Fishery surveys in the tidal Potomac, and elsewhere in the Chesapeake Bay, clearly document an increase in abundance and diversity of fish species. Juvenile fish survey data, collected between 1965 and 1987 at Indianhead and Fenwick in the tidal river, were analyzed using the Index of Biotic Integrity (IBI) (Figure 8-22). The IBI, developed by Karr (1981) for use in midwestern streams, has been adapted for use in other areas. This index is a composite of 12 ecological attributes of fish communities, including species richness, indicator taxa (both intolerant and tolerant), trophic guilds, fish abundance, and incidence of hybridization, disease, and abnormalities (Karr et al., 1986). IBI scores range from a low of 12 to a high of 60. A score of 12 is assigned to conditions where no fish are present even after repeated sampling; a score of 60 is assigned to conditions comparable to the best habitats without human disturbance (Karr et al., 1986). The trend in the IBI at Indianhead (Jordan, 1992) shows that the river quality for fish increased from poor, indicating an impaired or restricted habitat (IBI scores in the 20 to 30 range), to fair, indicating slightly impaired habitat (IBI scores in the 40 to 50 range).
These data indicate that, in the late 1960s and early 1970s, the fish community at Indianhead was dominated by a few tolerant species, with few fish present at all in some years. In the last 20 years, a general upward trend in river quality for fish has been observed, evidenced by increasing numbers of pollution-intolerant species and a species mix suitable to provide for a reasonably balanced trophic structure. Indicator variables currently measured at Indianhead are at about two-thirds of their expected level in undisturbed habitats. The rise in the IBI at Indianhead, where a wastewater treatment plant discharge is located, is in contrast to stable or declining trends observed at other locations that lack wastewater treatment plant outfalls (Jordan, 1992).
In addition to the direct effect on the survival and condition of fish populations due to low DO concentrations due to high organic loadings (Tsai, 1991), fish populations
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