. The changing Illinois environment : critical trends : technical report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. LAKES AND IMPOUNDMENTS SUMMARY Lakes and impoundments have reflected the effects of intensive agriculture, urbanization, industrialization, and species introductions, all of which have accelerated since World War II. Significant changes in fish species compositions have occurred in Lake Michigan due to deliberate and accidental species introductions (exotics now constitute 17% of all fish species),
. The changing Illinois environment : critical trends : technical report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. LAKES AND IMPOUNDMENTS SUMMARY Lakes and impoundments have reflected the effects of intensive agriculture, urbanization, industrialization, and species introductions, all of which have accelerated since World War II. Significant changes in fish species compositions have occurred in Lake Michigan due to deliberate and accidental species introductions (exotics now constitute 17% of all fish species), shoreline habitat and drainage area deterioration, and over- exploitation of some fish stocks. Recreational fishing in the Illinois part of Lake Michigan is now dominated by a native yellow perch fishery and a subsidized, introduced salmonid fishery. Because impoundment construction accelerated after many land-use changes had begun, data indicating trends in impoundments are limited. However, soil loss due to row-crop and livestock farming, in addition to the removal of stream riparian vegetation, has resulted in high sedimentation rates that are capable of filling some impoundments in less than 100 years. Most impoundments are also artificially eutrophicated due to agricultural fertilizers and urban waste. There is no evidence for multispecies overexploitation in impound- ments by recreational or commercial fishermen, when overexploitation is defined as a decrease in total yield when fishing effort increases. The definition of overexploitation of individual fish species stocks depends on the size structure of fish desired, but there is no evidence of recruitment failures or extinctions due to excessive exploitation. Major increases in expenditure on the stocking of fish have occurred, but the benefits of stocking to specific angling groups compared to the overall benefits of improving habitat and other management alternatives have not been explored. INTRODUCTION True lakes are
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