Quantitative description of predator diets prior and subsequent to walleye (Steizostedion vitreum) stocking in Otsego Lake Brian McDonnell 1 and Mark Cornwell 2 ABSTRACT This study was designed to evaluate survival of pond fingerling walleye (Steizostedion vitreum) stocked into Otsego Lake. The recent limnological changes in Otsego Lake may partially be a result of the 1986 introduction of alewives (Alosa pseudoharengus) because they are efficient planktivores (Foster, 1989; Harman et al., 1997). Reintroduction of the walleye began in 2000 when 80,000 pond fingerlings were added and continued this year with an additional 45,000 fish. Walleye, generally ranging between 45-60 mm in length, were stocked throughout 36 ha (89 ac) of littoral habitats in the lake. The rationale for their re-establishment was that they might effectively utilize the forage base provided by the alewives. An additional benefit might be that a reduced alewife population would, through trophic cascade changes, lead to increased zooplankton size and biomas allowing for greater grazing of algae (Cornwell, in prep.). Haul seining at six sites within the stocked area provided results indicating high predation on stocked walleye 0-48 hours subsequent to stocking. Using pulsed gastric lavage, five species were determined to be responsible. Yellow perch (Perca flavescens), largemouth bass (Micropterus salmoides), smallmouth bass (Micropterus dolimieui), chain pickerel (Esox niger) and rockbass (Amblopleites rupestris) were found to be predators of stocked walleye. Areal abundance estimates for each of these predators in the stocked littoral, coupled with the average number of walleye in each stomach, yields a rough estimate of 278 walleye consumed per hector (112/acre) during this two day post stocking experiment. Total walleye consumed in the stocked region was extrapolated to be approximately 10,000. INTRODUCTION Otsego Lake (42 40' N-70 00 W) is located in the glacially over-deepened headwaters of the Susquehanna River valley within the northern Appalachian Plateau (Harman et al., 1997). The 1986 introduction of the alewives into Otsego Lake has led to limnological changes (Foster, 1989; Harman et al., 1997). Alewives are efficient, selective planktivores that have altered the zooplankton community from one dominated by larger crustaceans to dominance by smaller cladocera and rotifers (Warner, 1999). Current information shows decreased transparency resulting from decreased grazing by zooplankton on algae. Also, regenerating nutrients and decreased ambient N: P ratio 1 Robert C. Mac Watters internship in aquatic sciences, summer 2000. Present affiliation: Department of Fisheries and Wildlife Technology, SUNY Agriculture and Technical College, NY 12043. 2 SUNY Oneonta biology MA candidate. Biological Field Station, Cooperstown, NY.
allows for greater algal production (Warner, 1999). This situation has also caused depressed hypolimnetic oxygen levels during summer stratification (Albright, 2002). The reduction in the population of the whitefish (Coregonus clupeaforms) and cisco (C. artedii) could be the result of competition with alewives for larger zooplankton (Linhart, 1999). Walleye have historically been a common gamefish in Otsego Lake but suffered declines through the 1970s-80s and were virtually extirpated by 1990. That decline is believed to be resultant of the accidental stocking of cisco, which were first reported in 1955 (McBride and Sanford, 1996). That species efficiently consumes pelagic fry of other fish, particularly walleye. Beginning in 2000, cooperative efforts by the Biological Field Station, the NYS Department of Environmental Conservation and the Cornell Warmwater Fisheries Unit lead to attempts to re-establish the walleye fishery (Cornwell, 2002). That year, 80,000 pond fingerlings were stocked, while only 45,000 were available in 2001. The expectation was that alewives would provide an excellent forage base for this gamefish. An additional benefit might be that a resultant reduced alewife population would release the once abundant large-bodied crustacean zooplankton populations, which would increase lake wide algal filtering rates. If so, it would be expected that Secchi transparencies would increase and rates of hypolimnetic oxygen depletion would decrease. Current late summer oxygen concentrations (Albright, 2002) are approaching the minimum required to maintain Otsego s cold water fishery (Harman et al., 1997; Nichols, 1995). Alewives spawn from late May or early June until August in the shallow waters of the littoral zone (Smith, 1985). Walleye stocking followed the spawning of the alewives. Introduced walleyes are expected to feed on alewife fry and walleye growth should maintain a pace that would allow them to continue to utilize that year class, resulting in a depressed alewife population. Stocked walleye have to survive heavy predation, which is important variable determining the survival of stocked fish (Clapp et al., 1995; Santucci and Wahl, 1993). Hoyle and Keast (1987) indicated the maximum size of the slender bodied prey was 50% of bass body weight and the optimal prey size approximately 35% of total body length. Stocked Otsego Lake walleye average approximately 50 mm in total length. The Hoyle and Keast model projects that stocked walleye could be vulnerable to predators as small as 100 mm (minimum) and very vulnerable to 150 mm predators (optimal). Brooking et al. (2001) explored the possibility that stocked walleye survival would be lower in lakes with higher densities of large predators. Others have hypothesized that walleye predation may be occurring within a larger suite of predators outside the usual bass-esocid model (Brooking et al., 2001; Meehan, 2002). The goal of the study was to evaluate the extent of predation on pond fingerling walleye over two days post stocking, to identify specific predators and to determine whether those predators seem to be selecting for walleye over other forage fish present in the community.
MATERIALS AND METHODS In June of 2001, approximately 45,000 pond fingerling walleye (mean length average = 53.6 mm, range = 41 65 mm) were stocked throughout the littoral zone from Five Mile Point counterclockwise to Point Judith (Figure 1). Efforts were made to distribute the fish within the 5 m (15 ft) contour. Six sample sites were used in collecting data of predation data, Five Mile Point, Three Mile Point, Brookwood Point, Rat Cove, Point Judith and Point Florence (Figure 1). The six sites were chosen for easy accessibility and where permission to gain access to shore lines was granted. Sampling was conducted both prior to and within 48 hours following the stocking using a 200 ft (61 m) haul seine following methods described by Neilson and Johnson (1983). Collected fish were measured, weighed and stomach contents of fish 100mm or larger removed using pulsed gastric lavage described by Lagler et al. (1963) and Foster (1977). Stomach contents were collected plastic bags, labeled and enumerated in the lab. Stomach contents for predators are expressed in two ways following Murphy and Willis (1996). The proportion of the fish that contained one or more of a given food type is the frequency of occurrence. The percent composition by number is the number of items of a given food type expressed as a percentage of the total items counted of all food types. Selection indices for predators are based on a comparison between the relative abundance of a prey type in the diet of the predator and the relative abundance of that prey type in the environment. A selection index can then be made to find if a prey type is selected for, selected against or eaten in the same frequency as it occurs in the environment. The Strauss index (1979) is used to describe prey selection by predators in this study and is represented by the formula: L= r i p i Where L is the expression of the Strauss index from 1 to 1, r i is the relative abundance of prey type in the predator diet and p i is the relative abundance of that prey fish in the environment (Murphy and Willis, 1996). Ration (r i ) was determined directly from the predator stomachs in percent composition by number and prey proportion ( p i ) was determined through boat electrofishing catch per shoreline length sampled during June 2000 all fish collections. Walleye were stocked only in an abbreviated littoral (36 ha; 89 acres) and were assumed evenly distributed in this area. June 2000 electrofishing data were used to estimate proportions of littoral prey fish species. The area of the field of the electrofisher was determined by multiplying the shoreline length sampled with the electrofisher (4,400 m; 14,440 ft) by the field width of the shocker (3.7 m; 12ft). The area fished by the shocker was 1.61 ha (3.97 ac). The catch in this zone was then multiplied by 22.4 (ratio of stocked littoral/sampled littoral) to arrive at a rough relative abundance estimate for each prey fish species. One assumption made by this study is that the proportion of prey fish in the littoral during June 2001 is similar to June 2000.
143 New York Stale Shadow Brook Blackbird Bay Otsego Lake Willow Brook Susquehanna River Figure 1. Map of Otsego Lake, Otsego County, NY. Walleye stocking in June 200 1 occurred within the 15' contour from Five Mile Point counterclockwise to Point Judith.
The proportion of walleye in the environment was calculated by electrofishing after stocking fall fingerlings in October 2001. Electrofishing efficiency was determined using 8,000 fin clipped fingerlings (TL= 112-144) stocked at Springfield Public Landing in the north littoral. Electrofishing only samples a narrow swath of the littoral and therefore only collects a small fraction of the population. Night electrofishing recaptured five (5) walleye of the 8,000 fish two days post stocking. The area of the north shoreline sampled by the electrofisher was 0.4 ha (1.0 ac). The rate of return was 12.35 walleye per ha (5 per ac). This return rate was used to estimate the proportion of walleye in south littoral. The stocked littoral was 36 ha; that was multiplied by the return rate (12.35/ha), giving an expected return of 445 walleye throughout the stocked littoral. This assumes walleye were distributed evenly in both the north and south stocked littorals and occurred in the same density. This resulted in the estimation that walleye comprised 10.8% of the prey species throughout the stocked littoral (Table 1). Prey fish proportions for each species was found by dividing that population by the total number of prey fish estimated in the abbreviated littoral, including walleye. The area swept by the haul seine was used to extrapolate the abundance of fish species that were found to prey upon the stocked walleye. The area covered by the 61 m haul seine was assumed to be that of a half ellipse, determined by Wilson et al. (1999) to be 310 m 2. Average numbers of walleye eaten for predators was calculated then extrapolated to the entire littoral community of all predators of walleye based on the littoral area and a population estimate of the abundance of those predators by the area swept. RESULTS AND DISCUSSION Table 1 provides an overview of Strauss index of selectivity indicates that stocked walleye pond fingerlings are selected for by chain pickerel, largemouth bass, yellow perch and rock bass. A Strauss index of +1 indicates perfect selection for a prey item and an index of 1 indicates perfect selection against (Murphy and Willis, 1996). Chain pickerel and rock bass have a very high selection index (.772 and.802 respectively) for walleye. Additionally, yellow perch frequency of occurrence (44%) is high particularly when considered with the Stauss index (.492), indicating yellow perch are significant predators of stocked walleye. Walleye predation is not limited to this suite of predators. Foster (pers. comm.) observed walleye in bluegill stomachs soon after stocking in 2000 shore seines, indicating that the potential suite of predators may be even larger. Figure 2 graphically provides results of stomach analysis of gamefish. With the exception of smallmouth bass, which targeted primarily young-of-year alewife, the studied predators fed primarily upon walleye when they were available.
Table 1. Predator stomach contents before and after stocking expressed as frequency of occurrence and percent composition by number. Predator Stomach Freq.Occ. (%) % by # Proportion Strauss* Selected for, against Contents Prior After Prior After In Environ. Index or same proportion Yellow Perch Walleye 0 44 0 60 10.8.492 Selected for Fathead 0 12.5 0 12 - - - Alewife 0 4.1 0 2 80-0.78 Selected against Inverts 18.18 8.3 66.67 4.00 - - - Tessellated darter 4.5 0 16.66 0 1.1 - - Smallmouth bass 4.5 0 16.66 0 - - - Unknown Fish 0 21 0 22 - - - Empty 73 33 - - - - - Largemouth Bass Walleye 0 33 0 44 10.8.332 Selected for Bluntnose minnow 28.5 17 66.6 22 2.2.198 Selected for Alewife 14.2 17 33.3 22 80.5-5.85 Selected against Tesselated darter 0 17 0 11 1.1.10 Selected for Empty 57.1 17 - - - - - Rock Bass Walleye 0 29 0 91 10.8.802 Selected for Bluntnose minnow 0 14 0 9 2.2.068 Same Proportion Bullhead 10 0 17 0 - - - inverts 30 0 50 0 - - - Smallmouth bass 10 0 17 0 - - - Alewife 10 0 17 0 80 - - Empty 40 71 - - - - - Smallmouth Bass Walleye 0 14 0 6 10.8 -.048 Same Proportion Fathead 0 7 0 3 - - - Alewife 14.3 14 22 75 80 -.05 Same Proportion Inverts 21.4 7 33.3 6 - - - Tesselated darter 0 7 0 3 1.1.019 Same Proportion Smallmouth bass 14.3 7 33.3 6 - - - Empty 42 35.7 - - Chain Pickerel Walleye 0 40 0 88 10.8.772 Selection for Tesselated Darter 12.5 0 33.3 0 1.1 - - Banded Killifish 12.5 20 33.3 12.5.07 Same Proportion Small mouth bass 12.5 0 33.3 0 - - - Empty 62.5 60 - - - - - *The proportion of prey fish in the environment and the % by # after walleye stocking are used to calculate Stauss Index of selectivity.
These data may be used to calculate a rough estimate of walleye predation in the stocked littoral. From the area swept by each seine (310 m 2, or 0.031 ha) and the number of seines it would take to cover the stocked littoral (36 ha/ 0.031 ha = 1,160 seines) and average number of predators (107 predators in 11 seines = 9.72) caught per seine, the total number of predators in this area can be estimated to be 11,275. Sixty-one (61) total predator stomachs sampled contained 54 walleye. Therefore, during this time each predator consumed an average of 0.885 walleye each (54/61). If all littoral predators ate.885 walleye the potential predation would be approximately 10,000 (11,275 X 0.885) or 278 walleye (10,000/36) eaten per littoral hectare stocked (or 112 walleye/ac). This method crudely demonstrates the potential for predation on walleye 0-48h post stocking. Walleye stocked sparsely form the boat may decrease the chances of survival. When the walleye are stocked they are disoriented allowing them to be an easy target for predators in the vicinity of the stocking. Stocking in larger numbers might increase the chances for survival by overwhelming the predators, allowing the majority of stocked walleye to orient themselves and seek refuge toward the bottom of the littoral zone. Ideas for reducing predation include using a pipe from the boat to deliver the fish to the bottom, to avoid predators located in the water column and stocking walleye at night when sight oriented feeders do not forage as actively. Another option may be to stock larger Fall fingerling walleye. However stocking fall fingerlings may put optimal foraging on alewife in jeopardy because Otsego YOY alewife may be too large for stocked walleye (Cornwell, 2002). REFERENCES Albright, M.F. 2002. Otsego Lake limnological monitoring, summer 2001. In 34 th Ann. Rept. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Brooking, T.E., J.R. Jackson. J.R. Rudstam and A.J. VanDevalk. 2001. Factors affecting survival of stocked walleye in New York Lakes. Progess report. Cornell University Biological Field Station. Cochran, W.G. 1953. Sampling techniques. 1 st edition. Wiley Co. New York. Cornwell, M.D. In prep. M.A. thesis. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Cornwell, M.D. 2002. Walleye (Stizostidion vitreum) Reintroduction Update: Walleye Stocking and Electrofishing Summary 2000-1. In 34 th Ann. Rept. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Clapp, D.F., C.S. Kohler, D.H. Wahl, Harrison, J. Brooks, J. Wadell and R. Heidinger. 1995. Evaluation of walleye stocking program. Federal Aid Project F-118-R, Illinois Natural History Survey. Aquatic Ecology Technical Report 95/3. Foster, J.R. 2002. Personal communication. Professor of fisheries and aquaculture, SUNY Cobleskill.
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