Muskellunge Esox masquinongy are an important sportsfish that are commonly stocked throughout Illinois and much of the Midwestern United States. In Illinois, as in many other states, the demand for these fishes far exceeds the supply. Stocking has become the primary management tool for establishing and maintaining muskellunge populations. The high costs associated with producing these fishes create the need for efficient management practices. Previous research efforts have determined the size of fish and timing of stocking to maximize growth and survival. However, additional information on muskellunge stocking strategies is needed. Specifically, more biological data on different genetic stocks of muskellunge is needed to determine the best population to stock in a particular body of water to maximize growth and survival. In addition little research has focused on the response of fish communities and lake ecosystems to muskellunge stocking. As muskellunge increase in popularity and stocking becomes more widespread, potential impacts of muskellunge introduction on existing fisheries and aquatic communities must be considered. Morphological and geographic characteristics have suggested multiple distinct groups of muskellunge. More recently, genetic analysis identified several different genetic stocks of muskellunge (Ohio River drainage, Upper Mississippi River drainage, and the Great Lakes drainage stocks), each with multiple populations. Previous work with young-of-year from these populations found differences in growth and food consumption as a function of temperature. As a trophy species, anglers and managers are interested in utilizing populations of fish that grow the fastest, live longest, and obtain a largest maximum size. Because muskellunge populations are either not naturally found or have been extirpated in many Illinois lakes and reservoirs, it is not clear which population to use in stocking efforts. The muskellunge population currently used as brood stock for the stocking program in Illinois is of an unknown origin and may be made up of several different populations. Muskellunge stocks from various populations may perform differently in Illinois waters in terms of growth and survival. Additional information is needed on differences in growth and survival among stocks in waters at varying latitudes in Illinois before management recommendations can be made on which stock is most appropriate. Determining which stock has the highest levels of growth and survival under the various conditions found in Illinois waters will increase stocking success and angler satisfaction. The first two jobs of this study examine differences in growth and survival among different stocks of muskellunge in order to make recommendations regarding stocking in Illinois. Previous research on interactions of muskellunge with the aquatic community has been sparse or generally inconclusive. In addition, the existing literature on muskellunge diet focuses on natural lakes in northern states which limit the utilit y o f this informat ion to managers in the lower Midwest. A few studies exist in the literature which report fishery effects of muskellunge introductions. For example one study attributed muskellunge with the decline of largemouth bass populations in two Wisconsin lakes and another study documented a decline in black crappie and white sucker populations in Iron Lake, Michigan in response to muskellunge stocking. There also exists a conflicting body of literature on interactions between muskellunge and northern pike. The utility of these studies to inform managers about the potential effects of muskellunge introduction in lakes of the lower Midwest is limited by a lack of replication or adequate comparison to control systems. The third job o f this study will provide a rigorous evaluation of the diet and community effects of muskellunge across a number of Illinois lakes in order to inform managers about the potential effects of muskellunge introductions. During segment six, all activities outlined in the annual work plan were accomplished and were completed within the specified budget. During this segment, two jobs related to 6 muskellunge stock evaluation and one job related to food habits and effects of muskellunge introduction were completed. In previous segments of the study, we compared initial growth and survival of muskellunge from the Upper Mississippi River drainage stock, the Ohio River drainage stock, and the Illinois North Spring Lake progeny in two Illinois lakes. During this segment muskellunge fingerlings from four sources were introduced into Pierce Lake, Lake Mingo, and Sam Dale Lake at rates ranging from 3.3 ñ 4.9 fish per hectare during fall 2007. Electrofishing was conducted during fall 2007 and spring 2008, and combined with modified fyke net surveys during spring 2008 in all lakes. Across years and lakes, the Ohio River drainage stock and the Illinois population appear to have similar growth rates; both consistently higher than the Upper Mississippi River drainage stock. Results from lake introductions suggest that after the first summer, the Ohio River drainage stock and Illinois population typically have similar survival and both are higher than the Upper Mississippi River drainage stock. These, and future introductions will need to be monitored over additional years to further assess long-term growth and survival differences among stocks. Muskellunge diet samples were collected from 280 fish across 4 Illinois lakes. These lakes included Lake Shelbyville, Lake Mingo, Ridge Lake, and Pierce Lake. Diet analysis showed that where present, gizzard shad dominated muskellunge diet in both numbers and biomass across all size classes and seasons. Diet of muskellunge in Ridge Lake consisted primarily of bluegill although a small percentage of the samples contained largemouth bass. Diet in this lake is limited by low species diversity and the lack of preferred prey such as gizzard shad. Diet breadth was highest in Lake Shelbyville likely due to its large size and diverse fish community. Preliminary results from diet analysis indicate that where available gizzard shad are the primary forage of muskellunge in Illinois lakes followed by bluegill. While this data provides a preliminary analysis of muskellunge diets in these lakes over the past year, more data is required to adequately characterize annual and seasonal fluctuations occurring over time. Specifically it is unclear how food habits of muskellunge may change in response to annual fluctuations in prey availability or whether consistent seasonal trends are present. In the current segment we began examining two sets of analyses on the community and fishery effects of muskellunge introductions. The first analysis utilizes a community data set collected as part of previous Federal Aid in Sport Fish Restoration Projects (F-135-R, Factors influencing largemouth bass recruitment and stocking and F-128-R, Quality management of bluegill populations). Data from each trophic level including fish communities, zooplankton, larval fish, benthic macroinvertebrates, and nutrients has been collected on a series of control lakes as well as lakes Mingo and Ridge which have received muskellunge stockings. Lake Mingo has received muskellunge since fall of 2002 and Ridge Lake has been stocked since fall 2006. Data from Mingo and Ridge Lake will be compared to control lakes before and after muskellunge introduction to determine if these stockings cause any changes in the aquatic communities of these two lakes. Preliminary results indicate that largemouth bass and bluegill abundance have not been negatively affected by muskellunge stocking in Lake Mingo. In addition gizzard shad abundance has remained relatively constant although the average size of gizzard shad sampled in Lake Mingo has increased relative to controls. The increase in size structure of gizzard shad in Lake Mingo may be due to increased predation pressure on smaller size classes by juvenile muskellunge. The second set of analyses on effects of muskellunge stocking involves a larger sample of lakes taken from the state FAS database. Examination of muskellunge stocking records has identified several lakes that received concurrent initial stockings of muskellunge. This analysis 7 will focus specifically on fish communities and creel data comparing trends before and after muskellunge introduction with a series of control lakes. Controls will be selected by choosing lakes which have similar geographic, physiochemical, morphometric and fishery characteristics to lakes receiving muskellunge stockings. This analysis will provide a more rigorous examination of muskellunge effects on existing fisheries due to a larger number of replicate lakes. In future years we will continue to monitor populations of muskellunge in lakes Mingo, Pierce and Sam Dale to evaluate long term growth and survival differences between stocks and populations. The results obtained from initial years will be combined with those from future years to identify the long-term growth and survival differences among genetic stocks of muskellunge. Results will be used to develop guidelines for future muskellunge stockings that maximize growth, survival, and angler satisfaction in lakes throughout Illinois. As the management of muskellunge fisheries improves due to increased understanding of intraspecific variation, the effects of these highly predacious fishes on the existing aquatic community also needs to be considered. In future segments we will continue to examine the food habits and effects of muskellunge on existing and native communities. This information, combined with an increased understanding of appropriate stocks, will contribute to a more informed and holistic approach to muskellunge management in Illinois and the lower Midwest.