Muskellunge Esox masquinongy are an important and increasingly popular sportfish that are commonly stocked throughout Illinois and much of the Midwestern United States. Stocking has become the primary management tool for establishing and maintaining muskellunge populations in Illinois and across the Midwestern United States. Great demand for these fish and the high costs associated with producing them 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 data on performance of different genetic stocks of muskellunge, both within and outside their native range, is needed to determine the best population to stock in a particular body of water to maximize growth and survival. Little research has focused on the response of fish communities and lake ecosystems to muskellunge stocking. As the muskellunge range is artificially expanded by more widespread stocking it becomes increasingly important to understand the potential impacts of muskellunge introduction on existing fisheries and aquatic communities. 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. Genetically distinct stocks are quickly becoming the new operational unit in fisheries management to optimize performance regionally. Understanding stock differentiation is especially important with a trophy species like muskellunge where 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. 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. 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 rest of the aquatic community has been sparse and generally inconclusive. In addition, the existing literature on muskellunge diet focuses on natural lakes in northern states, which limits the utility of this information to managers in the lower Midwest. Few studies report fishery effects of muskellunge introductions. 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 Michigan in response to muskellunge stocking. 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 of this study provides 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 eight, all activities outlined in the annual work plan were accomplished and were completed within the specified budget. During this segment, two jobs related to muskellunge stock evaluation and one job related to food habits and effects of muskellunge introduction were continued. 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. In this segment electrofishing was conducted during fall 2009 and spring 2010. These data were combined with modified fyke net surveys during spring 2010, including the first trap netting effort on Sam Dale Lake. Across years and lakes, the Ohio River drainage stock and the Illinois population generally appear to have similar growth rates; both consistently higher than the Upper Mississippi River drainage stock. Results thus far 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 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 fish across Illinois lakes from fall 2007 to spring 2010. These lakes included Lake Shelbyville, Lake Mingo, Ridge Lake, Pierce Lake, Lake of the Woods, Otter Lake, and Sam Dale Lake. Thus far food habits data has shown that where present, gizzard shad dominates muskellunge diet in both numbers and biomass across all size classes and seasons. Gizzard shad are not present in Ridge Lake where muskellunge diets consist primarily of bluegill, although a small percentage of the samples contained largemouth bass. Results thus far 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, 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 or size related trends are present. In the current segment we continued two sets of analyses focused 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 two reference 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 is being compared to reference 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 muskellunge stocking in Lake Mingo or Ridge Lake has not negatively affected largemouth bass and bluegill abundance and no significant effects on other parts of the aquatic food web have been detected. The second set of analyses on effects of muskellunge stocking will involve a larger sample of lakes taken from the state Fishery Analysis System (FAS) database. Examination of muskellunge stocking records has identified a series of lakes that received concurrent initial stockings of muskellunge. To ensure that lakes selected for analysis have substantial muskellunge populations, we are selecting lakes where stockings have been successful based on both standardized electrofishing and spring trap netting surveys. This analysis will focus specifically on fish communities comparing trends before and after muskellunge introduction with a series of reference lakes. Reference lakes have been selected, which have similar geographic, physiochemical, morphometric and fishery characteristics to the study 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. Our results suggest that across 8 lakes in a time series ranging from 3-10 years after muskellunge introduction the abundance of largemouth bass and primary prey species have not been negatively affected. 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. These long-term data will be used to examine attributes such as longevity, maximum size-at-age, and size-at-maturity. 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 stock 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 fish populations. 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.