Survival and Growth of Intensively Reared Large Walleye Fingerlings ...

North American Journal of Fisheries Management 20:337–348, 2000 q Copyright by the American Fisheries Society 2000

Survival and Growth of Intensively Reared Large Walleye Fingerlings and Extensively Reared Small Fingerlings Stocked Concurrently in Small Lakes MARK H. OLSON,1 THOMAS E. BROOKING,* DAVID M. GREEN, ANTHONY J. VANDEVALK, AND LARS G. RUDSTAM Department of Natural Resources and Cornell Biological Field Station, Cornell University, 900 Shackelton Point Road, Bridgeport, New York 13030, USA Abstract.—Size of walleyes Stizostedion vitreum at the time of stocking is thought to play a key role in determining the success of stocking programs. Because starvation and predation risk are size-dependent processes, fingerlings stocked at larger sizes are expected to survive better than fingerlings stocked at smaller sizes. However, large fingerlings often require different culture techniques that make them more expensive to produce. Therefore, large fingerlings should only be used if returns are high enough to offset costs. We evaluated the relative success of two sizes of stocked walleye fingerlings in four lakes in New York State. For four consecutive years (1993– 1996), each lake was stocked with equal densities (50 fish/ha) of small (30–50-mm), extensively reared pond fingerlings in June and large (120–140-mm), intensively reared fall fingerlings in September. In October, we electrofished each lake to determine growth and survival of stocked fingerlings. Despite being stocked at a much later date, average catch rates of fall fingerlings were not significantly higher than pond fingerlings. The relative survival of fingerlings varied among lakes, and in two lakes survival of fall fingerlings was actually lower than pond fingerlings. Similar patterns of survival were observed for walleyes caught as yearlings and for catches of older walleyes in gill nets. Fall fingerlings were also significantly smaller than pond fingerlings at the end of the first growing season, and this disparity persisted 3 years after stocking. Overall, our results indicate that relative success of the two sizes of stocked walleyes varies among lakes and that stocking large walleye fingerlings will not necessarily lead to higher returns.

The success of a stocking program is often thought to depend on the size of fish at the time of stocking (Carline et al. 1986; Stroud 1986; Madenjian et al. 1991). In general, survival of fish stocked as larvae or small fingerlings is low because of factors such as physical stress, predation, or starvation (Houde 1987; Ludsin and DeVries 1997). Because vulnerability to these factors typically decreases with size (Werner and Gilliam 1984; Miller et al. 1988), survival and return rates of fish stocked at larger sizes are expected to be high relative to smaller fish (Hanson et al. 1986; Parker 1986). However, the expected benefits of stocking large fish must be balanced by increased rearing expenses associated with longer keeping of fish in a hatchery (Weithman 1986; Madenjian et al. 1991). Furthermore, production of large fish may be more limited if intensive culture techniques are required (Conover 1986; Fenton et al. 1996). Therefore, decisions to stock large fish rath-

* Corresponding author: [email protected] 1 Present Address: Department of Biology, Franklin and Marshall College, Post Office Box 3003, Lancaster, Pennsylvania 17604-3003, USA. Received June 22, 1999; accepted November 1, 1999

er than small fish in a given system must be made carefully. A key to developing an effective stocking strategy is to test the generality of the hypothesis that fish stocked at larger sizes will have substantially higher survival than smaller fish. If the expected pattern of differential survival is widespread, then stocking decisions can be based on priority or need (i.e., large fish can be stocked where a fishery is most desired). However, if exceptions to the expected pattern are common, then compared with smaller fish, large fish must be stocked strategically into waters where they are more likely to survive (Noble 1986). In that case, an understanding of the factors that influence stocking success of different fish sizes is critical for the identification of suitable candidate waters (Bennett and McArthur 1990). Currently, empirical evaluations of stocking success provide no consensus about the most effective strategy for stocking of walleyes Stizostedion vitreum (Laarman 1978; Ellison and Franzin 1992). One commonly used strategy is to extensively rear walleye fingerlings to total lengths of 30–50 mm before being stocked in midsummer (Buttner et al. 1991). Although this strategy has

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TABLE 1.—Description of study lakes, which were all located in New York. Conductivity was measured in spring 1993. Walleye catch per unit effort (CPUE), determined by electrofishing in fall 1992, measured the prestocked population and excluded walleyes stocked that year.

Lake

County

Area (ha)

Cayuta Eaton Brook Findley Sixtown Pond

Schuyler Madison Chautauqua Jefferson

152 103 111 70

been successful in some systems, returns have been highly variable and sometimes very low (McWilliams and Larscheid 1992). Therefore, an alternative strategy is to intensively raise walleyes through their first growing season and release them in their first fall at total lengths of 100 mm or larger (Laarman 1981; Green 1986). Intensively reared fingerlings are expected to have higher survival and return rates than the smaller extensively reared fingerlings. Because of differences in culture technique, however, these fingerlings are more expensive to produce than extensively reared fingerlings. For example, production of intensively reared fingerlings in New York in 1995 cost approximately US$0.80/fish, whereas extensively reared fingerlings cost $0.30/fish. In a meta-analysis of stocking evaluations, Ellison and Franzin (1992) found that large fingerlings were considered to be successfully stocked in 50% of stocking efforts, compared with only 32% for small fingerlings. However, in a direct comparison of the two stocking sizes, Koppelman et al. (1992) found that survival of small fingerlings was actually higher than large fingerlings in two Missouri reservoirs. These conflicting results call into question the assumed superiority of large fingerlings and suggest that the generality needs to be explicitly tested. In this study, we evaluated the stocking success of two sizes of walleye fingerlings in a set of four lakes that had been identified by the New York State Department of Environmental Conservation as a high priority for restoration of the walleye fishery (Festa et al. 1987). In each lake, we stocked extensively reared small fingerlings and intensively reared large fingerlings at equal densities for a period of 4 consecutive years (1993–1996). At the end of each growing season, we evaluated relative survival and growth of age-0 and older walleyes stocked at the two sizes. Our main objective was to test the hypothesis that walleyes stocked as fall fingerlings are more successful than pond fingerlings.

Maximum depth (m)

Conductivity (mS/cm)

Walleye CPUE (number/h)

7.0 16.0 11.6 7.3

120 100 170 220

0.0 1.1 0.0 15.6

Study Sites The four lakes in this study were in central and western New York State. Physical characteristics of each lake are summarized in Table 1. All lakes were small (70–152 ha) with maximum depths ranging from 7 to 16 m. Fish assemblages were predominantly warmwater species, such as bluegill Lepomis macrochirus, pumpkinseed L. gibbosus and yellow perch Perca flavescens. Cayuta Lake also contained high densities of alewives Alosa pseudoharengus. The predominant predators were largemouth bass Micropterus salmoides, smallmouth bass M. dolomieu, chain pickerel Esox niger and northern pike E. lucius. At the start of the experiment, adult walleyes were present in Eaton Brook Reservoir and Sixtown Pond but were not present Cayuta Lake or Findley Lake (Table 1). However, Miller-type sampling for larval fish (Noble 1970) in early May of each year indicated that natural reproduction was extremely rare in our study lakes (Brooking et al. 1997). Depth-stratified sampling of at least 900 m3 of water produced no wild walleye larvae in Cayuta or Findley lakes or in East Brook Reservoir. In Sixtown Pond, wild larvae were collected in 1994 at an estimated density of 0.008/m 3 (Brooking et al. 1997). No wild larvae were collected in Sixtown Pond in any other year. During the 4 years of this study (1993–1996), all lakes were also stocked with walleye larvae at a density of 12,000/ha. Before stocking, these larvae were immersed in 500 mg/L oxytetracycline hydrochloride (OTC) for 6 h to mark their otoliths with a fluorescent ring (Brooks et al. 1994). Otolith analysis indicated that stocked larvae composed less than 0.7% of the walleyes collected in fall and were absent in 13 out of 16 cases (Brooking et al. 1997). Methods Fingerling culture and stocking.—All fingerlings in this study originated from spawn collected

SURVIVAL AND GROWTH OF WALLEYE FINGERLINGS

from adults in Oneida Lake, New York. Eggs were fertilized and hatched at the Oneida Fish Cultural Station (OFCS) in late April of each year. After hatching, larvae were divided into two groups, one extensively pond-reared to a small size and stocked in June (pond fingerlings) and the other intensively reared to a large size and stocked in September (fall fingerlings). Pond-fingerling larvae were transported to the South Otselic Fish Cultural Station within 3 d of hatching and stocked in either 0.3- or 0.5-ha earthen ponds at a density of 20,000 or 40,000 larvae per pond, respectively. Larvae in these ponds fed on zooplankton until late June, when large-bodied zooplankton species (e.g., Daphnia spp.) became rare. By that time, pond fingerlings had reached 43–51 mm total length (TL) and were stocked into study lakes at a density of 50/ha. In each lake, pond fingerlings were released from boats along the outer edge of the littoral zone. The second group of larvae were raised as fall fingerlings in 4,200-L hatchery tanks (approximate dimensions: 7.6 m long 3 1 m wide 3 0.5 m deep) at OFCS. These walleyes were housed in tanks at an initial density of approximately 50,000 per tank and fed a progression of food items beginning with brine shrimp and continuing with increasing sizes of dry pellets. Mortality reduced densities over time to approximately 6,000/tank by mid-September (R. T. Colesante, New York State Department of Environmental Conservation, personal communication). At that time, the surviving walleyes, at sizes of 115–140 mm TL, were stocked at a density of 50/ha using the same stocking techniques as were used for pond fingerlings. Before stocking, we clipped the left pelvic fin of fall fingerlings in 1993 and 1995 and their right pelvic fin in 1994 and 1996. Sampling.—Relative survival of walleye fingerlings was evaluated each fall by boat electrofishing. Sampling began at least 3 weeks after fall fingerlings had been stocked. Most or all of the shoreline was electrofished in each lake, beginning after sundown using DC with voltage, frequency, and pulse width adjusted to produce a current of approximately 8 A. For each stocking strategy (pond or fall fingerling), a single estimate of catch per unit effort (CPUE) was calculated as the number of age-0 or age-1 walleyes caught per hour of electrofishing, based on an average of 5.6 h of electrofishing (range: 3.8–8.9 h). For each walleye caught, length (TL to the nearest 1 mm) and mass (to the nearest 2 g) were recorded and pelvic fins were examined for a clip.

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Up to 30 unmarked age-0 walleyes were kept for OTC analysis to distinguish pond fingerlings from walleyes stocked as larvae. In 1995 and 1996, up to 30 age-0 walleyes of both stocking strategies were sacrificed and stomach contents were examined for evidence of piscivory. For older walleyes, a sample of scales was taken from behind the depressed left pectoral fin to compare growth rates of pond and fall fingerlings. To verify indices of abundance from electrofishing and to determine which stocking strategy recruited more walleyes to the fishery, abundance was also estimated from gill nets set in mid-August 1996. In each lake, five to seven variable-mesh monofilament gill nets (45.7 m long 3 1.8 m deep, mesh sizes were 3.8, 5.1, 6.4, 7.6, 8.9, and 10.2 cm stretch) were set overnight for 18 h. Each walleye was measured, weighed, and inspected for a fin clip. Scales were also taken from each fish for age and growth determination. Data analysis.—Differences in catch rates of age-0 and age-1 walleyes stocked as pond or fall fingerlings were evaluated using mixed-model analysis of variance (ANOVA). In these models, lakes and years were treated as random effects (or blocks) and stocking strategy was considered a fixed effect. Denominator degrees of freedom for tests of main effects were approximated using Satterthwaite’s formula and rounded down to the nearest whole number (Littell et al. 1991). In our analyses of catch rates, each combination of lake, year, and stocking strategy was represented by a single measure of walleye catch rate. Because we had no replication within each lake– year stocking-strategy combination, we could not test for the three-way interaction among these main effects. Therefore, ANOVA could only be used to examine the main effects of each factor and the three two-way interactions. To better interpret significant two-way interactions involving stocking strategies and lakes, we also used t-tests that compared pond and fall fingerlings separately for each lake (with years serving as replicates). Analyses of catch rates were based on a total of 32 observations for age-0 walleyes (two stocking strategies by four lakes by 4 years) and 24 observations for age-1 walleyes (fish stocked in 1996 were not sampled in 1997). Because we had few denominator degrees of freedom in our analyses, we considered P , 0.10 to be significant. Before analysis, catch rates of age-0 and age-1 walleyes were log-transformed, loge(x 1 1) and loge(x 1 0.1) respectively, to homogenize variance and normalize residuals. We used a smaller constant to

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transform catch rates of age-1 walleyes because most values were between 0 and 1. All analyses were performed using the Statistical Analyses System (SAS Institute 1989). Walleye CPUE in gill nets was also analyzed with a mixed-model ANOVA. In this analysis, stocking strategy was considered a fixed effect and lake was a random effect. Each gill-net set in 1996 was treated as a replicate (i.e., each net was treated as an independent estimate of walleye abundance in a lake) and catch rates of all walleyes older than age-0 were pooled to give a single estimate of walleye abundance per net. To interpret the interaction of stocking strategy and lake, we used ttests that separately compared differences between stocking strategies for each lake. Before analysis, data were log-transformed, loge(x 1 1), to homogenize variance and normalize residuals. Growth of age-0 walleyes was estimated as mean total length at the end of the growing season (in October). Differences in mean length of walleyes stocked as pond or fall fingerlings were analyzed with ANOVA using the same model used for catch rates. In two study lakes, we collected sufficient numbers of older walleyes to directly compare growth rates beyond the first year. In these two lakes, growth rates were back-calculated from scales using the Fraser-Lee method with an intercept of 55 mm (Carlander 1982). Total lengths at each age were converted to mass using regressions developed for each stocking strategy–lake combination. Using analysis of covariance (ANCOVA), we separately compared size-dependent growth rates (i.e., annual change in mass versus mass at the start of the year; see Osenberg et al. 1988) of the two stocking strategies in each lake. Changes in mass and initial mass were both logtransformed (logex) before analysis to linearize relationships. Whenever fingerlings of both stocking strategies were collected in the same lake and year, we used a chi-square test to compare numbers of walleyes with fish in their stomachs to numbers with empty stomachs. These tests determined whether the frequency of piscivory differed between pond and fall fingerlings. Results Electrofishing and Gill Net Catch Rates Electrofishing catch rates of age-0 walleyes were similar for the two stocking strategies. Mean (6 SE) catch rates for the 16 lake–year combinations were 3.36 6 0.93/h for pond fingerlings

and 3.71 6 1.49/h for fall fingerlings. Pondfingerling catch rates ranged from zero, which occurred in three cases, to 12.9/h in Eaton Brook Reservoir in 1994. Fall-fingerling catch rates ranged from zero (in five cases) to 20.9/h in Findley Lake in 1995 (Figure 1). Across all lakes and years, catch rates of pond and fall fingerlings were not correlated (by using non-transformed data, r 5 0.24, N 5 16, P 5 0.36). ANOVA confirmed the lack of a difference in overall catch rates of pond and fall fingerlings. Although the overall ANOVA model for age-0 catch rates was significant, the main effect of stocking strategy was not (Table 2). Rather, ANOVA detected a stocking strategy 3 lake interaction (Table 2). This interaction was caused by lake-specific differences in relative abundance of pond and fall fingerlings (Figure 1). In Cayuta and Findley lakes, mean catch rates of fall fingerlings over the 4 years of stocking were higher than pond fingerlings (Figure 1a, c). In contrast, mean catch rates of pond fingerlings exceeded fall fingerlings in Sixtown Pond and Eaton Brook Reservoir (Figure 1b, d). Because of high interannual variability in catch rates and generally low numbers of walleyes collected, within-lake differences in relative abundance of pond and fall fingerlings were significant (paired t-tests) only in Sixtown Pond (Cayuta: t4 5 20.70, P 5 0.51; Eaton Brook: t4 5 1.37, P 5 0.22; Findley: t4 5 20.52, P 5 0.63; Sixtown: t4 5 2.37, P 5 0.06). In addition to the interaction of stocking strategy and lake, the ANOVA on age-0 catch rates also detected a significant lake 3 year interaction (Table 2). This interaction indicated that maximum combined catch rates of pond and fall fingerlings were observed in different years among lakes (Figure 1). Mean catch rate, averaged across stocking strategies, was highest in Cayuta Lake in 1996 (Figure 1a), in Eaton Brook Reservoir in 1994 (Figure 1b), and in Findley Lake in 1995 (Figure 1c). In Sixtown Pond, mean catch rates were similar in 1994–1996 (Figure 1d). In Findley Lake, maximum catch rates of pond fingerlings also occurred in different years than the maximum catch rates of fall fingerlings (Figure 1c, d). Within each stocking strategy, annual variation in catch rates was not significantly related to variation in total length at the time of stocking. Although mean size at the time of stocking varied among years, 43– 51 mm for pond fingerlings and 115–140 mm for fall fingerlings, ANCOVA conducted separately for each stocking strategy (with lake as a main effect and size-at-stocking as a covariate) was not


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FIGURE 1.—Fall electrofishing catch rates (number [N]/h) of pond and fall fingerlings in (a) Cayuta Lake, (b) Eaton Brook Reservoir, (c) Findley Lake, and (d) Sixtown Pond. Catch rates are presented for each year separately. Mean catch rates of pond and fall fingerlings over the 4 years of stocking are also presented on the right side of each figure. Error bars denote 1 SE. TABLE 2.—Analysis of sources of variance for catch rates of age-0 and age-1 walleyes. For both age-classes, overall analysis of variance models (i.e., tests of full models with all main effects and interaction terms) were significant (age-0: F 22.9 5 2.64, P 5 0.06; age-1: F17,6 5 4.25, P 5 0.04). Degrees of freedom are listed for the numerator and denominator. For random effects and interactions that involved random effects, denominator degrees of freedom were approximated using Satterthwaite’s formula (Littell et al. 1991) and rounded down to the nearest whole number. Significant results are in bold italic. Fvalue

P

Stocking strategy Lake Year Strategy 3 lake Strategy 3 year Lake 3 year

Age-0 walleyes 1, 2 0.19 3, 6 1.36 5, 5 1.13 3, 9 1.00 3, 9 0.23 9, 9 1.16

0.22 0.76 1.09 2.77 0.64 3.21

0.69 0.56 0.42 0.10 0.61 0.05

Stocking strategy Lake Year Strategy 3 lake Strategy 3 year Lake 3 year

Age-1 walleyes 1, 2 0.04 3, 4 0.49 2, 4 0.68 3, 6 0.56 2, 6 0.09 6, 6 0.24

0.06 0.68 2.71 6.56 1.10 2.83

0.82 0.60 0.18 0.03 0.39 0.12

Source

df

Mean square

significant (pond: F7,8 5 1.23, P 5 0.38; fall: F7,8 5 2.20, P 5 0.15). For both pond and fall fingerlings, catch rates of age-0 walleyes were correlated with catch rates of age-1 walleyes collected in the following year (r 5 0.52, N 5 24, P 5 0.01). This relationship between catch rates of age-0 and age-1 walleyes was the same for both stocking strategies. The ANCOVA on age-1 catch rates detected a significant effect of age-0 catch rates from the preceding year, whereas the effect of stocking strategy and the interaction of stocking strategy and age-0 catch rates were not significant (overall model: F3,20 5 2.44, P 5 0.09; age-0 catch rate: F1,20 5 7.08, P 5 0.02; stocking strategy: F1,20 5 0.03, P 5 0.85; interaction: F1,20 5 0.05, P 5 0.83). Therefore, patterns of age-1 abundance were similar to those observed for age-0 walleyes. As was the case for age-0 walleyes, there was a significant interaction between lake and stocking strategy (Table 2) due to lake-specific patterns of relative abundance of age-1 walleyes stocked as pond or fall fingerlings (Figure 2). Lake-specific differences in relative survival of

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OLSON ET AL.

FIGURE 2.—Mean (6SE) electrofishing catch rates (number[N]/h) of pond and fall age-1 walleye fingerlings in the four study lakes. Means are based on 3 years of sampling in each lake.

pond and fall fingerlings were also evident from walleye CPUE in gill nets set in 1996 (Figure 3). The ANOVA on gill-net CPUE was highly significant (F7,36 5 7.75, P 5 0.0001) and included significant main effects of lake and stocking strategy and the interaction of these two factors (lake: F3,36 5 5.47, P 5 0.003; stocking strategy: F1,36 5 13.74, P 5 0.0007; interaction: F3,36 5 7.85, P 5 0.0004). In Sixtown Pond and Eaton Brook Reservoir, catch rates of walleyes stocked as pond fingerlings were significantly higher than fall fingerlings (Figure 3; t-test with unequal variance; Eaton Brook: t5 5 8.72, P 5 0.0003; Sixtown: t5 5 10.92, P 5 0.0004). In these two lakes, we failed to catch a single walleye in gill nets that was stocked as a fall fingerling. In Cayuta and Findley lakes, catch rates of fall fingerlings in gill nets were not significantly different from pond fingerlings (t-test with equal variance; Cayuta: t10 5 0.25, P 5 0.81; Findley: t10 5 0.67, P 5 0.52). Because catch rates of fall fingerlings were similar to pond fingerlings in two lakes and significantly lower in the other two lakes, overall CPUE of fall fingerlings across all lakes was lower than pond fingerlings, as indicated by the significant main effect of stocking strategy.

FIGURE 3.—Mean (6SE) gill-net catch rate (number[N]/gill net) of stocked pond and fall walleyes, based on 5–7 nets set per lake. Only age-1 and older walleyes were counted, and age-classes were not differentiated. Pond and fall fingerlings were identified by the presence or absence of a fin clip. Asterisks denote significant differences (a 5 0.10).

egy were both significant (overall model: F21,4 5 7.76, P 5 0.03; stocking strategy: F1,3 5 9.29, P 5 0.04; lake: F3,4 5 0.57, P 5 0.67; lake 3 strategy interaction: F3,4 5 5.33, P 5 0.07). The main effect of stocking strategy was significant because mean total length of pond fingerlings was higher than fall fingerlings (Figure 4). The interaction term indicated that lakes also differed in the size disparity between pond and fall fingerlings. In all lakes but Cayuta, the difference was significant (t-

Growth Rates In contrast to patterns of survival, differences in relative growth rates of pond and fall fingerlings were more consistent across lakes. The ANOVA for mean total length at the end of the first growing season indicated that the main effect of stocking strategy and the interaction between lake and strat-

FIGURE 4.—Mean (6SE) total length (mm) of pond and fall fingerlings at the end of the first growing season in the four study lakes. Means are based on 4 years of sampling. Estimated mean total lengths for each year were based on an average of 26.2 6 6.6 fish. Asterisks denote significant differences (a 5 0.10).


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were significantly different from one another (ANCOVA; overall model: F3,171 5 200.07, P 5 0.0001; size: F1,171 5 401.02, P 5 0.0001; strategy: F1,171540.18, P 5 0.0001, strategy 3 size interaction: F1,171 5 29.50, P 5 0.0001). The significant interaction of strategy and size was caused primarily by slow growth of fall fingerlings in their second year. Growth of age-1 walleyes averaged 181.0 6 12.1 g/year for pond fingerlings and only 95.5 6 5.4 g/year for fall fingerlings. Because of the low growth by fall fingerlings in their second growing season, the size disparity between pond and fall fingerlings increased. After the second growing season, growth rates of the two stocking strategies were more similar, thereby maintaining the size advantage of pond fingerlings (Figure 5a). Diets

FIGURE 5.—Mean (6SE) mass-at-age (g) for pond and fall fingerlings in (a) Cayuta and (b) Findley lakes. Means are based on 33.8 6 7.9 fish for each age-class.

test with equal variance; Cayuta t3 5 0.92, P 5 0.41; Eaton Brook: t4 5 5.87, P 5 0.001; Findley: t4 5 6.75, P 5 0.0005; Sixtown: t2 5 7.28, P 5 0.018). In the two lakes where growth of older walleyes could be directly compared (Cayuta and Findley), the size advantage of pond fingerlings observed at the end of the first growing season persisted at least 3 years after stocking (Figure 5). In Findley Lake, growth rates of the two strategies increased with size but were not significantly different from one another (ANCOVA; overall model: F3,38 5 16.26, P 5 0.0003; size: F1,38 5 16.26, P 5 0.0003; strategy: F1,38 5 0.15; P 5 0.70; strategy 3 size interaction: F1,38 5 1.28, P 5 0.27). Consequently, size differences set in the first year were maintained over time because the two strategies grew at similar rates (Figure 5b). In contrast, growth rates of pond and fall fingerlings in Cayuta Lake

In five of the seven comparisons of pond and fall-fingerling diets within the same lake and year, pond fingerlings had a significantly higher frequency of piscivory than fall fingerlings (Table 3). Overall, mean frequency of piscivory was 70.9 6 7.3% for pond fingerlings and 16.4 6 6.0% for fall fingerlings. Even though they were sampled approximately 4 weeks after stocking, 74.8 6 8.5% of fall-fingerling stomachs were empty, compared with 24.8 6 6.1% for pond fingerlings. When piscivorous, both stocking strategies fed primarily on age-0 Lepomis spp. Across all lakes combined, pond and fall fingerlings also consumed prey of similar sizes. Measurable fish prey in pondfingerling stomachs averaged 42.4 6 1.5 mm TL (N 5 47), whereas fish prey in fall-fingerling stomachs averaged 40.0 6 3.9 mm TL (N 5 4). These differences in prey size were not significant (t-test; t46,3 5 0.47, P 5 0.64) and were within the preferred predator–prey size ratio (32–36% of predator length) for walleyes (Parsons 1971). Discussion Despite being stocked later in the season and at a larger size, intensively reared fall walleye fingerlings stocked in September did not have a consistent advantage in survival over extensively reared pond fingerlings stocked in June. Estimates of walleye abundance based on gill-net sampling indicated that fall fingerlings had survival rates similar to pond fingerlings in two of the study lakes and significantly lower survival than pond fingerlings in the other two lakes, where no fall fingerlings recruited to the fishery. In all lakes, these patterns of relative abundance were established within 4 weeks of fall-fingerling stocking. There-

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TABLE 3.—Frequency of occurrence of piscivory and empty stomachs in age-0 walleyes in 1995 and 1996. Chisquare statistics were calculated for each year separately. Chi-square values greater than 3.84 are significant (P , 0.05) and are printed in bold italic. No test was performed for Sixtown Pond in 1996 because no fall fingerlings were collected for diet analysis. Pond Lake Cayuta Eaton Brook Findley Sixtown Pond Mean

Year

N

1995 1996 1995 1996 1995 1996 1995 1996

5 25 33 4 12 12 10 16 14.6

With fish (%) 60 52 88 50 92 100 50 75 70.9

fore, the abiotic and biotic factors that determine survival of fall fingerlings began operating very soon after stocking. In addition to the lack of a survival advantage, fall fingerlings also had a significantly smaller mean body size relative to pond fingerlings of the same age. Across all study lakes, mean lengths of fall fingerlings were an average of 19.3% smaller than pond fingerlings at the end of their first growing season. Larscheid (1995) observed similar results for walleyes stocked in East Okoboji Lake, Iowa. Because vulnerability to predators such as largemouth bass is size-dependent (Santucci and Wahl 1993), fall fingerlings may have been more vulnerable to predation than pond fingerlings in the fall (Madenjian et al. 1991). Furthermore, the size discrepancy between pond and fall fingerlings persisted for at least 3 years after stocking (based on direct comparisons in the two lakes in which both pond and fall fingerlings survived beyond their first year). Because low growth rates can reduce survival (Forney 1976; Mittelbach and Chesson 1987) and delay recruitment (Madenjian and Carpenter 1991), fall fingerlings may be less effective than pond fingerlings at creating or enhancing a fishery. Finally, fall fingerlings had a lower frequency of piscivory than pond fingerlings. Although prey within their preferred predator–prey size ratio were available to both types of fingerlings, the majority of fall fingerlings examined had empty stomachs up to 4 weeks after stocking. In contrast, pond fingerlings in the same lakes exhibited a high frequency of piscivory, feeding heavily on age-0 Lepomis spp. These patterns were consistent across lakes and therefore independent of observed patterns of relative survival. Nevertheless, the lack

Fall Empty (%)

N

With fish (%)

40 48 12 25 8 0 40 25

37 21 10 5 18 1 3

5 0 20 40 17 0 33

24.8

13.6

16.4

Empty (%)

Chi square

78 95 80 60 78 100 33

10.32 14.63 17.58 0.53 15.43 13.00 0.44

74.8

of success in making a transition from pellets to fish may have contributed to the reduced survival of fall fingerlings in two study lakes (Knight et al. 1984; Wahl 1995). Together, results of this experiment indicate that stocking the larger fall fingerlings cannot be assumed to be a superior strategy to stocking the smaller pond fingerlings. Koppelman et al. (1992) found similar results in two Missouri impoundments where survival of small (25–51 mm) fingerlings was 2.7 times greater than large (91–122 mm) fingerlings. In our study, recruitment of fall fingerlings was equal to or lower than pond fingerlings in all lakes (based on CPUE in gill nets), and fall fingerlings were consistently smaller. Given that fall fingerlings are approximately 2.5 times more expensive to produce than pond fingerlings (costs in New York in 1995), our study does not support the general hypothesis that the benefits of stocking large fingerlings outweigh the increased costs. It is important to note that our estimates of pond-fingerling abundance could have included walleyes that were produced naturally, particularly in Sixtown Pond in 1994. Also, other studies have demonstrated that large intensively reared fingerlings can be used to establish walleye populations (Laarman 1981; Green 1986; McWilliams and Larscheid 1992; Santucci and Wahl 1993). Therefore, fall-fingerling stocking may be a more effective strategy than pond-fingerling stocking in certain lakes even if their regionwide level of success does not differ from pond fingerlings. Although we have emphasized differences in size between pond and fall fingerlings, the two stocking strategies also differ in culture technique. Pond fingerlings are raised in ponds and feed primarily on zooplankton (Fox and Flowers 1990).


In contrast, fall fingerlings are raised in tanks and, following a brief period in which they are fed brine shrimp, are raised on a diet of dry pellets. Consequently, fall fingerlings have little experience with live prey at the time of stocking. This culture technique may influence walleye behavior in ways that decrease feeding efficiency and increase vulnerability to predation (Wahl 1995). As a result, the advantages gained by being stocked at a larger size could be negated by disadvantages in behavior. Another difference between pond and fall fingerlings was the time at which stocking occurred. Many conditions at the time of pond-fingerling stocking in June were different from when fall fingerlings were stocked in September. For example, water temperatures in June averaged 22.68C, compared with 19.98C in September. Santucci and Wahl (1993) found that high water temperatures can increase mortality associated with stocking stress. High temperatures, or a large temperature difference between the holding tank and the lake, could have led to high mortality of walleye fingerlings in our study lakes. Separating the effects of body size, culture technique, and stocking date on walleye fingerling survival would require an experiment in which each factor is manipulated independently. From a practical standpoint, however, these factors are not independent of one another. Therefore, our experiment evaluated growth and survival based on production of pond versus fall fingerlings. The lake-specific patterns of relative survival suggest that factors influencing fall fingerlings differed from those affecting pond fingerlings. In two of the four study lakes, fall-fingerling densities were lower than pond-fingerling densities within 4 weeks of stocking. This rapid decline in abundance suggests that predation may have been an important source of fall-fingerling mortality. Larscheid (1995) reached a similar conclusion for the rapid decline in abundance of large fingerlings in East Okoboji Lake. Starvation is an unlikely mechanism because fall-fingerling survival was not related to patterns of growth or piscivory (which were consistent across lakes) and because starvation typically operates over a time scale of months for fingerling walleyes (Jonas and Wahl 1998; Copeland and Carline 1998). The most abundant predators in our study lakes were largemouth bass and Esox spp. Previous studies have identified largemouth bass as an important predator of walleye fingerlings (Santucci and Wahl 1993; Seip 1995). Fall electrofishing catch rates

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of largemouth bass were high in our study lakes, but among-lake variation was low (mean catch rates of largemouth bass over the 4-year study were 20.7/h in Cayuta, 21.9/h in Eaton Brook, 14.5/h in Findley, and 13.0/h in Sixtown). Therefore, largemouth bass do not appear to be responsible for the observed among-lake differences in relative survival of pond and fall fingerlings. However, largemouth bass may still play an important role in determining overall survival of both pond and fall fingerlings. Indeed, catch rates of both types of fingerlings in our study lakes were much lower than reported elsewhere (Hauber 1983; McWilliams and Larscheid 1992; Brooking et al. 1995). In contrast to largemouth bass, catch rates of Esox spp. were consistent with patterns of relative survival. Esox spp. were rare in Cayuta and Findley lakes (mean catch rates over 4 years were 2.7/ h in Cayuta and 1.2/h in Findley), where survival of fall fingerlings was similar to pond fingerlings. In contrast, Esox spp. were abundant in Eaton Brook and Sixtown (mean catch rates were 17.8/ h and 11.9/h, respectively), where survival of fall fingerlings was lower than pond fingerlings. Johnson et al. (1994) found that northern pike were important predators of fall-stocked walleye fingerlings in Lake Mendota, Wisconsin, where adult walleyes and northern pike accounted for 33% of the mortality of walleye fingerlings between October 1988 and May 1989. Interestingly, adult walleyes were present in Eaton Brook Reservoir and fairly abundant in Sixtown Pond at the start of the study (Table 1). Therefore, the low survival of fall fingerlings in these two lakes may have been due to the combined effects of both predators. Because our study was based on four lakes, we have little power to detect effects of different predators on walleye survival. To further investigate these patterns more rigorously, we are currently examining survival patterns of pond and fall fingerlings in a set of eight lakes that span a wider gradient of walleye, Esox spp. and largemouth bass abundance. Survival of fall fingerlings may also have been influenced by interactions with pond fingerlings. Because of an advantage in size, pond fingerlings may have been superior competitors for food or preferred habitats. Consequently, the presence of pond fingerlings, particularly in Eaton Brook Reservoir and Sixtown Pond, may have reduced survival of fall fingerlings. However, survival of both types of fingerlings in Cayuta Lake suggests that intraspecific competition was not the only source

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of mortality for fall fingerlings. Furthermore, the correlation between pond- and fall-fingerling catch rates across all lakes and years was poor, indicating that some factors affecting survival of pond fingerlings differed from the factors affecting survival of fall fingerlings. In addition to surviving poorly in some lakes, fall fingerlings also exhibited difficulty in making a diet transition from dry pellets to fish. This result was somewhat surprising because laboratory feeding trials have indicated that fall fingerlings switched to piscivory within 5 d of being offered fish prey (Wahl et al. 1995; Brooking 1998). Furthermore, the fall fingerlings that were piscivorous consumed age-0 Lepomis spp. within their preferred predator–prey size ratio of 32–36% (Parsons 1971), suggesting that prey were available. Nevertheless, piscivory by fall fingerlings in our study was rare. The successful piscivory observed by Wahl et al. (1995) and Brooking (1998) in smallscale laboratory experiments and the low frequency of piscivory observed in our study lakes suggests that fall fingerlings may experience problems encountering prey, perhaps as a result of utilizing habitats where prey are rare. To train walleyes to forage more effectively, fall fingerlings may need to be switched to fish prey before stocking. This strategy could also potentially improve survival of fall fingerlings (Wahl 1995; Wahl et al. 1995). In a laboratory experiment, Szendrey and Wahl (1995) found that predation rates by largemouth bass were lower on muskellunge Esox masquinongy raised on fathead minnows Pimephales promelas than muskellunge raised on dry pellets (see also McKeown et al. 1999). If walleyes are actively foraging for fish, they may increase use of vegetated refuges, which could in turn lower vulnerability to predators (Werner and Hall 1988). Many stocking strategies are developed based on the assumption that larger fish have higher survival. Our results indicate that this assumption is not necessarily true for walleye fingerlings. However, the low survival of fall fingerlings in some lakes may have been a result of culture technique rather than size. Therefore, we believe that future research on this issue should be aimed in two directions. First, behavior of intensively reared fall fingerlings should be examined in more detail to determine whether patterns of habitat use, prey selection, and predator avoidance differ from pond fingerlings. Second, improved understanding of factors influencing survival of small and large walleye fingerlings is needed (Bennett and McArthur 1990). That information can then be used

to determine whether small, extensively reared fingerlings or large, intensively reared fingerlings would be most successful in a given lake. Acknowledgments We thank Scott Prindle and Brian Young for assistance in the field or laboratory. Comments and suggestions by Janet Fischer, John Forney, Rob Klumb, and Brian Young greatly improved this manuscript. This project was supported by the Federal Aid to Sport Fish Restoration Act through the New York State Department of Environmental Conservation, Project FA-5-R and is contribution number 190 of the Cornell Biological Field Station. References Bennett, D. H., and T. J. McArthur. 1990. Predicting success of walleye stocking programs in the United States and Canada. Fisheries 15(4):19–23. Brooking, T. E. 1998. Feeding transition of intensively reared fall fingerling walleye from pellets to fish. New York State Department of Environmental Conservation, Albany. Brooking, T. E., A. J. VanDeValk, D. B. MacNeill, D. M. Green, and L. G. Rudstam. 1995. Evaluation of walleye fingerling stocking in Port Bay, New York. Great Lakes Research Review 2:11–15. Brooking, T. E., A. J. VanDeValk, L. G. Rudstam, and M. H. Olson. 1997. Five year summary of factors affecting survival of stocked walleye in New York Lakes: 1992–1996. New York State Department of Environmental Conservation, Albany. Brooks, R. C., R. C. Heidinger, and C. C. Kohler. 1994. Mass-marking otoliths of larval and juvenile walleyes by immersion in oxytetracycline, calcein, or calcein blue. North American Journal of Fisheries Management 14:143–150. Buttner, J. K., D. B. MacNeill, D. M. Green, and R. T. Colesante. 1991. Angler associations as partners in walleye management. Fisheries 16(4):12–17. Carlander, K. D. 1982. Standard intercepts for calculating lengths from scale measurements for some centrarchid and percid fishes. Transactions of the American Fisheries Society 111:332–336. Carline, R. F., R. A. Stein, and L. M. Riley. 1986. Effects of size at stocking, season, largemouth bass predation, and forage abundance on survival of tiger muskellunge. Pages 151–167 in G. E. Hall, editor. Managing muskies. American Fisheries Society, Special Publication 15, Bethesda, Maryland. Conover, M. C. 1986. Stocking cool-water species to meet management needs. Pages 31–39 in R. H. Stroud, editor. Fish culture in fisheries management. American Fisheries Society, Fish Culture Section and Fisheries Management Section, Bethesda, Maryland. Copeland, T., and R. F. Carline. 1998. Overwinter survival and lipid content of walleye fingerlings. North


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