The Impact of Waterfowl Production Area (WPA) Fish Communities Upon the Invertebrate Food Base of Waterfowl. P. Kelly McDowell

Transcription

1 The Impact of Waterfowl Production Area (WPA) Fish Communities Upon the Invertebrate Food Base of Waterfowl by P. Kelly McDowell A Thesis submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE College of Natural Resources UNIVERSITY OF WISCONSIN Stevens Point, Wisconsin February, 989

2 APPROVED BY THE GRADUATE COMMITTEE OF: Professor of Wildlife Dr. James W. Hardin Professor of Wildlife Dr. Fredrick A. Copes Professor of Fisheries i

3 PREFACE This paper is part of study 36 entitled "Duck and Pheasant Management in the Pothole Region of Wisconsin." Study 36 was initiated in 982 by the Wisconsin Department of Natural Resources to determine methods of increasing waterfowl and pheasant production on private and public lands. As a part of study 36, 2 experiments were conducted to evaluate impacts of fish communities on waterfowl food resources. A paired pen study was conducted to evaluate impacts of fathead minnows on waterfowl invertebrate foods. Two paired pond experiments were conducted to further evaluate impacts of minnow and other wetland fish species which occur on federal and state Waterfowl. Production Areas. Pen study data are included in the first paper and appendix A. Paired-pond data are included in appendices B-K. ii

4 ACKBOWLEDGEMENTS This project was multi-disciplinary by design. In the same manner, assistance to complete this project was multidisciplinary. I thank my advisor Dr. Lyle E. Nauman and Richard A. Lillie, for help in all stages of this project. Eldon McLaury facilitated and encouraged research in this area. Drs. Fredrick A. Copes and James W. Hardin provided editorial assistance and were members of my graduate committee. Gerry Wegner and Greg Quinn constructed and installed enclosures. Hannibal Bolton and Jim Milligan, U.S. Fish and Wildlife Service, provided the original fish stock and recommended stocking rates. Special thanks to Thomas Neuhauser and Dr. Robert Rogers for statistical and computing assistance. Dr. Robert Freckmann verified aquatic plant samples and Jeff Demick identified unknown aquatic insects. Labor, logistical, and technical support was provided by the Wisconsin Department of Natural Resources Research and Management Staff, including James Evrard, Scott Stewart, Cindy Swanberg, Bill Fannucchi, and Bruce Bacon. I also extend a special thanks to the many University work study students who spent endless hours sorting invertebrates, particularly Bob Brua, Shelly Thilleman, and Gene Klees for laboratory and field assistance. A special word of appreciation is extended to my wife, Sandy, for her interest, support, labor, and help in the preparation of this report. Principle funding was provided by the Wisconsin Department of Natural Resources and the Federal Aid in Wildlife Restoration iii

5 - Act under Pittman-Robertson project W-4-R. Additional support was provided by the U.S. Fish and Wildlife Service and the Wetlands Conservation League of Stevens Point. iv

6 ~ABLE OF CON~ENTS PREFACE ii ACKNOWLEDGEMENTS iii TABLE OF CONTENTS v LIST OF TABLES vi LIST OF APPENDICES viii ABSTRACT INTRODUCTION OBJECTIVES STUDY SITE MERTODS Enclosures ~ Fish Collections Aquatic Invertebrate Sampling Taxa Composition Water Chemistry and Select Physical Measurements Aquatic Plant Sampling Data Analysis 0 RESULTS Invertebrate Composition Vegetation Fathead Minnow Food Habits Water Chemistry DISCUSSION MANAGEMENT IMPLICATIONS LITERATURE CITED APPENDICES v

7 LIST OF TABLES Table. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. Student t-tests between stocked and control enclosure invertebrates (numbers and biomass) in 96 water column and benthic samples in Oakridge WPA, 985. Values for and probabilities represent results of nonparametric tests after!n conversions Paired t-tests between stocked and control enclosure invertebrates (numbers and biomass) in 07 water column and benthic samples in Oakridge WPA, 986. Values for and probabilities represent results of nonparametric tests after ln conversion Comparison of percentage similarity of community (PSC) and coefficient of similarity (B) indices of taxa composition (numbers) in stocked and control enclosures in Oakridge WPA, Comparison of percentage similarity of community (PSC) and coefficient of similarity (B) indices of taxa biomass (mg) in stocked and control enclosures in Oakridge WPA, Percentage similarity of community (PSC) indices of taxa in stocked and control enclosures at depths {8-48 em) in Oakridge WPA, Percentage similarity of community (PSC) indices of taxa in stocked and control enclosures at depths {49-59 em) in Oakridge WPA, Percentage similarity of community (PSC) indices of taxa in stocked and control enclosures at depths of >59 em in Oakridge WPA, Paired t-test results of midge (Chironomidae), mayfly (Caenidae), and gastropod (Planorbidae, Lymneadae, and Physidae) populations between stocked and control enclosures in Oakridge WPA, Paired t-test results of midge (Chironomidae), mayfly (Caenidae), and gastropod (Planorbidae, Lymneadae, and Physidae) biomass between stocked and control enclosures in Oakridge WPA, vi

8 Table 0. Table. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. Table 20. Paired t-test results between stocked and control enclosures of midge (Chironomidae) and mayfly (Caenidae) average biomass (mg) in Oakridge WPA, Results of 985 t-test, and 986 paired t-test analysis of mean total vegetation stem numbers and biomass/m2 in stocked and control enclosures in Oakridge WPA Frequency of occurrence and aggregate % dry weight of plant taxa in stocked and control enclosures in Oakridge WPA, Frequency of occurrence and aggregate % dry weight of plant taxa in stocked and control enclosures in Oakridge WPA, Paired t-test results of dominant vegetation taxa/m2 between stocked and control enclosures in Oakridge WPA, Fathead minnow food habits during May-August 985 at Oakridge WPA, northwest Wisconsin, <n = 62) Fathead minnow food habits during May-August 986 at Oakridge WPA, northwest Wisconsin, (n = 4) Limnological analysis from 6 stocked and control enclosures in May and July on Oakridge WPA, Student t-test results from weekly water chemistry data from Oakridge WPA, 985. Values for were not significant at the ~ < 0.05 level Limnological analysis from 6 stocked and control enclosures on Oakridge WPA, May Student t-test results from weekly water chemistry data from Oakridge WPA, 986. Values for ~ were not significant at the ~ < 0.05 level vii

9 LIST OF APPENDICES Appendix A. Appendix B. Appendix c. Appendix D. Appendix E. Appendix F. Appendix G. Appendix H. Appendix I. Appendix J. Appendix K. Number of undesired central mudminnows and pumpkinseeds in stocked and control enclosures in Oakridge WPA, Mean number and biomass (g/m ) of invertebrates collected from fish complex (FC) and fathead minnow (FM) stocked and control ponds on Kostka WPAs in northwest Wisconsin, Mean biomass (mg) of Chironomidae from fish complex (FC) and fathead minnow (FM) stocked and control ponds on Kostka WPAs in northwest Wisconsin, Mean Diptera and non-diptera emergence (#/m ) from fish complex stocked and control Kostka ponds in northwest Wisconsin, 986, (N = 3) Mean Diptera and non-diptera emergence (#/m ) from fathead minnow stocked and control Kostka ponds in northwest Wisconsin, 986, (N = 3). 58 Mean number of invertebrates/liter in 3 zooplankton samples collected from stocked and control fish complex ponds on Kostka WPA in northwest Wisconsin, Mean number of invertebrates/liter in 3 zooplankton samples collected from stocked and control fathead minnow ponds on Kostka WPAs in northwest Wisconsin, Vegetative characteristics of stocked and control fish complex ponds on Kostka WPAs in northwest Wisconsin, Vegetative characteristics of stocked and control fathead minnow ponds on Kostka WPAs in northwest Wisconsin, Weekly water chemistry data from fish complex (FC) and fathead minnow (FM) stocked and control ponds on Kostka WPAs in northwest Wisconsin, Limnological analysis for fish complex (FC) and fathead minnow (FM) stocked and control ponds in northwest Wisconsin, viii

10 Impact of Waterfowl Production Area (WPA) fish communities on the invertebrate food base of waterfowl. Patrick Kelly McDowell, University of Wisconsin-Stevens Point Stevens Point, WI 5448 Lyle E. Nauman, University of Wisconsin-Stevens Point Stevens Point, WI 5448 AbStract: Invertebrate populations were monitored in the presence and absence of fathead minnow (Pimephales promelas) populations within a series of paired enclosures. Stocking condition did not affect invertebrate numbers and biomass in water column and benthic samples. Chemical and vegetation parameters were simdlar between stocked and control treatments. Periphyton was present in 98% of fathead minnow stomachs and comprised 89% aggregate dry weight (APDW) while invertebrates occurred in 56% and comprised 4% APDW in 985. In 986, invertebrates occurred in 89% of fathead stomachs and made up 32% APDW while periphtyon was present in 73% and made up 56% APDW. Fathead minnows have the potential for competition with waterfowl but did not appear to impact invertebrate populations in this study.

11 IRlRODUCTIOM 2 Researchers are aware of the importance of invertebrates as protein in duckling and breeding waterfowl diets (Chura 96, Perret 962, Dirschl 969, Schroeder 973, Krapu 974, Krapu and Swanson 975, Reinecke 977, Street 978, Drobney and Fredrickson 979, Pehrsson 979). Past studies based on gizzard and gullet material over-estimated the importance of plant foods in diets of waterfowl and under-estimated the importance of animal foods (Swanson and Bartonek 970). Reported biases were related to differences in digestion rates of plant and animal foods and the time lag between feeding and collection. Therefore, past management of wetland habitat for waterfowl has emphasized seed production rather than invertebrate production. Seasonal variations in waterfowl feeding behavior are dependent on nutritional needs, stage of development and availability of food items (Chura 96, Bartonek and Hickey 969, Krapu 974, Swanson and Meyer 977, Drobney and Fredrickson 979, Pehrsson 979). The availability of invertebrates appears to influence duck brood movements (Ball 973, Talent 980, Ringleman and Longcore 982, Talent et al. 982). Low invertebrate numbers may have contributed to low duckling survival in English gravel quarries (Street 977). The abundance, composition and availability of invertebrate food items are directly related to aquatic plant characteristics (Krecker 939, Berg 949, McGaha 952, Rosine 955, Krull 970, Mauser 985). Aquatic plant characteristics

12 3 are related to various abiotic and biotic factors including water chemistry, sediment or soil characteristics, water level fluctuations, muskrat (Ondatra zibethicus) eatouts, and weather. Swanson and Nelson (970) expressed concern that wetland fisheries may adversely impact waterfowl breeding habitat through the alteration or destruction of aquatic vegetation and/or direct competition for invertebrate food items. This concern has been voiced more recently by wildlife professionals in the Midwest (E. McLaury, pers. comm.), in particular, the possible impact of minnow stocking or removal by bait dealers on waterfowl production. The sale of live bait in Wisconsin generates $.5 millon annually and is increasing (Threinen 982). The potential conflict or incompatability in the use of existing resources by both wildlife and fisheries interests has been explored recently with varing degrees of success. Pehrsson (984) documented that mallards (Anas platyrhynchos) in Sweden tended to select smaller, fishless lakes over lakes with fish. Erickson (979) demonstrated that fledged goldeneyes (Bucephala clanqula) preferred to lakes lacking fish populations. He speculated that the mechanism responsible for this selection was related to the availability of invertebrate food organisms as influenced by fish predation. Carmichael (983) studied the dietary overlap of largemouth bass (Micropterus salmoides) and rainbow trout (Salmo gairdneri) with canvasback (Aythya americana) and redhead ducks (Aythya valisineria). He estimated co-utilization of food resources of

13 4 <40\, with greater overlap between canvasbacks and fish than between redheads and fish. The adverse effect of acidification on fish may produce temporary positive impacts on duckling food resources (Hunter et - al. 985 and 986). They reported invertebrate numbers and biomass were greater on acidified ponds without fish populations in Maine. Ducklings gained more weight and spent less time searching for food and more time feeding on acidified lakes. This was presumably caused by the increased invertebrate availability due to fish extirpation. While the impacts of particular fish species or communities on invertebrate communities are well documented (Galbraith 967 and 982, Crowder and Cooper 982, Gilinsky 984, Mittlebach 984), little has been reported relative to minnow species resident to Wisconsin WPAs. Food habit studies of fathead minnows (Held and Peterka 974, Isaak 96, Saylor 973, Zischke et al. 983) suggest a potential dietary overlap with waterfowl during later life stages of minnows. Results of many food habits studies of the fathead minnow are conflicting. Pearse (98) and Held and Peterka (974) reported that fathead minnows feed primarily on zooplankton. Bottom ooze or slime was reported as an important food items of fathead minnows (Starrett 950, Simon 95, Copes 970). Vegetation was the primary food source used by fatheads in Beckmans {952) study, while Isaak (96) found that invertebrates were important only to larger fry. In recent years, the fathead minnow has been used in

14 5 Minnesota for mosquito control (Becker 983}. This study was designed to explore the potential impacts of fathead minnow populations on the invertebrate food base of waterfowl. OBJECTIVES Research objectives related to documenting interactions between fathead minnow and waterfowl populations are as follows: ) Compare invertebrate abundance, diversity, and composition in the presence and absence of fathead minnows. 2) Determine the preference for invertebrate food items by fathead minnows. 3) Compare invertebrate parameters and minnow dietary preference to waterfowl diet literature to estimate overlap with resident waterfowl. S~DY SITE The study site is located in the Wisconsin pothole region in northern St. Croix County, 6 (km} northeast of New Richmond, Wisconsin. Experiments were conducted on the south shore of Oakridge WPA. Oakridge is a groundwater depression or slope wetland with continuous groundwater flow to adjacent down slope wetlands (Evrard and Lillie 985). The Oakridge basin is large (65 Ha) with a narrow margin of littoral zone (<0m). Fish populations were dominated by fathead minnows and mudminnows (Umbra limi). Pumpkinseeds (Lepomis gibbosus) and golden shiners (Notemigonus crysoleucas) were also common in Oakridge WPA.

15 6 METHODS This study evaluated the impacts on invertebate populations by the dominant minnow species present in most WPAs, the fathead minnow. enclosures. Evaluations were conducted within a series of paired Fathead minnows were stocked at densities comparable to those generally found on WPAs in the pothole region of Wisconsin. present. Adjacent enclosures were established with no fish The vegetation and invertebrate communities within the enclosures were monitored from early May to mid July to document the direction and degree of changes occurring. limnological data were likewise monitored. Associated Approximately 0 fish were removed every 2 weeks for analysis of gullet contents. Enclosures 2 In 985, 6 pairs of 3. m, 3 sided, screened, enclosures were placed parallel to the shoreline in areas characteristic of depths and vegetation which are used often by waterfowl broods {< m}. Each enclosure was divided into 2 cells open on the shoreward side with one side in common with an adjacent cell. The sides of enclosures were approximately {O.Sm} above the water to prevent waves from transporting organisms into or out of the enclosures. mill drying felt {200 CFM}. Sides of the enclosures consisted of pulp The felt allowed free circulation of water, but prevented movement of most organisms into or out of the enclosures. Sides of the enclosures were extended and buried in the subtrate approximately 30 em to prevent

16 7 movement of fish and invertebrates into or out of enclosures. Algae growth on the sides of the barriers was removed as needed to maintain water circulation and minimize affects of shading. In 986, the enclosures were modified to maintain adequate water levels in all the enclosures. Enclosures used during 986 had 4 sides and were placed in deeper water (X=56 em) compared to the shallower 3 sided enclosures used in 985 (X=20 em). A stocking rate of 50 adult fathead minnows/cell was used to replicate densities normally found on many WPAs in the pothole region of Wisconsin, (H. Bolton per. comm.) as supported by unpubl. data. Only males were stocked to prevent reproduction within enclosures. Stocking was done in early May. Male fatheads are generally larger than females of the same age class (Becker 983), and may eat larger food items. Hence, the experimental design likely represents a "worst-case" scenario. Dead minnows or those removed for stomach analysis were replaced from existing male WPA populations. All fish were fin clipped initially before release. Bi-weekly sampling around enclosures and within the control portion of the enclosure was conducted to determine if minnows were escaping. Fin clipping was discontinued after 7 June 985, because of fungal development. Fish Collections Minnows were collected by shocking within enclosures every 2 weeks during crepuscular hours. Minnows were preserved in 70% ethyl alcohol and stomach contents were removed later to identify contents. Food items were identified, counted, dried at 65 C for

17 8 24 hours and weighed on a H5 Mettler analytical balance to the nearest 0.0 mg. Minnows which were shocked and removed from the enclosures were replaced to maintain a constant stocking level. Aquatic Invertebrate Sampling Water column and benthic samples were collected on 6 dates between 24 May and 5 July in 985 and 986. Cells were subdivided into 6 transects perpendicular to the shoreline. One transect was selected randomly on each sample date. Each transect was sampled once at 3 locations: near the shoreline, midway and at the deeper end. One water column and one benthic sample was collected at each location. Benthic and water column samples were separated to detect differences that might arise. from fish predation. Benthic organisms were collected with a modified core sampler described by Swanson (978A). Water column samples were collected with a column sampler similar to the one described by Swanson (978b). Water column and benthic samples were poured through a #30 sieve and subsequently preserved in 70% ethyl alcohol. Samples were sorted and keyed to the taxonomic level of family for Diptera, Ephemeroptera, Coleoptera, Hemiptera, and Gastopods; to Suborder for Odonata: and to Order for other invertebrates. Invertebrates were counted, air dried and weighed to the nearest 0.0 mg on a H5 Mettler analytical balance. Analysis was not done for Copepods, Cladoceran, Oligacheates, and Collembolla, because of their potential to pass through a #30 sieve.

18 9 Taxa Composition Taxa composition in stocked and control enclosures were compared using 2 similarity indices; the percentage similarity of community (PSC) as discussed by Whittaker {952) and the coefficient of similarity {B) Pinkham and Pearson (976). The coefficient of community was used to compare taxa similarity between stocked and control enclosures. These indices were selected because of favorable reviews (Brock 977, Whittaker and Fairbanks 958, and Washington 984} and their ability to evaluate species occurrence and abundance. Two similarity indices were used because of reported inconsistencies between indices {Brock 977). The B index is sensitive to changes in rare taxa numbers (Brock 977), while the?sc index emphasizes changes in abundant taxa (Whittaker 952). Brock (977) concluded that B index may over emphasize changes in rarer species and underestimate changes in more abundant species. The abundance of an invertebrate taxon is important in determining its availability to waterfowl {Swanson 984, Serie and Swanson 976}. Samples were divided into 3 depths, shallow (8-48 em), medium (49-59 em). and deep ( > 59 em) to evaluate potential impacts of fish predation on taxa at different depths. Water Chemistry and Selected Physical Measurements Water samples were collected in May and August in 985 and in June of 986. Parameters measured included total alkalinity, color, turbidity, conductivty, ph and nutrients including total

19 0 nitrogen, total phosphorus, calcium, magnesium, sulfate and chloride. Analysis was conducted by the State Laboratory of Hygiene, Madison, Wisconsin. Total alkalinity (titrations}, conductivity (meter), ph (indicator dye-color comparator), dissolved oxygen (titration), and temperature were done weekly from May to July both years. Aquatic Plant Sampling Aquatic plant samples were collected in conjunction with water column samples. All living portions of plants in the water column samples were washed to remove attached invertebrates.. Aquatic plants were separated by species, force air dried at 80 C for 24 hours, and weighed to the nearest 0.00 g. Major plant communities were mapped in each enclosure in early July in both years. Data Analysis Data were analyzed by year because of modification to the enclosures between 985 and 986. Invertebrate data were converted to n for non-parametric testing. Non-parametric testing was neccessary because of the contiguous distribution of invertebrates (Elliott 977}. Paired t-tests were used to analyze 986 invertebrate data. In 985 Student t-tests were used because of missing sample sites due to low water levels and poor preservation of many samples. In presentations of invertebrate statistical analysis, ~values and probabilities represent results of ln transformed values, however means equal

20 actual values. Invertebrate water column number and biomass are reported per cubic meter and per square meter for comparison with other studies. Weekly water chemistry data were analyzed with a Student t-test. Vegetation was analyzed with paired t-tests. All analysis was done on a SPSSx statistical programming package. RESULTS 3 Average densities of total invertebrates/m in 985 and 986 were (X= 42,69) and (X= 24,330), respectively. Invertebrate numbers and biomass in 985 water column or benthic samples were not significantly different (~ < 0.05) between stocked and unstocked enclosures (Table ). The stocking condition in 986 did not influence (~ < 0.05} invertebrate numbers and biomass in water column or benthic samples (Table 2). The central mudminnow was present in both stocked and control portions of enclosures in 985. In 986 both pumpkinseeds and mudminnows were present in stocked and control enclosures. The study design did not account for this type of intrusion. Stocked pens in 986 contained more pumpkinseeds and mudminnows than did control pens. In 985 effort of removal was not equal between stocked and control enclosures, so it was not possible to determine what influence mudminnows may have had on the study. Because it is impossible to determine what degree mudminnows could potentially influence the experiment in 985, only basic statistical comparisons will be made.

21 2 Table. Student t-tests between stocked and control enclosure invertebrates {numbers and biomass) in 96 water column and benthic samples in Oakridge WPA, 985. Values for ~ and probabilities represent results of nonparametric tests after ln conversions. Sample Type Stocked Control.t. Value Water Column 3 n/m 36,68 28, n/m 4,647 4,88 3 Biomass/m Biomass/m Benthic 2 n/m 9,889 0, Biomass/m

22 3 Table 2. Paired t-tests between stocked and control enclosure invertebrate (numbers and biomass) in 07 water column and benthic samples in Oakridge WPA, 986. Values for and probabilities represent results of nonparametric tests after ln conversions. Sample Type Stocked Control.t. Value Water Column 3 n/m 2 n/m 9,456 0,590 7,668 0, Biomass/m 2 Biomass/m Benthic 2 n/m 2 Biomass/m 5, ,

23 Invertebrate Composition 4 Comparison of the PSC and B similarity indices displayed different results between taxa composition in stocked and control enclosures, 986. The PSC taxa composition indicies for numbers {95%) and biomass {63%) were much higher than the B indices for numbers {54%) and biomass {36%) {Table 3-4). Percent similarity coefficient {numbers) were similar at shallow {89%), medium {86%) and deep {88%) depths between stocked and control enclosures {Table S-7). Biomass PSC indices for taxa composition between stocked and control enclosures were 82 % at shallow, 27% at medium, and 59% at deep depths. During 986, midge and mayfly densities comprised 59 and 58\ of total invertebrate numbers in stocked and control enclosures, respectively. Total gastropoda biomass comprised 6% of stocked and 3% of control enclosures. Midge, mayfly, and gastropod taxa were analyzed separately because of their importance to waterfowl diets and dominant presence in St. Croix County WPAs. Numbers and biomass of the 3 invertebrate taxa were not significantly different (P< 0. OS) between stocked and control -enclosures in the water column or benthic samples during 986 (Tables 8-9). Average biomass of midge and mayfly in the water column were less in stocked enclosures than in controls, however differences were not significant at the~< 0.05 level (Table 0).

24 5 Table 3. Comparison of percentage similarity of community (PSC) and coefficient of similarity (B) indices of taxa composition (numbers) in stocked and control enclosures in Oakridge WPA, 986. Taxa Mayfly (Caenidae) Caddisfly (Trichoptera) Lepidoptera Dragonfly (Anisoptera) Damselfly (Zygoptera) Pigmy backswimmer (Pleidae) Water boatmen (Corixidae) Water scorpion (Nepidae) Water treader (Mesoveliidae) Leaf hopper (Homoptera) Midge (Chironomidae) Biting Midge (Ceratopogonidae) Cranefly (Tipulidae) Soldierfly (Stratiomyidae) Deerfly (Tabanidae) Marshfly (Sciomyzidae) Mosquito (Culicidae) Scavenger beetle (Hydrophilidae) Crawling water Beetle (Haliplidae) Predacious diving Beetle (Dytiscidae) Scirtidae Staphylinidae Leaf Beetle (Chrysomelidae) Weevil (Curculionidae) Leech (Hirundinae) Water mite (Hydracarina) Spider {Aranae) Ant (Hymenoptera) Scud (Amphipods) Orb Snail (Planorbidae) Pouch Snail (Physidae) Pond Snail {Lymnaeidae) Pelecypoda Total PSC = 95%, B = 54\ Stocked n n Control

25 Table 4. Comparison of percentage similarity of community (PSC) and coefficient of similarity (B) indices of taxa biomass (mg) in stocked and control enclosures on Oakridge WPAs, 986. Taxa Stocked Biomass (mg) Control Biomass (mg) 6 Mayfly (Caenidae) Caddisfly (Trichoptera) Lepidoptera Dragonfly (Anisoptera) Damselfly (Zygoptera) Pigmy backswimmer (Pleidae) Water boatmen (Corixidae) Water scorpion (Nepidae) Water treader (Mesoveliidae) Leaf hopper (Homoptera) Midge (Chironomidae) Biting midge (Ceratopogonidae) Cranefly (Tipulidae) Soldierfly (Stratiomyidae) Deerfly (Tabanidae) Marshfly (Sciomyzidae) Mosquito (Culicidae) Scavenger beetle (Hydrophilidae) Crawling water Beetle (Haliplidae) Predacious diving Beetle (Dytiscidae) Scirtidae Staphylinidae Leaf beetle (Chrysomelidae) Weevil (Curculionidae) Leech (Hirundinae) Water mite (Hydracarina) Spider (Aranae) Ant (Hymenoptera) Scud (Amphipods) Orb snail (Planorbidae) Pouch snail (Physidae) Pond snail (Lymnaeidae) Pelecypoda Total PSC = 63\, B = 36\ T T T T = Taxa with <0.0 \ of the total invertebrate biomass T T T T T T

26 7 Table 5. Percentage similarity of community (PSC) indices of taxa in stocked and control enclosures at depths (8-48 em) in Oakridge WPA, 986. Taxa Numbers Stocked % Control % Biomass Numbers Biomass Mayfly (Caenidae) Caddisfly (Trichoptera) Lepidoptera Dragonfly (Anisoptera) Damselfly (Zygoptera) Pigmy backswimmer (Pleidae) Water boatmen (Corixidae) Water treader (Mesoveliidae) Leaf hopper (Homoptera) Midge (Chironomidae) Biting Midge (Ceratopogonidae) Cranefly (Tipulidae) Deerfly (Tabanidae) Marshfly (Sciomyzidae) Scavenger beeetle (Hydrophilidae) Crawling water Predacious diving Beetle (Dytiscidae) Leech (Hiriundinae) Water mite (Hydracarina) Spider (Aranae) Scud (Amphipods) Orb snail (Planorbidae) Pouch snail (Physidae) Pond snail (Lymnaeidae) PSC Number = 89%, PSC Biomass = 82%

27 8 Table 6. Percentage similarity of community (PSC) indices of taxa in stocked and control enclosures at depths (49-59 em) in Oakridge WPA, 986. Taxa Stocked % Control \ Numbers Biomass Numbers Biomass Mayfly (Caenidae) Caddisfly (Trichoptera) Lepidoptera Dragonfly (Anisoptera) Damselfly (Zygoptera) Pigmy backswimmer (Pleidae) Midge (Chironomidae) Biting Midge (Ceratopogonidae) Deerfly (Tabanidae) Marshfly (Sciomyzidae) Scavenger beetle (Hydrophilidae) Crawling water Beetle (Halipidae) Predacious diving Beetle (Dytiscidae) Leaf beetle (Chrysomelidae) Weevil (Curculionidae) Leech (Hiriundinae) Water mite (Hydracarina) Spider (Aranae) Ant (Hymenoptera) Scud (Amphipods) Orb snail (Planorbidae) Pouch snail (Physidae) Pond snail (Lymnaeidae) Pelecypoda T T T T PSC Number = 86%, PSC Biomass = 27% T = Taxa which had number or biomass that made up <0.0 % of the total invertebrate number or biomass

28 9 Table 7. Percentage similarity of community (PSC) indices of taxa in stocked and control enclosures at depths of >59 em in Oakridge WPA, 986. Taxa Stocked % Control % Numbers Biomass Numbers Biomass Mayfly (Caenidae) Caddisfly (Trichoptera) Dragonfly (Anisoptera) Damselfly (Zygoptera) Pigmy backswimmer (Pleidae) Water boatmen (Corixidae) Water scoirpion (Nepidae) Water treder (Mesoveliidae) Midge (Chironomidae) Biting Midge (Ceratopogonidae) Cranefly (Tipulidae) Soldierfly (Stratiomyidae) Deerfly (Tabanidae) Marshfly (Sciomyzidae) Mosquito (Culicidae) Scavenger beetle (Bydrophilidae) Predacious diving Beetle (Dytiscidae) Scirtidae Staphylinidae Leaf beetle (Chrysomelidae) Leech (Biriundinae) Water mite (Bydracarina) Spider (Aranae) Ant (Hymenoptera) Scud (Amphipods) Orb snail (Planorbidae) Pouch snail (Physidae) Pond snail (Lymnaeidae) Pelecypoda T T T T T PSC Number = 88%, PSC Biomass = 59% T = Taxa which had numbers or biomass that made up <0.0 % of the total invertebrate number or biomass

29 20 Table 8. Paired t-test results of midge {Chironomidae}, mayfly (Caenidae}, and gastropod {Planorbidae, Lymneadae, and Physidae} populations between stocked and control enclosures in Oakridge WPA, 986. Sample Type 3 a/m 2 n/m. Value Midge Water column Stocked Control 7,959 6,98 4,650 4, Benthic Stocked Control 2,433 2, Mayfly Water column Stocked Control 3,924 3,827 2,292 2, Benthic Stocked Control,53, Gastropod Water column Stocked Control 4,560 4,430 2,667 2, Benthic Stocked Control,65,

30 2 Table 9. Paired t-test results of midge (chironomidae}, mayfly (Caenidae} and gastropod (Planorbidae, Lymneidae, and Physidae} biomass between stocked and control enclosures in Oakridge WPA, 986. Sample Type Biomass Biomass.t. Value g/m3 gfh2 Midge Water column Stocked Control Benthic Stocked Control Mayfly Water column Stocked Control Benthic Stocked Control Gastropod Water column Stocked Control Benthic Stocked Control

31 22 Table 0. Paired t-test results between stocked and control enclosures of midge (Chironomidae) and mayfly (Caenidae}, average biomass (mg} in Oakridge WPA, 986. Sample Type Stocked Control t Value Midge Water column Benthic Mayfly Water column Benthic

32 23 Vegetation Mo significant {~ < 0.05} differences in total number of stems and plant biomass between stocked and control enclosures were observed in 985 or 986 {Table ). In 985 emergent plant taxa were dominant {Table 2), while in 986 submergent plants were more common {Table 3}. Differences in plant communities between 985 and 986 were due probably to differences in the mean water column sample depths between 985 (X = 20cm) and 986 (X = 56cm). Vegetative composition and structure were similar between stocked and control enclosures within years of sampling. Watercolumn samples were dominated by 5 taxa; Myriophylum spp., Ceratophylum spp., Potamoqeton zosteriformis, ~. qramineus and natans, and Mymphea spp. in 986. Biomass and number of 2 stems/m of these species were compared separately from other taxa. Only biomass was compared for Myriophylum and Ceratophylum spp., because samples of these taxa often contained only portions of plants. 2 The number of stems and biomass/m of dominant plant taxa in stocked and control enclosures were not significantly {~ < 0.05} different (Table 4). Fathead Minnow Pood Habits Measurable amounts of food { >0. mg) were found in 62 fathead minnow stomachs in 985 and 4 in 986. Periphyton was the dominant food of fathead minnows in 985 and was important in 986. Periphyton appeared in 98% of 985 fathead stomachs and

33 24 Table. Results of 985 t-test, and 986 paired t-test analysis between mean total vegetation stem numbers and biomass/~in stocked and control enclosures in Oakridge WPA. Sample Type Stocked Control!. Value 985 Number stems Biomass Number stems Biomass

34 25!able 2. Frequency of occurrence and aggregate % dry weight of plant taxa in stocked and control enclosures in Oakridge WPA, 985.!ax a SlOCKED CON!ROL Freq. % Aggr. % Freq. % Aggr. % ~ lanuqinosa ~ rastrata! ~ camosa 5 ~ spp.!! Ceratophylum-Mvriophyllum Pbalaris arundinacea Potamoqeton zosteriformis 6 2 L qramineus & natans Sparaanium spp lt.!mni spp. 2! 7! Spartina pectinatas Agropyron respens! Saqittaria latifolia Circu\a bulbifera:!! Scirpus cyperinus 2 Scirpus validius 2 Juncus balticus! No vegetation 40 36

35 26 Table 3. Frequency of occurrence and aggregate \ dry weight of plant taxa in stocked and control enclosures in Oakridge WPA, 986. Taxa STOCI(ED rreq. \ Aggr. \ CONTROL rreq. \ Aggr. \ Unknown grass T Unknown 2 T Nymphea spp Carex comosa T Ceratophylum demersum Myriophyllum spp Potamogeton filiformis 2 T 2 Potamogeton natans T Potamogeton zosteriformis ~ gramineus & natans Sparganium spp. 2 3 Typha spp. T Sagittaria latifolia cirpus validius Naias spp T No vegetation 3

36 27 2 Table 4. Paired t-test results of dominant plant taxa/m between stocked and control enclosures in Oakridge WPA, 986. Sample Type Stocked Control.t. Value Myriophylum spp. Biomass {gm) Ceratophylum demersum Biomass {gm) Potamoqeton zosteriformis Humber stems Biomass {gm) Potamoqeton qragdneus & natans Number of stems Biomass {gm) Nymphea spp. Number of stems Biomass (gm)

37 28 made up 89% in APDW of their diet. Invertebrates followed in importance with 56% frequency of occurrence and 4% APDW. Ostracods were the most important invertebrate with 48% frequency of occurrence and 3% APDW followed by Ceratopogonidae with 5% frequency of occurrence and % APDW (Table 5). The diet of fathead minnows was different in 986. Invertebrates made up the major portion of fathead minnow diets in 986. Invertebrates were recorded in 89% of fathead minnow stomachs and comprised 32% APDW. Ostracods remained the major invertebrate food item {65% frequency of occurrence) but were lower in biomass (2% APDW). Chironomids were the most important invertebrate prey in 986, by weight (8% APDW) and occurred in 42% of the fathead minnow stomachs (Table 6). Periphyton remained the most important food item by weight {56% APDW) in 986, and occurred in 73% of the stomachs. Water Chemistry Water chemistry parameters remained constant between stocked and control enclosures in May and July 985. Alkalinity and conductivity decreased from May to July in 985. Turbidty was 4 higher in the July samples. Total N and P, Ca, Mg, So, and Cl concentrations were similar between stocked and fishless enclosures (Table 7). Nutrient concentrations were slightly higher between May and July samples. Turbity in stocked pens was higher due to the presence of a muskrat in one pen. Increased nutrient

38 29 Table 5. Fathead minnow food habits during May-August 985 at Oakridge WPA, northwestern Wisconsin, {.n = 62). Freq. of APDW Aggr. Taxa D. Occurrence Periphyton Algae 2 T T Detritus 37 4 Total invertebrates Ostracods Bydracarina 6 0 T T Caenidae 2 T T Ceratopogonidae 3 5 T Chironomidae 2 3 T T

39 Table Fathead minnow food habits during May-August 986 at Oakridge WPA, northwest Wisconsin, <n = 4). Taxa \ Freq. APDW Aggr. Occurrence Periphyton N/A Filirnentous algae N/A 7 T T Detritus N/A 35 9 Fish eggs Total Invertebrates Ostracods T 2 Cladocera T Copepoda T T Amphipods 8 6 T Hydracarina 9 5 T T Planorbidae Unknown Chironomidae Larvae Pupae T T Adults 5 9 T 3 Ceratopogonidae Larvae Adults T T Caenidae Ceongrionidae Tabanidae T Stratiomyidae T Notonectidae T T Coleoptera 2 2 T

40 3 Table 7. Limnological analysis from 6 stocked and control enclosures in May and July on Oakridge WPA, 985. Variable HI! liyli Mean Stocked Control Stocked Control Chemical Parameters Alkalinity (Mg/L) pb Conductivity (Umbos) Color Turbidity (JTU) Ion and Nutrient Concentrations (mg/l) Ca Mg so Cl lotal-p Total-N

41 32 levels in the 985 July control enclosures may have been associated with higher turbidity. Mean weekly water chemistry samples including ph, alkalinity, conductivity and dissolved oxygen did not significantly differ at the E < 0.05 level between stocked and control enclosures in 985 (Table 8). Water quality in 986 appear to be similar in stocked and control enclosures (Table 9). Analysis of 986 weekly water chemistry data showed no differences at the~ < 0.05 level for all parameters between stocked and control enclosures (Table 20). DISCUSSIOII Fathead minnow predation did not appear to impact invertebrate populations in benthic or water column samples. However, it is difficult to be certain what effects stocking had on invertebrate populations in 985 because of an undetermined number of central mudminnows in stocked and control enclosures. This species is a bottom dweller which burrows in the mud and is difficult to remove by electric shocking. Central mudminnows may also have burrowed under the enclosures. The central mudminnow is an opportunist, feeding mainly on invertebrates (Paszkowski 983). Mudminnows and pumpkinseeds were found in enclosures in 986. Pumpkinseeds spend much of their time in shallow water (Becker 983), feeding primarily on insect larvae and Gastropods (Sadzikowski and Wallace 976, Becker 983). Predation by mudminnows and pumpkinseeds could have reduced invertebrate numbers to an undetermined amount. The study

42 33 Table 8. Student t-test results from weekly water chemistry data from Oakridge WPA, 985. Values for~ were not significant at the~< 0.05 level. Chemical Variable X Standard Deviation t Value Temperature (C ) Stocked Control Alkalinity (mg/l) Stocked Control ph (units) Stocked Control Conductivity (umbos) Stocked Control Dissolved 0 (mg/l) Stocked Control

43 34 Table 9. Limnological analysis from 6 stocked and control enclosures on Oakridge WPA, May 986. Variable Mean Stocked Control Chemical Parameters Alkalinity (Mg/L) ph Conductivity (Umbos) Color Turbidity (JTU) Nutrient Ion Concentrations Ca Mg 804 Cl Total-P Total-N. 2 3 NO & NO

44 35 Table 20. Student t-test results from weekly water chemistry data from Oakridge WPA, 986. Values for!_ were not significant at the P < 0.05 level. Chemical Variable Mean Standard Deviation!. Probability Value Temperature {C) Stocked Control Alkalinity {mg/l) Stocked Control ph {units) Stocked Control Conductivity {umhos) Stocked Control Dissolved 02 {mg/l) Stocked Control

45 36 design did not account for this type of intrusion. In 985, accurate records of removal of undesirable fish were unavailable and removal efforts were more intense in stocked enclosures. In 986, the removal effort was constant between stocked and control enclosures and numbers of fish removed were recorded. Although more pumpkinseeds and mudminnows were found in stocked than in control enclosures, there were no differences (P < 0.05) in invertebrate numbers and biomass. Invertebrate populations may have already been impacted by an existing fish population, causing the fathead minnow to feed on more abundant available resources, particularly periphyton. The pens were in shallow water (X=20 em) within emergent vegetation and fathead minnow mortality was high ip 985 and they may have been displaced from their natural habitat. While electroshocking, few fish were captured in shallow water within emergent vegetation. Most fish were caught in slightly deeper water near the edge of the emergent vegetation and within submergent vegetation. This may account for the differences in food habits between 985 and 986. Differences in food habits from many studies suggest the fathead minnow is an opportunist selecting different food items in different regions. Regional differences in feeding have been observed in Wisconsin. Our 985 data agree with Williamson (939) who reported that stomach contents from fathead minnows from northern Wisconsin consisted of algae and organic matter. In southeastern Wisconsin, the fathead was observed feeding on

46 37 the bottom for insect larvae which formed over 90% of its diet (Cahn 927). In 986 more fathead minnow stomachs contained invertebrate foods, however, periphyton was more important by weight. Biomass of taxa varied between stocked and control enclosures, primarily at medium depth~. The biomass taxa composition coefficient showed only 27% similarity. The average weight of individual midge and mayfly larvae was less in stocked enclosures, although not significantly different at P < Numbers of invertebrates can increase in fish predated environments due to removal or reduction of invertebrate predator species (Gilinsky 984). Numbers may remain constant while size or biomass of invertebrates are reduced (Galbraith 967, Mittelbach 984). Fathead minnow or mudminnow predation on larger invertebrate instars may explain smaller midge and mayfly average biomass. If the experiment were to occur over a greater time period invertebrate size, composition and abundance may have become significantly different between stocked and control enclosures. The littoral zone of Oakridge WPA had dense stands of aquatic vegetation. Gilinsky (984) reported that macrophytes may serve.as cover from fish predators for invertebrates, and structural complexity decreases predator efficiency. Wetland zones with dense stands of aquatic vegetation may provide a barrier from fish predation to prevent significant reductions in invertebrate foods for waterfowl. Invertebrate numbers in 985 were higher than in 986. Waterfowl invertebrate food resources in areas

47 38 with sparse emergent vegetation may be impacted greater by fish predation because of greater mobility of fish in these areas. This study suggests that there is a potential dietary overlap between food habits of fathead minnows and waterfowl. The primary nesting waterfowl Qn the study area and in Wisconsin are blue-winged teal (Anas discors) and mallards (Evrard and Lillie 985, Jahn and Hunt 964). Waterfowl ducklings feed primarily on invertebrates in the first weeks of development but gradually change to a plant diet as they mature (Cottam 939, Mendall 949, Sugden 973}. Periphyton was the staple food of fathead minnows in this study. Fathead minnows consumption of periphyton and organic matter have little potential overlap with duckling diets. However, fathead minnow diets consisting of invertebrate foods may impact invertebrate food resources used by resident ducklings depending on abundance and availability of invertebrate prey. Ruddy duck (Oxyura jamaicensis) ducklings are considered excellent strainers, eating primarily zooplankton (Collias and Collias 963}. However, even the ruddy duck did not strain the very small ostracods and mites found in fathead stomachs. In 986, fathead food habits reflected greater potential for competition with waterfowl. Chironomids are often the dominant invertebrate food item in waterfowl producing wetlands (Maher 984, Maher and Carpenter 984, Mauser 985, this study). The availability of emerging chironomids coincide with waterfowl breeding in Sweden and Australia (Danell and Sjoberg 977, Sjoberg