Competition between Gizzard Shad and Sportfish

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MATERIALS AND METHODS

Study Area

Claytor Lake is a mainstream hydroelectric impoundment of the New River in Pulaski County, Virginia (Figure 1). Created by the Appalachian Power Company in 1939, Claytor Lake drains a 3862-km2 watershed, is 21.7 km long, and has a surface area of 1820 ha at a standard pool elevation of 663 m above mean sea level (Roseberry 1950). Maximum depth is 37.5 m, with a mean depth of 15 m (Kohler et al. 1986). Claytor Lake is distinctly riverine in morphometry, with a mean width of only 451 m and a retention time of approximately 33-63 days (Nigro 1980; DiCenzo 1996). The littoral habitat (< 5 m depth) along approximately 163 km of shoreline in Claytor Lake can be characterized as extremely narrow, rocky, and subject to continual wave action (Kelso 1983). Most littoral habitat suitable for shore-oriented species is found only in shallow coves and along the shoreline extending approximately 12 km upstream from the dam as well as in Peak Creek, Claytor Lake’s main tributary (Kelso 1983). An annual water level fluctuation of 1.6 m limits the establishment of rooted aquatic vegetation. Claytor Lake is dimictic (Nigro 1980), and the hypolimnion (> 5 m depth) frequently becomes anoxic during the summer (Boaze 1972). Claytor Lake averaged a total phosphorus concentration of 29.8 ppb, chlorophyll A concentration of 5.4 ppb, and secchi depth of 1.5 m from 1996 to 1998 (Thomas and Johnson 1998). Based on these values, the lake can be characterized as mesotrophic to moderately eutrophic (Carlson 1977; Hart 1981; Reckhow and Chapra 1983).
At least 14 fish species and two interspecific hybrids have been intentionally stocked in Claytor Lake since its formation in 1939, representing one of the primary management activities in this lake (Kohler et al. 1986). Certain species, such as threadfin shad, rainbow trout Oncorhynchus mykiss, and brown trout Salmo trutta, have failed to become established due to habitat limitations. Others, such as muskellunge Esox masquinongy, northern pike E. lucius, and tiger musky E. lucius X E. masquinongy, have failed due to minimal recruitment to the fishery. However, Claytor Lake now supports a diverse fishery of both native and introduced species. Popular sportfish species include largemouth bass, smallmouth bass Micropterus dolomieu, spotted bass M. punctulatus, white bass Morone chrysops, Lepomis spp., black and white crappie Pomoxis spp., channel catfish Ictalurus punctatus, flathead catfish Pylodictis olivaris, and walleye Stizostedion vitreum, which all reproduce naturally in the lake. Alewife and the recently introduced gizzard shad provide the major forage-fish base. In addition, annual stockings of fingerling striped bass Morone saxatilis and hybrid striped bass M. chrysops X M. saxatilis support a pelagic fishery in Claytor Lake

Field Collections

Limnetic larval (< 30-mm TL) gizzard shad, alewife, and Lepomis spp. were collected throughout the late spring and summer of 1997 and 1998 to determine the temporal distribution, growth, abundance, and diet of these fish species. The timing and intensity of peak larval abundances were specifically identified to assess the potential for interspecific competition based upon the extent of both temporal and spatial overlap among species. Cove (nearshore) and adjacent main channel (offshore) locations at three sites on the lake (Figure 1) were sampled at night on a weekly basis from mid-May to early/mid-August in each year. Cove and main channel locations at each site were sampled separately to evaluate differences in larval abundance in these distinctly different habitats. Sites were chosen to represent the upper lake and main tributary (Peak Creek – PK), middle lake (State Park – SP), and lower lake (Dam – DC). Sampling sites were selected to be representative of cove and main channel habitats in Claytor Lake. Cove habitat is somewhat limited in the lake, and those sites selected were chosen due to their lack of boat docks and other obstacles that would not have allowed enough area required for ichthyoplankton sampling. Additionally, I did not extend my efforts to the extreme uplake regions of the reservoir (uplake of the confluence of Peak Creek). Although evident on Smith Mountain Lake (Tisa 1988), research on large Missouri reservoirs found no consistent density gradients of larval shad based upon spatial location (Michaletz and Gale 1998). In Smith Mountain Lake, larval gizzard shad were almost exclusively limited to fertile, dendritic, uplake regions, while larval alewives were found in more oligitrophic, downlake locations near the dam (Tisa 1988). The high densities of larval gizzard shad found in uplake regions of Smith Mountain Lake were located in a region of the reservoir characterized by extensive coves and shallow flats favored by spawning gizzard shad. This type of habitat is extremely limited in Claytor Lake, especially in the uplake, riverine sections of the reservoir.
I also collected age-0 Micropterus spp. in 1998 from the littoral zone at each of the three sampling sites. Although young Micropterus spp. may be spatially segregated from larval clupeids in some reservoir systems (Jackson et al. 1990), Allen and DeVries (1993) found larval gizzard shad inshore and evenly distributed within 50 m of shore in West Point Reservoir, Alabama-Georgia, increasing the potential for interactions with littoral species. I therefore examined the diets of age-0 Micropterus spp. to quantify trophic overlap and utilization of larval gizzard shad as food by age-0 Micropterus spp. Adult alewives were collected in 1998 to evaluate potential diet overlap and resource competition between these fishes and larval gizzard shad.
I sampled zooplankton in Claytor Lake to relate temporal distribution, abundance, and composition of zooplankton to the abundance and diet composition of larval fishes. These data also were collected to determine whether larval gizzard shad appeared to depress the zooplankton population, or altered the species composition or size structure through selective feeding. I sampled zooplankton concurrently with all limnetic fish sampling

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Limnetic Larval Fish

Larval fish in the limnetic zone (> 5-m deep) were sampled with a neuston net (0.5-m x 1.0-m mouth, 4 m long, 1-mm bar mesh). A 5.5-m johnboat powered by a 60-hp outboard motor and equipped with a Lowrance model X70A (depth-finder and speedometer) was used to tow the net. The Lowrance calculated the speed of the boat, and together with the time traveled per tow and area of the mouth of the net, I was able to estimate volume of water filtered. A flow meter (General Oceanics model 2030) was mounted at the mouth of the net and also estimated volume of water filtered. I used this estimate to periodically check the accuracy of my original estimate provided by the Lowrance. The neuston net was towed at approximately 1.0-1.3 m/s for 5 min; other researchers (Kilch 1976; Nigro 1980; Cada and Loar 1982; Tisa 1988; Dettmers and Stein 1992; Jackson and Bryant 1993) have sampled larval shad Dorosoma spp. and alewife, and found that towing at approximately 1.0-1.8 m/s was an effective speed for capturing these clupeids. Because I was interested in reducing net avoidance by larger larvae, this speed appeared to be the fastest towing speed possible while minimizing the visible pressure wave in front of the net. Each tow filtered 150-195 m3 of water; 100 m3 is generally accepted as a minimum sampling volume in freshwater larval fish studies (Kelso and Rutherford 1996). The net was attached to the stern of the boat and towed approximately 18 m behind the boat in a circular pattern, thus keeping the net out of the boat’s wake and propwash. Because the neuston net floated and therefore sampled the top 0.5-m of the water column, all sampling for abundance estimates was conducted after dark to take advantage of surface-oriented and more randomly distributed larval fish while minimizing net avoidance (Kelso and Rutherford 1996). The surface-sampling neuston net was chosen instead of other larval nets because of its ease of handling and operation, and because other researchers had found success using the neuston net for collecting larval fishes on similar reservoir systems (Sammons and Bettoli 1998). Although the sampling depth of the neuston net was originally a concern, significant numbers of shad and alewife had previously been sampled at a depth of 1 m in nearby Smith Mountain Lake, Virginia (Tisa 1988), while larval gizzard shad were almost exclusively distributed in the top 1 m of the water column in Kansas reservoirs (Willis 1987). Three replicate tows were made at both nearshore and offshore locations at each of the three sites, equaling 18 tows per sampling night.
collected samples on approximately ten dates each year between mid-May and early/mid-August for a total of 180 tows per sampling season. Subsurface water temperature (approx. 0.25-m depth) was measured with a handheld thermometer at each site prior to sampling.
In both years, limnetic larval fish samples for stomach content analysis were obtained from the sampling methods just described. However, few larval fish sampled at night in 1997 contained identifiable (or any) food items in their digestive systems. Diet studies involving larval gizzard shad, alewife, and Lepomis spp. have presented mixed results concerning diel feeding patterns of these fish (Werner 1969; Barger and Kilambi 1980; Mallin et al. 1985; Dettmers and Stein 1992; Hayward and Hiebert 1993), suggesting that they may not always feed at night. However, through preliminary evening sampling in 1998, I found that many larval fish did feed at dusk. Therefore, in 1998, neuston net samples were also taken at dusk on a weekly basis in conjunction with nighttime tows. These dusk tows were taken exclusively for larval diet samples and were not designed to result in quantitative measures of fish abundance.
For the first two weeks of this study, I fixed larvae in approximately 40% ethanol (V. DiCenzo, VDGIF, and S. Sammons, Tennessee Tech University, personal communications). Unfortunately, this proved to be ineffective as a fixative as specimens exhibited deterioration of tissue and loss of body parts. Aldehyde-based solutions such as formaldehyde are better fixatives for preservation of ichthyoplankton because they immediately combine with tissue proteins and prevent proteins from reacting with other reagents (see Kelso and Rutherford 1996). Samples were therefore fixed in 10% formalin upon capture for the remainder of the study. Larvae were then transferred to 40% ethanol within 24 hrs. Although considerable attention has been given to the effects of preservation on the lengths of young fishes, results have been mixed (see Tisa 1988). Leslie and Moore (1986) reported that changes in body measurements of freshwater larval fishes associated with fixatives, including formalin and ethanol, were of little or no practical consequence for taxonomy and growth studies. Because measurements of larval fish were taken within 24 to 48 hrs after collection, I assumed the measurements in this study reflected live-state conditions

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Littoral Fish

In 1998, I sampled littoral age-0 Micropterus spp. during the day using seines (3 m X 1.3 m, 1.5-mm mesh; 4 m X 1.3 m, 5-mm mesh) and larval dip nets (45-cm mouth diameter, 500-µm mesh) for food habit analyses. Two to four seine hauls of approximately 20 m were made parallel to shore in the littoral zone at each site. Larval dip nets were periodically used to sample littoral fishes when larval fish aggregations were visually located in our sample sites. These samples were made both while wading and from the boat. However, this sampling method was neither efficient nor effective in capturing larger (> 20 mm TL) and more mobile age-0 fish. Littoral samples were taken once per week, usually on the same date as limnetic larval tows. Age-0 fish collected for stomach analysis were suffocated in air to prevent regurgitation (Kohler 1980), fixed in 10% formalin, and transferred to 40% ethanol within 24 hrs

INTRODUCTION 
Competitive Interactions between Gizzard Shad and Resident Fishes
Zooplanktivory.
Competition between Gizzard Shad and Sportfish
Competition between Gizzard Shad and Alewife
Goals and Objectives
MATERIALS AND METHODS
Study Area
Limnetic Larval Fish
Littoral Fish
Adult Alewife.
Crustacean Zooplankton
RESULTS 
Larval Distribution
Distribution in 1997
Distribution in 1998
Diet Composition
Larval Diet
1997 – General Description and Composition
Trends in Biomass of Gizzard Shad, Lepomis spp., and Micropterus spp.
Trends in Growth Rates of Lepomis spp. and Micropterus spp
DISCUSSION
Larval Distribution
Diet Composition
Zooplankton Density
Response of Resident Fish Populations to Gizzard Shad Introduction
SUMMARY AND CONCLUSIONS
REFERENCES
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