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Great Blue Herons

Figure 12-20 Colonial bird nest counts for Pigs Eye Lake. Source: Galli, 1992.

Electrofishing samples from Spring Lake, a backwater area affected by the Metro plant, were collected in 1981, 1986, and 1991. These samples showed an increase in species diversity and abundance of certain species (Gilbertson, 1992). The ecological quality of Spring Lake, as expressed by the Index of Biotic Integrity (IBI) (Karr, 1981; Karr et al., 1986), has improved since the mid-1980s (Figure 12-21). Species that have returned to Pool 2 include blue sucker and paddle fish; this is particularly noteworthy because paddle fish had not been observed in Pool 2 since the 1950s.

As in many other urban waterways of the United States, detectable levels of PCBs, a toxic organic chemical that adsorbs to sediment particles, have been identified in fish tissue and sediments as a result of contamination from industrial sources, transport of contaminated sediments, atmospheric deposition, stormwater runoff, and wastewater discharges. In 1975, PCB residues found in common carp and other fish species taken from the Upper Mississippi River exceeded the FDA action level of 5 mg/kg. The Minnesota Department of Health issued fish consumption advisories for a number of species including common carp, catfish, walleye, and smallmouth buffalo (MDH, 1998). Since the ban on production of PCBs in 1979, the level of PCBs in fish tissue in the Upper Mississippi River, as well as in many other rivers and lakes in the United States, has been declining. In the Upper Mississippi River, median PCB levels in common carp and walleye dropped by over 80 percent during the period between 1975-1979 and 1988-1995. Dramatic decreases have been recorded in fish tissue levels of PCBs from common carp collected in Pool 2 and Pool 4 (Lake Pepin) of the river. Lipid-normalized median PCB concentrations have declined in Pool 2 from 121 ^g/g in 1975-1976 to 18 ^g/g in 1987-1988 and in Lake Pepin from 62 ^g/g in 1973-1974 to 16 ^g/g in 1987-1988 (Biedron and Helwig, 1991)

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