white perch at multiple sites in the Roanoke River (Ridgeway Farms, Watkins Bridge, and Staunton View), indicating that larval predation occurred throughout the river. Although predation occurred in more than one location, striped bass larvae were infrequently found in the stomachs of white perch (FOO < 2% at any site), and never exceeded 1% of white perch diets by weight. The occurrence of larval predation may have been underestimated due to the increased digestion rate of larval prey (Dowd 1986; Rogers and Barley 1991), but it did not seem to be rampant enough in the river to have any substantial effect on striped bass recruitment. Although successful hatches of striped bass eggs occur in the lower reaches of the Roanoke River (Whitehurst 1982), striped bass larvae quickly descend to the upper part of Kerr Reservoir (between 2 to 3 days depending on river flows). Because the majority of my white perch collection was concentrated in the Roanoke River, food habits of white perch should be examined in the upper end of Kerr Reservoir after the peak spawn to fully assess the intensity of striped bass larval predation.

Crappie eggs were found in the diets of a small sample (N = 11) of white perch collected during April of 2005 in Kerr Reservoir. The small sample size did not allow quantification of white perch food habits during spring months (March-May) in Kerr Reservoir, so the intensity of eggs or larval predation on early spring spawners (crappies and largemouth bass) in Kerr Reservoir is unknown. Future research should focus on examining food habits of white perch in Kerr Reservoir during the March-through-May period.

Juvenile Fish Predation by White Perch. - From June through December, age-1 and older white perch (> 80 mm TL) fed on a variety of fish species in Kerr Reservoir during both 2004 and 2005. From June through December, Lepomis species were the only sportfish species found in the diets of white perch, occurring in low proportions (< 7% by weight in any month) in both years. Consumption of centrarchid species by white perch has been observed in Nebraska reservoirs (Zuerlein 1981). In Kerr Reservoir, all Lepomis specimens consumed by white perch were small (< 30 mm TL) age-0 fish. Because other sportfish (crappie, largemouth bass, and striped bass) are spawned early in the spring compared to continuous summer spawning of Lepomis species, it is possible that they have outgrown vulnerability to predation by white perch before the summer months (June-August), when I was collecting white perch in Kerr Reservoir. To determine the predation potential of white perch on the juveniles of other sportfish, white perch food habits in Kerr Reservoir need to be examined in spring when age-0 sportfish are newly hatched and relatively small in size.

Predation on fish prey by white perch in Kerr Reservoir differed between 2004 and 2005. In 2004, fish were relatively low proportions of white perch diets from June through December (< 12% by weight for any month), while fish prey averaged 47% of white perch stomach contents by weight during these same months in 2005. During both years, the majority of fish consumed by white perch in Kerr Reservoir were small (< 50 mm) clupeids. Predation on other fish species (cyprinids, percids, and centrarchids) was also observed, but was infrequent. In 2005, temporal shifts in clupeid consumption by white perch were observed. From June through October, clupeids were smaller proportions of white perch diets compared to months of November and December (> 90% by weight). A strong year-class of threadfin shad was produced in 2005, and small age-0 clupeids were very abundant in fall months compared to other prey items. Temporal shifts in clupeid consumption by white perch have also been observed in Lake Ontario when alewives were abundant (Danehy et al. 1991) and in Lake Erie when gizzard shad were abundant (Schaeffer and Margraf 1986).

In Kerr Reservoir, white perch were piscivorous at relatively small sizes (by 80 mm in 2004 and by 150 mm in 2005), and fish were consumed consistently by all sizes classes of perch. This is contrary to the reported food habits of white perch in other waters where size-dependent shifts in fish consumption by white perch were observed. Fish were not regularly consumed by white perch until the perch reached sizes of 200 mm in Nebraska Reservoirs (Zuerlein 1981), Maine lakes (Warner 1974), and in the Susquehanna River, Maryland (Weisburg and Janicki 1990).

Trophic Competition

Food Habits of Age-0 Fishes. - Food habits of age-0 sportfishes varied between species and temporally with each species. Age-0 crappies displayed primarily invertivorous food habits, consuming mixtures of zooplankton and aquatic insects throughout the growing season; however age-0 crappie also ate small amounts of fish prey. The observed occurrence of fish in the diets of age-0 crappies was unusual compared to other studies. In other reservoirs, age-0 crappies have been documented consuming only zooplankton prey (Ewers and Boesel 1935; Schael et al. 1991; Devries et al. 1998; Pope and Willis 1998; Pine and Allen 2001; Dubuc and Devries 2002), or combinations of zooplankton and aquatic insects (Mathur and Robbins 1971; Ellison 1984; O’Brien et al. 1984). Generally, crappies do not regularly consume fish until they reach lengths greater than 150 mm (TL), typically reached in their second growing season (Marthur and Robbins 1971; Ellison 1984; O’Briean et al. 1984; Pine and Allen 2001). In Kerr Reservoir, age-0 crappies consumed fish prey at much smaller sizes (by 30 mm TL). In each year of the study, clupeids had very strong year classes (alewife in 2004 and threadfin shad in 2005), and all fish consumed by juvenile crappies were small age-0 clupeids. The occurrence of fish in the diets of age-0 crappies in Kerr Reservoir can be attributed to the abundance of small age-0 clupeids present in the reservoir.

Food habits of age-0 white perch were described for June through December 2004 but only for September and October in 2005, due to low collections of juveniles in 2005. It appeared that white perch experienced low reproductive success in 2005, as opposed to 2004 when age-0 white perch were very abundant. High interannual variability in year-class strength of white perch has been reported in other waters (Cacela 1989; Pace et al. 1993; Gopalan et al. 1998; O’Gorman and Burnett 2001); variability in year-class strength of white perch is thought to be a function of spring temperatures (O’Gorman and Burnett 2001). In both years when diets were quantified, age-0 white perch fed only on aquatic invertebrates (mostly copepods, daphnia, dipterans, and ephemeropterans). Food habits of age-0 white perch in Kerr Reservoir were similar to diet data collected on white perch in coastal waters, where key diets items were copepods, insect larvae and pupae, isopods, and mysids (Bath and O’Conner 1985; Rudershausen and Loesch 2000; St-Hilaire 2002), and in other inland systems where primary diet items were amphipods, copepods, daphnia, chironomids, and trichopterans (Leach 1962; Elrod et al. 1981; Prout et al. 1990; Hurley 1992; Gopalan et al. 1998).

Both age-0 largemouth bass and striped bass fed on a mixture of aquatic invertebrates and fish in June of both 2004 and 2005, but they consumed mostly fish prey from July through December of both years. Similar temporal shifts in food habits of age-0 largemouth bass have been documented in other systems (Pasch 1975; Shelton et al. 1979; Timmons et al. 1980; Keast and Eadie 1985; Mathews et al. 1992; Olsen 1996; Sutton 1997; Garvey et al. 1998; Olsen and Young 2003), and for striped bass in coastal waters (Merriman 1941; Heubach et al. 1963; Markle and Grant 1970; Schaefer 1970; Boynton et al. 1981; Rudershausen and Loesch 2000), and in other inland reservoirs (Stevens 1958; Gomez 1970; Ware 1970; Weaver 1975; Axon 1979; Higginbotham 1979; Saul 1981; Richardson 1982; Humphreys 1983; Van Den Avyle et al. 1983; Mathews et al. 1992; Sutton 1997; Rash 2003). Shifts in diets from invertebrates to fish for age-0 piscivores are dependent on predator size and the availability of ingestible prey (Pasch 1975; Shelton et al. 1979; Timmons et al. 1980; Keast and Eadie 1985; Garvey et al. 1998; Sutton 1997; Rash 2003). Previous studies in other Virginia reservoirs showed that this shift usually occurs at sizes between 120 mm and 130 mm for both largemouth bass and striped bass (Sutton 1997; Rash 2000). In this study, shifts occurred at similar or smaller sizes. Ontogenetic shifts to piscivory of age-0 largemouth were exhibited in fish at sizes of approximately 100 mm in 2004 and at 50 mm in 2005; age-0 striped bass exhibited shifts to piscivory when fish reached approximately 130 mm in 2004 but only 70 mm in 2005. These earlier ontogenetic shifts in diets of largemouth and striped bass were most likely a function of the abundance of age-0 threadfin shad present in Kerr Reservoir in 2005 as opposed to 2004 when the larger age-0 gizzard shad were more abundant.

Potential for Trophic Competition. - High similarities (Schoener’s Overlap >0.5) in diets between age-0 white perch and the three species of age-0 sportfish occurred in early June when all were largely invertivorous, but only overlap between age-0 crappies and white perch exceeded 0.6. Additionally, high diet similarities between white perch and sportfish were displayed in small largemouth bass (< 60 mm, Schoener’s Overlap > 0.53) and striped bass (< 100 mm, Schoener’s Overlap between 0.45 and 0.64). At these small sizes largemouth and striped bass consume large amounts of invertebrates similar to age-0 white perch, and have not made the ontogenetic shift to fish prey. Previous studies have found high trophic overlap between age-0 white perch and striped bass in their native estuaries when both species relied heavily on invertebrate prey (Setzler-Hamilton 1982; Rudershausen and Loesh 2000; St-Hilaire et al. 2002). Diet overlap of white perch versus crappies, largemouth bass, and striped bass has not been reported in inland waters, but white perch have been reported to have high trophic overlap with other age-0 species (alewife; yellow perch, and white bass) that have highly invertivorous diets in inland waters (Elrod et al. 1981; Emmons 1986; Parrish and Margraf 1990; Prout et al. 1990; Gopalan et al. 1998).

High trophic overlap values were also found between age-1 and older white perch versus adult crappies, with overlap values of 0.50 in 2004 and 0.73 in 2005. Both of these species fed on mixtures of zooplankton, aquatic insects, and fish in both years, causing high similarities in diets between these species. Catfish species were the only other species that regularly consumed aquatic invertebrates, but ictalurids fed more heavily on Asiatic clams and organic matter than on the zooplankton and insects consumed by white perch. All other adult sportfish (largemouth bass, striped bass, walleye, and white bass) were primarily piscivorous, and their diets had low overlap (< 0.38) with the diets of the primarily invertivorous white perch.

Similarities in diets indicated that competitive trophic interactions could occur between white perch and all age-0 species of sportfish, especially early in the growing season, and between adult white perch and crappies throughout the year. Determination of the degree and intensity to which trophic competition actually occurs depends on knowledge of the abundance of food resources (Wiens 1977), which was beyond the scope of this study.

The results of trophic competition are manifested in the performance of the competing species. Competitive interactions between fish have often been found to lead to reduced growth and increased mortality (Werner and Hall 1977; Hanson and Leggett 1985, 1986; Osenberg et al. 1988; Frank and Leggett 1994; Cushing 1988; Dong and DeAngelis 1998). Comparisons in temporal trends of sportfish growth and abundance (before vs. after white perch establishment) conclude this discussion.

Sportfish Utilization of White Perch

An analysis of the food habits of Kerr Reservoir sportfishes revealed that white perch were not utilized as a prey source by any species. In all instances, when sportfish preyed on fish they consumed primarily clupeids. Between 2000 and 2004, the mean catch rate of clupeids from fall gillnet samples was 23 fish/929 m² of net, which was nearly 2.5 times greater than catch rates of white perch (9 fish/929 m² of net). In Kerr Reservoir, clupeids are the most abundant forage fish and are readily available for consumption by piscivorous predators.

Kerr Reservoir catfishes were omnivores, feeding on mixtures of aquatic invertebrates, fish, and filamentous algae. This generalist feeding behavior has been previously reported in other waters for blue catfish (Brown and Deny 1961; Minckly 1962; Perry 1969), channel catfish (Menzel 1943; Bailey and Harrison 1948; Armstrong and Brown 1983; Weisberg and Janicki 1990), and white catfish (Stevens 1959; Crumpton 1999). Although each catfish species was omnivorous, blue catfish consumed primarily clupeids (>52% by weight in both 2004 and 2005), whereas channel and white catfish diets were comprised large mixtures of invertebrates and filamentous algae. The difference in degree of fish consumption among the ictalurids was most likely a function of habitat preference and species-specific behavior. In reservoir systems, blue catfish prefer deep pelagic areas (Graham 1999) where clupeids may be abundant, while channel and white catfish prefer the shallower littoral areas (Klaassen and Marzolf 1971; Hubert and O’Shea 1992).

Kerr Reservoir crappies consumed mainly clupeids and aquatic invertebrates in both 2004 and 2005, with these prey categories representing approximately 98% of stomach contents by weight in both years. Crappie diets consisting of aquatic invertebrates and fish have also been reported in Florida Lakes (Reid 1949), Nebraska reservoirs (Ellison 1984), and Illinois reservoirs (Heidinger 1977, Heidinger et al. 1985). Although crappies consumed fish in the present study, they did not prey on white perch. The absence of white perch in crappie diets was most likely due to prey size, habitat differences, and alternative prey (clupeid) abundance. Gape-limited piscivores often consume deep-bodied prey much smaller than their maximum potential (Hoogland et al. 1956; Lawrence 1958; Kerby 1979; Gillen et al. 1981; Ott and Malvestuto 1981; Humphreys 1983; Hoyle and Keast 1987; Dennerline 1990; Hambright 1991; Juanes 1994; Dettmers et al. 1996; Sutton 1997). In Kerr Reservoir, adult crappies only ate small (< 80 mm) age-0 clupieds, and Kerr Reservoir white perch are only in this size range during summer months of their first growing season (June through September, see Figures 4 and 5). In reservoir systems, crappies primarily suspend in deep offshore areas near submerged structure during summer months (Grinstead 1969; O’Brien et al. 1984; Durfey 1986; Markham et al. 1991; Johnson and Lynch 1992; Guy et al. 1994). During my summer collections, small (< 100 mm) white perch were only collected in unvegetated, littoral areas where adult crappies were not present. Crappies appeared to rely on the more abundant, soft-rayed clupeid species that were available in offshore areas.

Kerr Reservoir largemouth bass were almost completely piscivorous, with clupeids (mostly threadfin and gizzard shad) being the primary prey fish consumed by adult and juvenile bass in both years. When abundant, soft-rayed clupeids have been shown to be the preferred prey of largemouth bass (Storck 1986), and their importance to diets of adult largemouth has been reported in many other waters (Aggus 1973; Lewis et al. 1974; Pasch 1975; Michaletz 1997; Carline et al. 1984; Moore 1988; Timmons et al. 1981; and Yako et al. 2000).

Over the two years of this study, only one captured largemouth bass contained a white perch (FOO < 2%). In contrast, Ward and Neumann (1998) reported that white perch were the most important and seasonally consistent prey item consumed by largemouth bass in Lake Lillinonoh, Connecticut, which lacked a clupeid prey base.

Striped bass fed almost exclusively on clupeids in both years. The high importance of clupeids to the diets of adult striped bass has previously been documented in their native estuarine environment (Gardinier and Hoff 1982; Walter and Austin 2003) and in inland waters (Stevens 1958; Combs 1978; Deppert 1979; Morris and Follis 1978; Kohler and Ney 1981; Persons and Bulkley 1982; Filipek and Tommey 1984; Moore et al. 1985; Bonds 2000). As opposed to shad, white perch were rarely found in diets of striped bass (FOO < 2%), and were of low importance (< 2% by weight) in both years. Gardinier and Hoff (1982) reported low occurrences (FOO approximately 11%) of white perch consumption by mature striped bass in the Hudson River Estuary in the presence of an alternative clupeid prey base. Additionally, Manooch (1973) found that striped bass preferred soft-rayed fish over spiny-rayed fish (e.g., white perch).

Food-habits analysis for walleye and white bass were questionable due to low sample sizes for each species. In other southern reservoirs, reliance on clupeid prey has been documented for both walleye (Dendy 1946; Momot et al. 1977; Fitz and Holbrook 1978; Moore 1988; Bonds 2002) and white bass (Olmstead and Kilambi 1971; Ruelle 1971). Only one white perch was found in stomachs of Kerr Reservoir walleye (FOO approximately 17%) and none were found in white bass. In Kerr Reservoir, walleye and white bass seem to prefer and depend on the more-abundant clupeids; however it has been documented that both species will prey on white perch when less desirable prey is not abundant (Forney 1974; Hartman and Margraf 1992; Hartman 1998).

Reservoir systems can be highly variable due to rapid fluctuations in water quality and levels (Summerfelt 1999), and shortages in clupeid prey can occur due to these changing conditions (Jenkins and Morais 1978; Jenkins 1979; Michaletz 1997). In times of low clupeid abundance, competition between sportfish species could intensify and white perch could add an alternative forage base for these predators (Forney 1974; Gardinier and Hoff 1982; Hartman and Margraf 1992; Ward and Neumann 1998). Additionally, white perch could be larger proportions of piscivores’ diets at different times of year. Piscivores were only collected in the fall for all species except largemouth bass, which were collected June through December. Hartman (1998) found that white perch were a seasonally important food resource for white bass during summer months (June-August) in Lake Erie. It is possible that piscivores fed on white perch at other times of the year, but it seems unlikely due to the high year-round abundant forage base of clupeids.

In summary, white perch are not utilized as a major food source for Kerr Reservoir sportfishes, but the occasional occurrence of white perch consumption indicates that they might become more important in the event of weak year classes of clupeids.

Sportfish Performance

Observed egg predation by white perch and potential trophic competition with white perch indicates that crappies are the sportfish species most likely to be adversely affected by white perch introductions. In B. Everrett Jordan Reservoir, North Carolina abundances of black crappie declined with concurrent increases in white perch abundance (Wong et al. 1999). Unfortunately, comparison in abundance and growth of Kerr Reservoir crappies before and after white perch establishment was not possible in the present study due to a lack of available or reliable data.

Relative abundance of pelagic fishes (striped bass and clupeid species) collected from annual gillnet samples were similar in years prior to and after white perch introduction, except for CPUEs of white bass (which declined after white perch establishment). DiCenzo and Duval (2002) determined that declines in abundance of Kerr Reservoir white bass were related to low April inflows from the Roanoke and Dan Rivers, which negatively affected year-class strength of white bass. Although interactions between Kerr Reservoir white bass and white perch were not featured in my study, their known interactions in other waters give cause to believe that white perch may have contributed to reductions in white bass populations. In Lake Erie, declines in white perch populations were correlated to the introduction and establishment of white perch (Madenjian et al. 2000), and were thought to be caused by some form of egg predation (Shaffer and Margraf 1987), competition (Gopalan et al. 1998), and/or hybridization (Todd 1986; Irons 2002). Although CPUEs of striped bass did not differ before and after white perch establishment, stocking history of striped bass in Kerr Reservoir could have affected comparative results. In years after white perch were established (1996-present), average numbers of striped bass fingerlings supplementary stocked in Kerr Reservoir were nearly four times greater than in years prior to white perch introductions (1974-87; see Table 1). Increased number of striped bass stocked in years after white perch compared to years before white perch could have facilitated similar CPUEs between the two periods even if white perch were affecting recruitment success of striped bass.

Largemouth bass relative abundances obtained from VDGIF electrofishing surveys were higher in years after white perch establishment (mean CPUE 55 fish/hour) versus years before white perch introductions (mean CPUE 25 fish/hour). It may seem that the rise in relative abundance of largemouth bass is an indication that interactions with white perch are minimal, but relative abundance increases were mostly likely due to enhanced capture efficiency of largemouth bass in years after white perch establishment (post 1995). In 1997, VDGIF personnel upgraded their electrofishing equipment and procedures, and a new modernized electrofishing boat and gear was used then and in later years (V. DiCenzo, VDGIF, personal communication). In Briery Creek Lake, Virginia, Wilson and Dicenzo (2002) found similar significantly higher higher catch rates of largemouth bass in years when the new modernized electrofishing gear (boat mounted anodes) was used as opposed to years when the older methods (hand-held anode) was used. Difference in capture efficiency appears to be the probable cause for the increase in relative abundance of largemouth bass in the period after white perch establishment, and comparison of abundance between the two periods may lead to faulty conclusions. Largemouth bass stock density estimates (PSD and RSD-P) were similar in years before and after white perch establishment.

Growth of largemouth bass and striped bass was evaluated by comparing length at age-3 fish for each species. Lengths of age-3 largemouth and striped bass collected in Kerr Reservoir were not statistically different between pre- and post-white perch years. Although no difference in growth of these age-3 sportfish was observed, differences in ageing procedures between time periods could have led to misleading results. In years prior to white perch establishment, all sportfish were aged using scales, but otoliths were used to age fish in years after white perch were established. Unlike otoliths, aging using scales has been found to lead to overestimation on ages of younger largemouth bass (Long and Fisher 2001) and reduced precision in age estimates with striped bass (Welch et al. 1993). Largemouth bass growth may have been greater than reported during years that scale-aging procedures were used, and therefore the comparison between age-3 largemouth bass before and after white perch establishment may have been faulty.

Although growth and abundance of Kerr Reservoir fishes were similar in periods before and after white perch, except abundance of white bass, observed predation on the eggs of sportfish and potential competition with the young of sportfish leads to some concern. It is possible that changes in resident fish dynamics were not observed due to sampling inconsistencies among the two periods. For future collections, sampling and recording procedures should be standardized to permit accurate annual comparisons.

Analysis of growth and abundance of largemouth bass and striped bass indicates that both parameters were similar before and after white perch introductions for each species. It does not appear that these species are competing for food resources with white perch, but differences in sampling and aging procedure could have lead to faulty comparisons. Crappie growth could not be evaluated, but overlap results suggest the potential for trophic competition.

SUMMARY AND CONCLUSIONS

1. Stomach contents of white perch and piscivorous sportfishes were collected in 2004 and 2005 to examine the trophic impacts (predation and/or competition) and benefits (as a forage resource) of the white perch in Kerr Reservoir. Adult white perch were collected throughout the Roanoke River and in Kerr Reservoir by boat electrofishing gear to determine whether they preyed on sportfish eggs, larvae, and juveniles. Throughout the growing season (June-December), age-0 white perch, crappie, largemouth bass, and striped bass were collected in Kerr Reservoir by seining and boat electrofishing to examine the potential for trophic competition between white perch and other sportfishes. Adult piscivores were sampled by gillnetting and electrofishing to determine whether they utilized white perch as a prey resource. Additionally, data from VDGIF annual sampling were used to compare growth and abundance of piscivores and forage fish of Kerr Reservoir before and after white perch establishment.

2. Age-and-growth analysis of white perch indicated that white perch growth does not appear to be stunted in Kerr Reservoir at this time. Kerr Reservoir white perch are approaching sizes of 150 mm TL by age-2 and surpass 200 mm TL by age-4. Growth of Kerr Reservoir white does not appear to limit the fishery for white perch at this time, because anglers start harvesting white perch at sizes of 150 mm TL.

3. Predation by white perch on striped bass eggs during the striped bass spawning run up the Roanoke River was observed in 2004 but was more intense in 2005. Striped bass eggs were eaten by up to 100% of the white perch collected at certain locations on days during the peak striped bass spawn in 2005. In both years, egg predation only occurred during the peak striped bass spawning period. Spatially, egg predation primarily occurred within or just below the mean spawning location of striped bass (Clarkton Bridge to Watkins Bridge). It appears that white perch opportunistically fed on striped bass eggs when and where eggs were in high abundance.

4. Although considerable egg predation was observed at times during 2005, projected population sizes of white perch large enough to significantly affect recruitment of striped bass likely were not present. Population densities greater than 5,000 perch/km would have been needed in the stretch of river where predation occurred to consume at least 50 % of the striped bass eggs spawned annually; electrofishing surveys captured fewer than 30 perch/km.

5. Predation by white perch on striped bass larvae was observed at several sites in the Roanoke River in May of 2005 during the striped bass spawning period, but appeared to be quite low. Only 4 of 986 (0.4 %) of white perch captured contained striped bass larvae. Instances of larval predation could have been higher than observed due increased digestion rates of larval prey, but still did not appear to be sufficiently intense to affect recruitment of striped bass. Striped bass larvae hatch in the 10-km stretch of the Roanoke River above Kerr Reservoir and are in the river less than one day. Larval predation by white perch may have been more intense in the upper end of Kerr Reservoir, but my sampling was not designed to assess this issue.

6. Predation by white perch on the eggs of crappies was detected during April of 2005 in Kerr Reservoir. White perch diets were not quantified during this period, so the frequency and impact of this predation to crappie populations in Kerr Reservoir is unknown. Further investigation into the food habits of white perch in Kerr Reservoir needs to be accomplished during the crappie spawning period (March-April) to determine the effect of egg and larval predation by white perch on this popular sportfish.

7. Instances of predation on the young of other sportfish by white perch were observed from June through December in Kerr Reservoir, but only age-0 Lepomis species were consumed. Frequency of predation appeared to be low with less than 5 % of 1165 white perch collected in both years containing Lepomis prey. Only small (< 30 mm TL) age-0 lepomids were observed in the diets of white perch; young of largemouth and striped bass had outgrown the size of prey targeted by white perch by June, when my sampling began. White perch food habits should be examined in Kerr Reservoir from April through May to determine whether white perch prey on the young of other sportfish when they are of vulnerable size.

8. White perch consumed large amounts of clupeid prey, especially at times when clupeids were more abundant than alternative invertebrate prey types. Clupeids comprised greater than 90% by weight of perch stomach contents in the fall of 2005, indicating that white perch will feed heavily on fish prey.

9. Both age-0 crappie species and age-0 white perch were highly invertivorous throughout their first growing season. Aquatic invertebrates represented greater than 90 % of age-0 crappie stomach contents by weight throughout the growing season of both years, while 100 % of age-0 white perch diets were composed of invertebrates during both growing seasons.

10. Age-0 largemouth bass and striped bass displayed ontogenetic shifts in food habits from invertebrates to fish in both 2004 and 2005, but shift occurred at smaller sizes than previously reported in other Virginia reservoirs. Diet shifts in largemouth bass occurred at sizes between 50 mm and 100 mm, while striped bass displayed shifts in diets at sizes between 70 mm and 130 mm.

11. Trophic overlap as an indicator of interspecific competition between age-0 white perch and young-of-year sportfishes was highest in early summer, when all fed heavily on zooplankton and aquatic macroinvertebrates. Largemouth bass and striped bass soon became piscivorous, but age-0 white perch did not. Significant diet overlap (>0.6, Schoener’s index) occurred only between white perch and crappie; both species were essentially invertivores throughout their first growing season.

12. Schoener’s (1970) Trophic Overlap Index revealed the potential for interspecific competition for food resources exists between adult white perch and adult crappies, but not between white perch and other adult sportfish species (catfishes, striped bass, largemouth bass, walleye, and white bass). Both adult white perch and crappies consumed considerable amounts of aquatic invertebrates whereas other sportfish were primarily piscivorous.

13. Analysis of food habits of Kerr Reservoir piscivores showed that utilization of white perch as forage was low. For both years, frequency of occurrence of white perch in piscivore diets was ~1% in a combined sampling of 526 striped bass, largemouth bass, and walleye. Most species fed primarily on the more abundant clupeid species, but the occurrence of predation on white perch indicates that perch could add an alternative forage base in times of low clupeid abundance.

14. Temporal comparisons of growth rates and relative abundance of major forage and sport fishes before and after white perch establishment in Kerr Reservoir revealed that white bass abundance has declined since white perch establishment. Low spring flows from the Roanoke and Dan Rivers are thought to be the primary cause for this reduction, but interactions between white perch and white bass in other systems is cause for concern. Growth and abundance of all other species (clupeids, striped bass, and largemouth bass) examined were either similar or greater in the period after white perch establishment.

The introduction of the white perch in Kerr Reservoir was cause for concern for management biologists due to the track record of the white perch in other waters, and reductions of year class strength of striped bass in Kerr Reservoir during the early 1990s. Results of this study indicated that there are potentially negative interactions between white perch and striped bass as well as other popular sportfish in Kerr Reservoir.

White perch preyed on the eggs of striped bass, and at times predation was intense. Allowing for low capture efficiency of white perch, it is unlikely that white perch densities were sufficiently high to result in consumption of a large fraction of striped bass eggs produced in the Roanoke River. The intensity of predation on eggs of other sportfish species by white perch is unknown, but the observance of predation on crappie eggs reveals that the potential for a substantial impact exists in Kerr Reservoir.

Potential competitive trophic interaction of white perch with largemouth bass and striped bass seem to be minimal and restricted to early months of the growing season. The abundant forage base of age-0 clupeids provides juvenile piscivores the ability to consume fish throughout the growing season. Abundance of clupeid species can be quiet variable in reservoir systems, and competitive interactions between these species might occur in years when the clupeid forage base is low.

This study indicated that crappies appear to have the highest potential for competitive interactions with white perch. Trophic overlap was high between these species for all life stages (juveniles and adults). Without some quantification of available food supply, it is not possible to assess whether these similarities in diets are suppressing the growth of crappies. Further, reliable data on crappie growth were not available in periods before white perch were introduced into Kerr Reservoir, preventing comparison of crappie growth before and after white perch establishment.

Growth and abundance of most sportfish have not declined since white perch establishment. It is possible that effects on performance of sportfish were not observed due to sampling inconsistencies between periods before and after white perch. Of all the sportfish examined, only white bass relative abundance has decline since white perch introduction. Interactions between these species were not examined during this study, but white perch introductions have preceded white bass decline in other waters.

The benefits of white perch as a forage fish to resident piscivores of Kerr Reservoir are minimal to nonexistent. Adult piscivores rely heavily on the abundant clupeid forage and do not readily consume spiny-rayed white perch, but might at times that clupeids are unavailable.

In summary, white perch are adding little benefit (with the exception of a small pan-fish fishery) to the overall Kerr Reservoir fishery, but appear to have some negative interactions with resident sportfish of Kerr Reservoir. Further research on white perch in Kerr Reservoir should be undertaken to better understand how these interactions are affecting the dynamics of these resident sportfish. Food habits of white perch should be examined in Kerr Reservoir and the Roanoke and Dan Rivers throughout spring months to truly assess the predation and competition potential of white perch in Kerr Reservoir.

MANAGEMENT RECOMENDATIONS

This study identified potential impacts of white perch on resident fishes of Kerr Reservoir, some of which merit more directed study. An alternative stocking regime for striped bass and consistency in assessment monitoring are among my recommendations to managers of the Kerr Reservoir fishery. My specific recommendations follow.

1. VDGIF should annually monitor year-class strength of striped bass in Kerr Reservoir and white perch relative abundance in the Roanoke and Dan Rivers. To assess year-class strength, monitoring of age-0 striped bass should take place in the upper end of Kerr Reservoir before supplemental stockings of striped bass occur (late May through June). During this time, age-0 striped bass could be collected with seining procedures described in Objective 2 of this study and/or with trawling techniques used by Whitehurst (1982). White perch relative abundance should be assessed during the peak spawning season of striped bass (April in the Dan River and May in the Roanoke River) using boat electrofishing gear.

Predation on striped bass eggs by white perch in the Roanoke River was observed in both years of this study. Although population sizes of white perch do not appear to be large enough in the Roanoke River to substantially affect natural recruitment of striped bass, the actual impact of egg predation is unknown. Further, the intensity of striped bass egg predation in the Dan River, the other important spawning tributary for Kerr Reservoir striped bass, was not examined. Having predictions on striped bass year-class strength and relative abundances of white perch in spawning waters of striped bass over multiple years would provide empiric evidence of a causal relationship (or its lack) between these two indices.

2. A study on white perch abundance and food habits should be conducted in the upper end of Kerr Reservoir (section of lake above mouth of Buffalo Creek) directly after the striped bass spawning period (May-June).

White perch fed on striped bass larvae in the Roanoke River, but larval predation appeared to be infrequent. Although larval predation was low in the Roanoke River, the overall intensity of striped bass larval predation by white perch is unknown because food habits of white perch were not examined in the upper end of Kerr Reservoir where striped bass larvae aggregate after hatching.

3. Future research should be accomplished to examine the food habits of white perch during spring months (April-May) and should be concentrated in the areas of Kerr Reservoir where Kerr Reservoir sportfish species (e.g. crappie and largemouth bass) primarily spawn.

The observance of crappie egg and juvenile centrarchid predation by white perch raise concern for how this predation affects populations of these species. In this study, diets of white perch were not examined in Kerr Reservoir during or directly after the spring spawning period of crappies and largemouth bass. Future research should be designed to determine the intensity of egg, larvae, and juvenile predation of these species by white perch and competitive trophic interactions between these species during this spring period.

I recommend stocking striped bass earlier in the growing season (May through early June) or at larger sizes (> 100 mm). Striped bass could be collected from the Roanoke River by Vic Thomas Hatchery personnel earlier in the spring than they have traditionally been collected, and spawning in these fish could be induced in hatchery facilities to provide earlier availability of fingerling striped bass for stocking (Sutton 1997).

In Kerr Reservoir, populations of striped bass are sometimes sustained by supplemental stockings of fingerlings. Potential competitive interactions between age-0 striped bass and white perch for food resources were observed between these species early in the growing season (June) and between smaller fish (< 100 mm TL). To minimize these potential interactions between these species, stocking strategies of fingerling striped bass could be adjusted. Traditionally, striped bass fingerlings have been stocked at the end of June at sizes less than 60 mm. Stocking striped bass earlier in the growing would allow juveniles to grow to lengths where ontogenetic shifts from invertebrates to fish prey occur before age-0 white perch are abundant in Kerr Reservoir.

5. VDGIF should continue to monitor abundance and growth of Kerr Reservoir crappies. Crappie species should be collected by traditional sampling gear and methods (spring trapnet surveys) for annual comparisons.

Potential negative interactions (predation and/or competition) were observed between Kerr Reservoir white perch and crappies, but comparisons of growth and abundance before and after white perch establishment were not performed due to insufficient data. To determine if these population characteristics are declining in the presence of white perch, indices growth and abundance of crappies should be monitored annually.

6. Future research should be conducted to examine the relationships between white bass and white perch in Kerr Reservoir. Specifically, intensity of white perch predation on eggs and juveniles of white bass and trophic competition with white bass should be accomplished.

Populations of white bass experienced drops in abundance after white perch were established, and published studies have shown that white perch can negatively affect white bass population dynamics through forms of predation, competition, and/or hybridization. This study did not evaluate interactions between these species in Kerr Reservoir.

7. VDGIF annual sampling should be performed with high consistency in regards to sampling gear and strategies, to assure future population data of Kerr Reservoir fishes can be used for comparative purposes.

Annual monitoring data collected by VDGIF personnel were essential to this research, but sampling inconsistencies between periods before and after white perch establishment in Kerr Reservoir created difficulties for comparisons between the two periods. In instances when sampling technologies are similar among years, consistency in sampling techniques ensures more accurate comparability in long-in data from long-term data set.

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INTRODUCTION

White Perch Biology and Ecology

Goal and Objectives

STUDY SITE

METHODS

Age and Growth Analysis

Objective 1: Predation

Objective 2: Trophic Competition

Objective 3: Sportfish Utilization of White Perch

Objective 4: Sportfish Performance

Statistical Analysis

RESULTS