. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. Streams and Rivers â 17 century pearl button industry, until over-exploitation depleted the resource. In the 1960s, revived populations of mussels supplied a new market as a raw material for Japan's cul- tured pearl industry. Demand pushed up prices, and the harvest of all mussel species increased dramatically between 1987 and 1990; the value of the catch from the Mississippi and Illinois rivers rose
. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. Streams and Rivers â 17 century pearl button industry, until over-exploitation depleted the resource. In the 1960s, revived populations of mussels supplied a new market as a raw material for Japan's cul- tured pearl industry. Demand pushed up prices, and the harvest of all mussel species increased dramatically between 1987 and 1990; the value of the catch from the Mississippi and Illinois rivers rose to nearly $3 million by 1990. (Figure 3-1) Harvests of threeridge mussels from the Illinois River more than doubled to more than 900,000 pounds, and 60% more live washboard mussels were hauled in during that time. Figure 3-1 Commercial Harvest of Miscellaneous Mussel Species In the Illinois and Mississippi Rivers 1920-1943,1975-1991 ,000 2,000,000 1,500,000 ,000 I I I I I 0 ! I I I I I I 1920 22 '24 26 '28 30 32 34 '36 '38 '40 '42 '75 '77 '79 '81 '83 '85 '87 '89 '91 2,500,000 -2,000,000 S o Q -s > Source: Ecological Resources, Illinois Natural History Survey, 1^94 Pollution The general quality of the water in Illinois streams has improved over the past 20 years. Measurements of metals, for example, show a trend toward decreasing concentrations, while concentrations of common pesticide compounds showed no significant trends over that time. Research also indicates decreasing trends in chemical oxygen demand, another measure of conventional water pollution. However, a highly significant trend is toward increased concentrations of substances in common agricultural use such as phosphorous and nitrate nitrogen, the latter showing up in the Rock, Illinois, and Kaskaskia river basins. During the first 60 to 70 years of this century, the discharge of sewage and factory wastes into Illinois streams was the major determinant of their wate
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