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Seasonal and Size-Related Changes in the Diet of Northern Pike from a Shallow Prairie Lake

Seasonal and Size-Related Changes in the Diet of Northern Pike from a Shallow Prairie Lake Seasonal and Size-Related Changes in the Diet of Northern Pike from a Shallow Prairie Lake Steven M. Sammonsa, Charles G. Scalet, and Robert M. Neumannb Department of Wildlife and Fisheries Sciences South Dakota State University Brookings, SD 57007 ABSTRACT Food habits of northern pike (Esox Iucius) were documented in Lake Thompson, a shallow, windswept, eutrophic lake in eastern South Dakota. The primary food items, assessed with the Relative Importance Index, eaten by northern pike were common carp (Cvorinus w, crappies (Pomoxis spp.), and fathead minnows (Pimeohales promeas). Common carp were eaten in midsummer and winter. Crappies were eaten primarily in the winter. Fathead minnows were an important food source throughout the year. Macroinvertebrates were consumed by northern pike up to 600 mm throughout the open-water months. Northern pike in the smaller length groups (401-500 mm, 501-600 mm) ate primarily fathead minnows, juvenile common carp, and macroinvertebrates. Diets of larger northern pike consisted primarily of larger common carp, juvenile crappies, leopard frogs (Rana oioiens), and occasionally walleyes (Stizostedion vitreum). INTRODUCTION Northern pike mox luciua, because they change food habits with size and grow to a relatively large size, utilize a wide range of prey organisms. Their general food habits have been described (e.g. Frost 1954, Diana 1979, Mann 1982, Chapman et al. 1989). Newly hatched northern pike feed almost exclusively on macroinvertebrates (Frost 1954, Franklin and Smith 1963). Frost (1954) recorded a shift in northern pike diets from macroinvertebrates to fishes when pike attained lengths of about 3 cm. Fishes constitute more of the diet as northern pike size increases, until the diet is almost exclusively fishes (Frost 1954). Few studies have documented northern pike diet selection over an entire year, and none have been conducted on an annual basis in natural lakes of the northern great plains. Because northern pike are opportunistic predators capable of modifying their feeding strategies in response to environmental changes (Chapman et al. 1989), how pike react in natural prairie lakes cannot necessarily be inferred from earlier studies at different locations. The purposes aPresent address: Department of Fisheries and Wildlife Sciences, School of Forestry and Wildlife Resources, V.P.I. and S.U., Blacksburg, VA 24061. b~resent address: Department of Natural Resources Management and Engineering, University of Connecticut, Storrs, CT 06269. Joumal of Freshwater Ecology, Volume 9, Number 4 - December 1994 of this study were to document the seasonal food habits of northern pike in a shallow prairie lake, and to describe diet changes associated with increasing pike size. METHODS A total of 632 northern pike stomachs were examined from Lake Thompson, a 6,573-ha natural lake located in Kingsbury County in east-central South Dakota. Lake Thompson is a typical prairie lake; it is shallow, windswept, eutrophic, and does not thermally stratify during the open-water season. The lake contains approximately 10 fish species, with northern pike, walleyes (Stizostedion vitreum), common carp (Cv~rinus car~io), and fathead minnows (Pime~hales promela3) being the most abundant. Sam~lina Protocol Lake Thompson was sampled monthly from July 1991 until February 1993, except for November and January of both years, and September 1992. Fish were collected with 45.0 by 1.8 m monofilament gill nets, with bar mesh sizes of 1.9, 2.5, 3.8, 5.1, 7.0, and 8.9 cm. During open-water periods, nets were set during the day and checked approximately every three hours to reduce digestion bias and mortality of nontarget fishes. Nets were set in nearshore environments, usually in water less than 2 m deep. During ice cover nets were set using a Murphy stick (Hubert 1983) and because catch rates were lower the nets were checked every three to five days depending on weather conditions. When ice was present nets had to be set in water 3 to 4 m deep to avoid having the nets freeze to the bottom of the ice. Northern pike were put into plastic bags and immediately placed on ice to slow digestion. Sampling continued until a sample size of approximately 50 fish was attained. Stomachs were removed from fish in the laboratory and prese~ed in 10% formalin. Stomach contents were identified to the lowest taxon possible, counted, measured to the nearest millimeter (standard length for fishes), and weighed to the nearest 0.1 g. In most cases, the lowest taxon for this study was family for aquatic insects, order for other invertebrates, and species for fishes. Food Habits Analvsis Frequency of occurrence, percent weight a taxon represented in the diet, and percent number a taxon represented in the diet have all been used to measure diets. However, each of these methods has certain biases (Bowen 1983). To avoid some of these biases, the Relative Importance (RI) index of George and Hadley (1979) was used; RI incorporates these three diet measurement techniques. The RI index ranges from 0 to 100, and the sum of all the RI values for prey eaten equals 100. The RI index decreases the importance of prey eaten in large quantities by a few fish and increases the importance of prey eaten in small quantities by many fish. Three length groups were used for RI calculations: 401 to 500 mm, 501 to 600 mm, and >600 mm (total lengths). These length groups were chosen to 322 ensure that an adequate sample size was attainable for each length-group calculation. The RI values were calculated each month for all fish sampled and Reconstructed weights were not used for RI for the three length groups. analysis, except for leopard frogs (Rana piwiens), because we found no differences in RI values calculated with and without reconstructed weights. spring (March, April, and May); Seasons referred to in this paper are: summer (June, July, and August); fall (September and October); and winter (December and February). Open-water samples were those taken from March through October, when the lake was unfrozen. Winter samples were those taken in December and February, when ice was present. RESULTS AND DISCUSSION The primary food items eaten by northern pike in Lake Thompson were common carp, fathead minnows, crappies (Pomoxis spp.), leopard frogs, leeches (Hirudinea), chironomids, and amphipods. While these were not the only items eaten, they were the ones most commonly found in stomachs. Seasonal food selection: Relative importance values for common carp peaked in late summer and in winter (Figure 1). The summer peak of common carp in northern pike diets consisted entirely of small carp (20 - 60 mm), which we assume were age-0 fish that occurred in shallow water at that time. The winter peak consisted of common carp of lengths up to 300 mm. Common carp use was lower in the second sampling year than in the first (Figure 1). Mauck and Coble (1971) determined that common carp >I10 mm were not vulnerable to northern pike 1316 mm and stated that carp likely outgrow pike predation in one or two years. However, in Lake Thompson, common carp up to 300 mm (approximately age 2) were found in northern pike stomachs during both winters. Northern pike growth in Lake Thompson was extremely fast (Neumann 1994). Northern pike with such high growth rates may be able to exert more of an influence on common carp populations than in other North American areas where pike do not grow as rapidly. Fathead minnows were an important food for northern pike in Lake Thompson throughout the year. They were the only food item that appeared in all monthly stomach samples, frequently in large numbers. However, fathead minnow RI values exhibited fluctuations (Figure 1). Declines corresponded to peaks in use of other food items, usually larger-bodied prey such as common carp and crappies (Figure 1). This suggested that while northern pike used fathead minnows to a high degree in Lake Thompson, they ate large-bodied prey when available. Northern pike have been reported to prefer larger-bodied prey (Frost 1954, Lawler 1965, Diana 1979). The increased use of fathead minnows in March 1992 (Figure 1) was likely due to the scarcity of other prey items available. There was an increase in the RI value of fathead minnows in northern pike diets the second year of sampling (Figure 1). This was probably linked to the increased abundance of fathead minnows we believe occurred during that period. Walleyes were infrequently preyed upon by northern pike in Lake Thompson (Figure 1). Of 632 stomachs examined, only seven walleyes were found. Analysis of gill-net catch data showed that walleyes and northern pike were temporally and spatially segregated in Lake Thompson, perhaps at least partially accounting for the low consumption of walleyes by pike. ..................................... OJ< 0 s o -. N;D - j P;M i M j i Stizostedion vitreum a 50, 5 25. ,................. n 0-I E J A s ODJ F~M A M j i Rana pipiens J F~M A M J . Invertebrates Month Fiaure 1, Relative Importance values for four fishes, leopard frogs, and invertebrates in northern pike diets in Lake Thompson, SD. The solid line represents the first year of sampling, July 1991 through June 1992. The dotted line represents the second year of sampling, July 1992 through February 1993. Underlined months denote those in which no sample was taken. No sample was taken in September 1992. With few exceptions, crappies were found in northern pike stomachs only during winter (Figure I), but they were an important diet component during that time. In the first winter of sampling, all but one of the 83 crappies eaten by northern pike were age-0 fish. However, in the second winter of sampling, 9 of 13 crappies eaten were age 1 and older. This was probably due to low recruitment of the 1992 crappie year class in Lake Thompson. Low crappie recruitment was observed in most natural lakes in eastern South Dakota in 1992 (C.S. Guy, Kansas Cooperative Fish and Wildlife Research Unit, personal communication) and was caused by unstable weather patterns during the spawning period. Many northern pike food habit studies have demonstrated that centrarchids are not a preferred food of pike (Seaburg and Moyle 1964, Beyerle 1971, Mauck and Coble 1971, Weithman and Anderson 1977). In a small pond stocked only with bluegills (Le~omis macrochirus) and northern pike, Beyerle (1971) found that despite the higher availability of bluegills, pike ate tadpoles as often as bluegills. Northern pike growth was slow, and he concluded that bluegills were not suitable food for pike. Beyerle and Williams (1968) found that soft-rayed fishes were selected over centrarchids by northern pike, and pike of all sizes selected the smallest centrarchid possible. This was probably the case in Lake Thompson, where northern pike ate the smallest crappies available each year. Leopard frogs were eaten primarily in October of both sampling years (Figure 1). This is when the frogs were moving from land and shallow water to deeper water for hibernation. A few leopard frogs were eaten during the winter of both years, but generally frogs were not eaten at any other time of year. There was a high use of invertebrates by northern pike in Lake Thompson (Figure 1). Invertebrates eaten by northern pike consisted of the orders Hirudinea, Amphipoda, Decapoda, and Insecta. Insects eaten were mostly chironornids, followed by odonates. Relative Importance of invertebrates averaged 18.8% during the open-water samples, and in May 1992, over 50% of northern pike diet consisted of this food item (Figure 1). Similarly, Chapman et al. (1989) found that the importance of macroinvertebrates in northern pike diets was high in May and June, then decreased throughout the rest of the year in nine Canadian lakes. Similarly, RI values of invertebrates in Lake Thompson were close to 0 during both winters (Figure 1). However, invertebrate RI values in the open-water samples were high compared to most other values reported in the literature. Diana (1979) reported that macroinvertebrate use by northern pike in Lac Ste. Anne, Alberta was less than 5% (by number) throughout the year. Lawler (1965) reported overall macroinvertebrate use by northern pike to be 4.4% (frequency of occurrence) in a Manit~ba lake. Several other studies reported low macroinvertebrate use by northern pike (Frost 1954, Seaburg and Moyle 1964, Wolfert and Miller 1978, Craig and Babaluk 1989). Chapman et al. (1989) reported northern pike diets where the frequency of occurrence of macroinvertebrates was 94% or greater in one or more sampling periods in three Canadian lakes. Mann (1976), working on two rivers in England, recorded a high use of mayflies (Ephemeroptera) by 325 Table 1. Relative Importance (RI) values for primary food items in diets of northern pike of three different length groups, sampled from July 1991 through February 1993. Length groups are: S - small (401-500 mm), M - medium (501-600 mm), and L - large (>601 mm). Seasons are: Su - summer, F - fall, W - winter, and Sp - spring. C. careio Length s 41.1 0.0 0.0 0.0 9.8 0.0 - groups M 72.3 2.1 22.3 1.9 9.5 0.0 18.2 L 0.0 5.4 44.8 0.0 8.6 0.0 16.0 P. promelas Length S 15.9 44.2 73.4 64.1 53.7 53.3 - groups M 9.5 10.5 45.5 48.3 54.6 51.6 73.8 L 0.0 6.6 9.8 40.7 11.7 26.9 48.6 Pomoxis spp. Length S 0.0 0.0 26.7 0.0 0.0 0.0 - groups M 0.0 0.0 18.6 0.0 2.6 0.0 1.1 L 0.0 9.3 31.9 0.0 0.0 0.0 17.5 Invertebrates Length S 24.5 29.4 0.0 18.3 9.4 8.3 - groups M 5.0 16.3 0.0 20.0 14.3 12.8 0.0 northern pike in April and September. Chapman and Mackay (1990) reported that northern pike ate macroinvertebrates almost exclusively in May, June, and July in an Alberta lake that had only yellow perch as a prey fish. Lake Thompson has a community of soft-rayed fishes that can be used as food for northern pike, yet they continued to eat invertebrates. Size-related Selection: Fathead minnows were a primary component in the diets of small northern pike (401 to 500 mm), were less of a component in larger pike diets (501 to 600 mm), and were of even less of a component in the largest pike diets (>600 mm) (Table 1). The fathead minnow component in the larger northern pike diets increased at the end of the sampling period; this was probably linked more to increased fathead minnow abundance than to a change in preference. Size of northern pike consuming common carp varied with season. Common carp were a primary component in small northern pike diets only during summer, and were more important in winter than in summer in the largest pike diets (Table 1). Northern pike in the 501-600 mm length group ate common carp in both seasons (Table 1). This was probably related to differences in the size of common carp consumed in summer and winter. In summer, all of the common carp consumed were juvenile fish between 20 and 100 mm, while in the winter, the majority of the carp consumed were larger than 100 mm. Frost (1954) reported that small fishes in Windermere, England were important in the diets of northern pike between 200 and 600 mm. Most other northern pike food habit studies dealing with fish in that length range have found similar results (Lawler 1965, Bregaui and Kennedy 1980, Mann 1982). Crappies were generally not eaten by small northern pike; however, in February 1992, age-0 crappies composed a large portion of the diets of all sizes of pike (Table 1). Crappies were found in winter diets of 501 to 600 mm northern pike and composed an even greater portion of the diets of the largest pike (Table 1). Macroinvertebrate occurrence in northern pike diet was primarily confined to pike under 600 mm. At times invertebrates were a major component in the diets of small northern pike (Table 1). However, they were usually a smaller component of the diets of medium-sized northern pike, although still a major food source for these fish (Table 1). Macroinvertebrate RI values were low in the diets of the largest northern pike, except for May 1992 when macroinvertebrates constituted a major food source for the entire sample. Lawler (1965) reported that macroinvertebrates were periodically important in the diets of northern pike up to 600 mm, but were not eaten by larger pike. Mann (1976) found a high use of macroinvertebrates by northern pike up to 700 mm. Chapman et al. (1989) found a high use of macroinvertebrates by northern pike in three lakes in northern Canada. However, these northern pike were all < 600 mm and the mean lengths of the pike sampled from the three lakes (360 mm, 400 mm, and 450 mm) were smaller than the mean length of pike in Lake Thompson that ate macroinvertebrates (519 mm). Water clarity may also have affected northern pike feeding in Lake Thompson. Northern pike food consumption peaked in winter months (Sammons 1993). Neumann (1994) reported that northern pike growth in Lake Thompson occurred during cool- and cold-water periods, and maximum female pike growth occurred during winter. Turbidity in Lake Thompson is primarily caused by wind. Ice formation eliminates this wind influence, the water becomes clear, and visibility is enhanced. Because northern pike are visual feeders, pike predation is facilitated by clear water, allowing higher food consumption and growth rates. Craig and Babaluk (1989) also found northern pike consumption and growth to increase in early winter in Dauphin Lake, Manitoba. They found no such increase in walleye consumption and growth rates. Since northern pike use sight more than walleyes in feeding, they also concluded that pike predation and growth was enhanced by water clarity. Diana (1979) found that in Lac Ste. Anne, Alberta, larger northern pike ate one large prey item, digested it, and then ate another. He theorized that the fish derived the maximum bioenergetic benefit from this action and northern pike eating larger food items grow optimally. Although they maintained rapid growth, northern pike in Lake Thompson did not follow this behavior. However, the largest northern pike tended to prey on the larger prey species available in the lake. Several other studies support the concept of the larger northern pike eating the largest prey fish found in the system (Frost 1954, Lawler 1965). ACKNOWLEDGEMENTS We thank M. Brown and D. Willis for critical review of this manuscript. Funding for this project was provided by the South Dakota Department of Game, Fish and Parks and the South Dakota Agricultural Experiment Station. This paper was approved for publication by the South Dakota Agricultural Experiment Station as Journal Series 2783. LITERATURE CITED Beyerle, G.B. 1971. A study of two northern pike - bluegill populations. Transactions of the American Fisheries Society 100:69-73. Beyerle, G.B., and J.E. Williams. 1968. Some observations on food selectivity by northern pike in aquaria. Transactions of the American Fisheries Society 97:28-31. Bowen, S.H. 1983. Quantitative description of the diet. Pages 325-336 jn L.A. Nielsen and D.L. Johnson, editors. Fisheries techniques. American Fisheries Society, Bethesda, Maryland. Bregazzi, P.R., and C.R. Kennedy. 1980. The biology of the pike, Esox lucius, in a southern eutrophic lake. Journal of Fish Biology 17:91-112. Chapman, L.J., and W.C. Mackay. 1990. Ecological correlates of feeding flexibility in northern pike (Esox lucius). Journal of Freshwater Ecology 5: 313-322. Chapman, L.J., W.C. Mackay, and C.W. Wilkinson. 1989. Feeding flexibility in northern pike (Esox lucius): fish versus invertebrate prey. Canadian Journal of Fisheries and Aquatic Sciences 46:666-669. Craig, J.F., and J.A. Babaluk. 1989. Relationship of condition of walleye (Stizostedion vitreum) and northern pike (Esox lucius) to water clarity, with special reference to Dauphin Lake, Manitoba. Canadian Journal of Fisheries and Aquatic Sciences 46:1581-1586. Diana, J.S. 1979. The feeding pattern and daily ration of a top carnivore, the northern pike (Esox lucius). Canadian Journal of Zoology 57:2121-2127. Franklin, D.R., and L.L. Smith. 1963. Early life history of the northern pike, &B Iucius L., with special reference to the factors influencing the numerical strength of the year class. Transactions of the American Fisheries Society 92191 -1 10. Frost, W.E. 1954. The food of pike Esox lucius L. in Windermere. Journal of Animal Ecology 23:339-360. George, E.L., and W.F. Hadley. 1979. Food and habitat partitioning between rock bass (Amblo~lites ru~estris) and smallmouth bass (Micro~terus clolomieui) young of the year. Transactions of the American Fisheries Society 1081253-61. Hubert, W.A. 1983. Passive capture techniques. Pages 95-1 11 h L.A. Nielsen and D.L. Johnson, editors. Fisheries techniques. American Fisheries Society, Bethesda, MD. Lawler, G.H. 1965. The food of the pike Esox lucius in Heming Lake, Manitoba. Journal of the Fisheries Research Board of Canada 22:1357-1377. Mann, R.H.K. 1976. Observations on the age, growth, reproduction, and food of the pike Esox lucius in two rivers in southern England. Journal of Fish Biology 8:179-197. Mann. R.H.K. 1982. The annual food consumption and prey preferences of pike from the River Frome, Dorset. Journal of Animal Ecology 51:81-95. Mauck, W.L., and D.W. Coble. 1971. Vulnerability of some fishes to northern pike (Esox lucius) predation. Journal of the Fisheries Research Board of Canada 28:957-969. Neumann, R.M. 1994. Growth, distribution, and movement of northern pike in a South Dakota natural lake, with sampling considerations. Ph.D. Dissertation. South Dakota State University, Brookings. 1993. Annual food habits of northern pike in an eastern South Sammons, S.M. Dakota natural lake. M.S. Thesis. South Dakota State University, Brookings. Seaburg, K.G., and J.B. Moyle. 1964. Feeding habits, digestive rates, and growth of some Minnesota warmwater fishes. Transactions of the American Fisheries Society 93:269-285. Weithman, A.S., and R.O. Anderson. 1977. Survival, growth, and prey of esocidae in experimental systems. Transactions of the American Fisheries Society 106:424-430. Wolfert, D.R., and T.J. Miller. 1978. Age, growth, and food of the northern pike in eastern Lake Ontario. Transactions of the American Fisheries Society 1 07:696-702. Received: 3 June 1994 Accepted: 10 October 1994 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Freshwater Ecology Taylor & Francis

Seasonal and Size-Related Changes in the Diet of Northern Pike from a Shallow Prairie Lake

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Taylor & Francis
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Abstract

Seasonal and Size-Related Changes in the Diet of Northern Pike from a Shallow Prairie Lake Steven M. Sammonsa, Charles G. Scalet, and Robert M. Neumannb Department of Wildlife and Fisheries Sciences South Dakota State University Brookings, SD 57007 ABSTRACT Food habits of northern pike (Esox Iucius) were documented in Lake Thompson, a shallow, windswept, eutrophic lake in eastern South Dakota. The primary food items, assessed with the Relative Importance Index, eaten by northern pike were common carp (Cvorinus w, crappies (Pomoxis spp.), and fathead minnows (Pimeohales promeas). Common carp were eaten in midsummer and winter. Crappies were eaten primarily in the winter. Fathead minnows were an important food source throughout the year. Macroinvertebrates were consumed by northern pike up to 600 mm throughout the open-water months. Northern pike in the smaller length groups (401-500 mm, 501-600 mm) ate primarily fathead minnows, juvenile common carp, and macroinvertebrates. Diets of larger northern pike consisted primarily of larger common carp, juvenile crappies, leopard frogs (Rana oioiens), and occasionally walleyes (Stizostedion vitreum). INTRODUCTION Northern pike mox luciua, because they change food habits with size and grow to a relatively large size, utilize a wide range of prey organisms. Their general food habits have been described (e.g. Frost 1954, Diana 1979, Mann 1982, Chapman et al. 1989). Newly hatched northern pike feed almost exclusively on macroinvertebrates (Frost 1954, Franklin and Smith 1963). Frost (1954) recorded a shift in northern pike diets from macroinvertebrates to fishes when pike attained lengths of about 3 cm. Fishes constitute more of the diet as northern pike size increases, until the diet is almost exclusively fishes (Frost 1954). Few studies have documented northern pike diet selection over an entire year, and none have been conducted on an annual basis in natural lakes of the northern great plains. Because northern pike are opportunistic predators capable of modifying their feeding strategies in response to environmental changes (Chapman et al. 1989), how pike react in natural prairie lakes cannot necessarily be inferred from earlier studies at different locations. The purposes aPresent address: Department of Fisheries and Wildlife Sciences, School of Forestry and Wildlife Resources, V.P.I. and S.U., Blacksburg, VA 24061. b~resent address: Department of Natural Resources Management and Engineering, University of Connecticut, Storrs, CT 06269. Joumal of Freshwater Ecology, Volume 9, Number 4 - December 1994 of this study were to document the seasonal food habits of northern pike in a shallow prairie lake, and to describe diet changes associated with increasing pike size. METHODS A total of 632 northern pike stomachs were examined from Lake Thompson, a 6,573-ha natural lake located in Kingsbury County in east-central South Dakota. Lake Thompson is a typical prairie lake; it is shallow, windswept, eutrophic, and does not thermally stratify during the open-water season. The lake contains approximately 10 fish species, with northern pike, walleyes (Stizostedion vitreum), common carp (Cv~rinus car~io), and fathead minnows (Pime~hales promela3) being the most abundant. Sam~lina Protocol Lake Thompson was sampled monthly from July 1991 until February 1993, except for November and January of both years, and September 1992. Fish were collected with 45.0 by 1.8 m monofilament gill nets, with bar mesh sizes of 1.9, 2.5, 3.8, 5.1, 7.0, and 8.9 cm. During open-water periods, nets were set during the day and checked approximately every three hours to reduce digestion bias and mortality of nontarget fishes. Nets were set in nearshore environments, usually in water less than 2 m deep. During ice cover nets were set using a Murphy stick (Hubert 1983) and because catch rates were lower the nets were checked every three to five days depending on weather conditions. When ice was present nets had to be set in water 3 to 4 m deep to avoid having the nets freeze to the bottom of the ice. Northern pike were put into plastic bags and immediately placed on ice to slow digestion. Sampling continued until a sample size of approximately 50 fish was attained. Stomachs were removed from fish in the laboratory and prese~ed in 10% formalin. Stomach contents were identified to the lowest taxon possible, counted, measured to the nearest millimeter (standard length for fishes), and weighed to the nearest 0.1 g. In most cases, the lowest taxon for this study was family for aquatic insects, order for other invertebrates, and species for fishes. Food Habits Analvsis Frequency of occurrence, percent weight a taxon represented in the diet, and percent number a taxon represented in the diet have all been used to measure diets. However, each of these methods has certain biases (Bowen 1983). To avoid some of these biases, the Relative Importance (RI) index of George and Hadley (1979) was used; RI incorporates these three diet measurement techniques. The RI index ranges from 0 to 100, and the sum of all the RI values for prey eaten equals 100. The RI index decreases the importance of prey eaten in large quantities by a few fish and increases the importance of prey eaten in small quantities by many fish. Three length groups were used for RI calculations: 401 to 500 mm, 501 to 600 mm, and >600 mm (total lengths). These length groups were chosen to 322 ensure that an adequate sample size was attainable for each length-group calculation. The RI values were calculated each month for all fish sampled and Reconstructed weights were not used for RI for the three length groups. analysis, except for leopard frogs (Rana piwiens), because we found no differences in RI values calculated with and without reconstructed weights. spring (March, April, and May); Seasons referred to in this paper are: summer (June, July, and August); fall (September and October); and winter (December and February). Open-water samples were those taken from March through October, when the lake was unfrozen. Winter samples were those taken in December and February, when ice was present. RESULTS AND DISCUSSION The primary food items eaten by northern pike in Lake Thompson were common carp, fathead minnows, crappies (Pomoxis spp.), leopard frogs, leeches (Hirudinea), chironomids, and amphipods. While these were not the only items eaten, they were the ones most commonly found in stomachs. Seasonal food selection: Relative importance values for common carp peaked in late summer and in winter (Figure 1). The summer peak of common carp in northern pike diets consisted entirely of small carp (20 - 60 mm), which we assume were age-0 fish that occurred in shallow water at that time. The winter peak consisted of common carp of lengths up to 300 mm. Common carp use was lower in the second sampling year than in the first (Figure 1). Mauck and Coble (1971) determined that common carp >I10 mm were not vulnerable to northern pike 1316 mm and stated that carp likely outgrow pike predation in one or two years. However, in Lake Thompson, common carp up to 300 mm (approximately age 2) were found in northern pike stomachs during both winters. Northern pike growth in Lake Thompson was extremely fast (Neumann 1994). Northern pike with such high growth rates may be able to exert more of an influence on common carp populations than in other North American areas where pike do not grow as rapidly. Fathead minnows were an important food for northern pike in Lake Thompson throughout the year. They were the only food item that appeared in all monthly stomach samples, frequently in large numbers. However, fathead minnow RI values exhibited fluctuations (Figure 1). Declines corresponded to peaks in use of other food items, usually larger-bodied prey such as common carp and crappies (Figure 1). This suggested that while northern pike used fathead minnows to a high degree in Lake Thompson, they ate large-bodied prey when available. Northern pike have been reported to prefer larger-bodied prey (Frost 1954, Lawler 1965, Diana 1979). The increased use of fathead minnows in March 1992 (Figure 1) was likely due to the scarcity of other prey items available. There was an increase in the RI value of fathead minnows in northern pike diets the second year of sampling (Figure 1). This was probably linked to the increased abundance of fathead minnows we believe occurred during that period. Walleyes were infrequently preyed upon by northern pike in Lake Thompson (Figure 1). Of 632 stomachs examined, only seven walleyes were found. Analysis of gill-net catch data showed that walleyes and northern pike were temporally and spatially segregated in Lake Thompson, perhaps at least partially accounting for the low consumption of walleyes by pike. ..................................... OJ< 0 s o -. N;D - j P;M i M j i Stizostedion vitreum a 50, 5 25. ,................. n 0-I E J A s ODJ F~M A M j i Rana pipiens J F~M A M J . Invertebrates Month Fiaure 1, Relative Importance values for four fishes, leopard frogs, and invertebrates in northern pike diets in Lake Thompson, SD. The solid line represents the first year of sampling, July 1991 through June 1992. The dotted line represents the second year of sampling, July 1992 through February 1993. Underlined months denote those in which no sample was taken. No sample was taken in September 1992. With few exceptions, crappies were found in northern pike stomachs only during winter (Figure I), but they were an important diet component during that time. In the first winter of sampling, all but one of the 83 crappies eaten by northern pike were age-0 fish. However, in the second winter of sampling, 9 of 13 crappies eaten were age 1 and older. This was probably due to low recruitment of the 1992 crappie year class in Lake Thompson. Low crappie recruitment was observed in most natural lakes in eastern South Dakota in 1992 (C.S. Guy, Kansas Cooperative Fish and Wildlife Research Unit, personal communication) and was caused by unstable weather patterns during the spawning period. Many northern pike food habit studies have demonstrated that centrarchids are not a preferred food of pike (Seaburg and Moyle 1964, Beyerle 1971, Mauck and Coble 1971, Weithman and Anderson 1977). In a small pond stocked only with bluegills (Le~omis macrochirus) and northern pike, Beyerle (1971) found that despite the higher availability of bluegills, pike ate tadpoles as often as bluegills. Northern pike growth was slow, and he concluded that bluegills were not suitable food for pike. Beyerle and Williams (1968) found that soft-rayed fishes were selected over centrarchids by northern pike, and pike of all sizes selected the smallest centrarchid possible. This was probably the case in Lake Thompson, where northern pike ate the smallest crappies available each year. Leopard frogs were eaten primarily in October of both sampling years (Figure 1). This is when the frogs were moving from land and shallow water to deeper water for hibernation. A few leopard frogs were eaten during the winter of both years, but generally frogs were not eaten at any other time of year. There was a high use of invertebrates by northern pike in Lake Thompson (Figure 1). Invertebrates eaten by northern pike consisted of the orders Hirudinea, Amphipoda, Decapoda, and Insecta. Insects eaten were mostly chironornids, followed by odonates. Relative Importance of invertebrates averaged 18.8% during the open-water samples, and in May 1992, over 50% of northern pike diet consisted of this food item (Figure 1). Similarly, Chapman et al. (1989) found that the importance of macroinvertebrates in northern pike diets was high in May and June, then decreased throughout the rest of the year in nine Canadian lakes. Similarly, RI values of invertebrates in Lake Thompson were close to 0 during both winters (Figure 1). However, invertebrate RI values in the open-water samples were high compared to most other values reported in the literature. Diana (1979) reported that macroinvertebrate use by northern pike in Lac Ste. Anne, Alberta was less than 5% (by number) throughout the year. Lawler (1965) reported overall macroinvertebrate use by northern pike to be 4.4% (frequency of occurrence) in a Manit~ba lake. Several other studies reported low macroinvertebrate use by northern pike (Frost 1954, Seaburg and Moyle 1964, Wolfert and Miller 1978, Craig and Babaluk 1989). Chapman et al. (1989) reported northern pike diets where the frequency of occurrence of macroinvertebrates was 94% or greater in one or more sampling periods in three Canadian lakes. Mann (1976), working on two rivers in England, recorded a high use of mayflies (Ephemeroptera) by 325 Table 1. Relative Importance (RI) values for primary food items in diets of northern pike of three different length groups, sampled from July 1991 through February 1993. Length groups are: S - small (401-500 mm), M - medium (501-600 mm), and L - large (>601 mm). Seasons are: Su - summer, F - fall, W - winter, and Sp - spring. C. careio Length s 41.1 0.0 0.0 0.0 9.8 0.0 - groups M 72.3 2.1 22.3 1.9 9.5 0.0 18.2 L 0.0 5.4 44.8 0.0 8.6 0.0 16.0 P. promelas Length S 15.9 44.2 73.4 64.1 53.7 53.3 - groups M 9.5 10.5 45.5 48.3 54.6 51.6 73.8 L 0.0 6.6 9.8 40.7 11.7 26.9 48.6 Pomoxis spp. Length S 0.0 0.0 26.7 0.0 0.0 0.0 - groups M 0.0 0.0 18.6 0.0 2.6 0.0 1.1 L 0.0 9.3 31.9 0.0 0.0 0.0 17.5 Invertebrates Length S 24.5 29.4 0.0 18.3 9.4 8.3 - groups M 5.0 16.3 0.0 20.0 14.3 12.8 0.0 northern pike in April and September. Chapman and Mackay (1990) reported that northern pike ate macroinvertebrates almost exclusively in May, June, and July in an Alberta lake that had only yellow perch as a prey fish. Lake Thompson has a community of soft-rayed fishes that can be used as food for northern pike, yet they continued to eat invertebrates. Size-related Selection: Fathead minnows were a primary component in the diets of small northern pike (401 to 500 mm), were less of a component in larger pike diets (501 to 600 mm), and were of even less of a component in the largest pike diets (>600 mm) (Table 1). The fathead minnow component in the larger northern pike diets increased at the end of the sampling period; this was probably linked more to increased fathead minnow abundance than to a change in preference. Size of northern pike consuming common carp varied with season. Common carp were a primary component in small northern pike diets only during summer, and were more important in winter than in summer in the largest pike diets (Table 1). Northern pike in the 501-600 mm length group ate common carp in both seasons (Table 1). This was probably related to differences in the size of common carp consumed in summer and winter. In summer, all of the common carp consumed were juvenile fish between 20 and 100 mm, while in the winter, the majority of the carp consumed were larger than 100 mm. Frost (1954) reported that small fishes in Windermere, England were important in the diets of northern pike between 200 and 600 mm. Most other northern pike food habit studies dealing with fish in that length range have found similar results (Lawler 1965, Bregaui and Kennedy 1980, Mann 1982). Crappies were generally not eaten by small northern pike; however, in February 1992, age-0 crappies composed a large portion of the diets of all sizes of pike (Table 1). Crappies were found in winter diets of 501 to 600 mm northern pike and composed an even greater portion of the diets of the largest pike (Table 1). Macroinvertebrate occurrence in northern pike diet was primarily confined to pike under 600 mm. At times invertebrates were a major component in the diets of small northern pike (Table 1). However, they were usually a smaller component of the diets of medium-sized northern pike, although still a major food source for these fish (Table 1). Macroinvertebrate RI values were low in the diets of the largest northern pike, except for May 1992 when macroinvertebrates constituted a major food source for the entire sample. Lawler (1965) reported that macroinvertebrates were periodically important in the diets of northern pike up to 600 mm, but were not eaten by larger pike. Mann (1976) found a high use of macroinvertebrates by northern pike up to 700 mm. Chapman et al. (1989) found a high use of macroinvertebrates by northern pike in three lakes in northern Canada. However, these northern pike were all < 600 mm and the mean lengths of the pike sampled from the three lakes (360 mm, 400 mm, and 450 mm) were smaller than the mean length of pike in Lake Thompson that ate macroinvertebrates (519 mm). Water clarity may also have affected northern pike feeding in Lake Thompson. Northern pike food consumption peaked in winter months (Sammons 1993). Neumann (1994) reported that northern pike growth in Lake Thompson occurred during cool- and cold-water periods, and maximum female pike growth occurred during winter. Turbidity in Lake Thompson is primarily caused by wind. Ice formation eliminates this wind influence, the water becomes clear, and visibility is enhanced. Because northern pike are visual feeders, pike predation is facilitated by clear water, allowing higher food consumption and growth rates. Craig and Babaluk (1989) also found northern pike consumption and growth to increase in early winter in Dauphin Lake, Manitoba. They found no such increase in walleye consumption and growth rates. Since northern pike use sight more than walleyes in feeding, they also concluded that pike predation and growth was enhanced by water clarity. Diana (1979) found that in Lac Ste. Anne, Alberta, larger northern pike ate one large prey item, digested it, and then ate another. He theorized that the fish derived the maximum bioenergetic benefit from this action and northern pike eating larger food items grow optimally. Although they maintained rapid growth, northern pike in Lake Thompson did not follow this behavior. However, the largest northern pike tended to prey on the larger prey species available in the lake. Several other studies support the concept of the larger northern pike eating the largest prey fish found in the system (Frost 1954, Lawler 1965). ACKNOWLEDGEMENTS We thank M. Brown and D. Willis for critical review of this manuscript. Funding for this project was provided by the South Dakota Department of Game, Fish and Parks and the South Dakota Agricultural Experiment Station. This paper was approved for publication by the South Dakota Agricultural Experiment Station as Journal Series 2783. LITERATURE CITED Beyerle, G.B. 1971. A study of two northern pike - bluegill populations. Transactions of the American Fisheries Society 100:69-73. Beyerle, G.B., and J.E. Williams. 1968. Some observations on food selectivity by northern pike in aquaria. Transactions of the American Fisheries Society 97:28-31. Bowen, S.H. 1983. Quantitative description of the diet. Pages 325-336 jn L.A. Nielsen and D.L. Johnson, editors. Fisheries techniques. American Fisheries Society, Bethesda, Maryland. Bregazzi, P.R., and C.R. Kennedy. 1980. The biology of the pike, Esox lucius, in a southern eutrophic lake. Journal of Fish Biology 17:91-112. Chapman, L.J., and W.C. Mackay. 1990. Ecological correlates of feeding flexibility in northern pike (Esox lucius). Journal of Freshwater Ecology 5: 313-322. Chapman, L.J., W.C. Mackay, and C.W. Wilkinson. 1989. Feeding flexibility in northern pike (Esox lucius): fish versus invertebrate prey. Canadian Journal of Fisheries and Aquatic Sciences 46:666-669. Craig, J.F., and J.A. Babaluk. 1989. Relationship of condition of walleye (Stizostedion vitreum) and northern pike (Esox lucius) to water clarity, with special reference to Dauphin Lake, Manitoba. Canadian Journal of Fisheries and Aquatic Sciences 46:1581-1586. Diana, J.S. 1979. The feeding pattern and daily ration of a top carnivore, the northern pike (Esox lucius). Canadian Journal of Zoology 57:2121-2127. Franklin, D.R., and L.L. Smith. 1963. Early life history of the northern pike, &B Iucius L., with special reference to the factors influencing the numerical strength of the year class. Transactions of the American Fisheries Society 92191 -1 10. Frost, W.E. 1954. The food of pike Esox lucius L. in Windermere. Journal of Animal Ecology 23:339-360. George, E.L., and W.F. Hadley. 1979. Food and habitat partitioning between rock bass (Amblo~lites ru~estris) and smallmouth bass (Micro~terus clolomieui) young of the year. Transactions of the American Fisheries Society 1081253-61. Hubert, W.A. 1983. Passive capture techniques. Pages 95-1 11 h L.A. Nielsen and D.L. Johnson, editors. Fisheries techniques. American Fisheries Society, Bethesda, MD. Lawler, G.H. 1965. The food of the pike Esox lucius in Heming Lake, Manitoba. Journal of the Fisheries Research Board of Canada 22:1357-1377. Mann, R.H.K. 1976. Observations on the age, growth, reproduction, and food of the pike Esox lucius in two rivers in southern England. Journal of Fish Biology 8:179-197. Mann. R.H.K. 1982. The annual food consumption and prey preferences of pike from the River Frome, Dorset. Journal of Animal Ecology 51:81-95. Mauck, W.L., and D.W. Coble. 1971. Vulnerability of some fishes to northern pike (Esox lucius) predation. Journal of the Fisheries Research Board of Canada 28:957-969. Neumann, R.M. 1994. Growth, distribution, and movement of northern pike in a South Dakota natural lake, with sampling considerations. Ph.D. Dissertation. South Dakota State University, Brookings. 1993. Annual food habits of northern pike in an eastern South Sammons, S.M. Dakota natural lake. M.S. Thesis. South Dakota State University, Brookings. Seaburg, K.G., and J.B. Moyle. 1964. Feeding habits, digestive rates, and growth of some Minnesota warmwater fishes. Transactions of the American Fisheries Society 93:269-285. Weithman, A.S., and R.O. Anderson. 1977. Survival, growth, and prey of esocidae in experimental systems. Transactions of the American Fisheries Society 106:424-430. Wolfert, D.R., and T.J. Miller. 1978. Age, growth, and food of the northern pike in eastern Lake Ontario. Transactions of the American Fisheries Society 1 07:696-702. Received: 3 June 1994 Accepted: 10 October 1994

Journal

Journal of Freshwater Ecology – Taylor & Francis

Published: Dec 1, 1994

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