Figure 11-14 Long-term trends in fish diversity in the Ohio River. Source: ORSANCO, 1982.
There is little long-term information on biological trends in the Ohio River (Pearson, 1992). Information on plants, invertebrates, and plankton is scarce or nonexistent. The only historical population data are for mussels, which were diverse and abundant in the 1800s but are less so now, even with water quality improvements in the river.
Data on fish populations in the middle section of the Ohio River have been collected since the 1950s and indicate that the populations have responded more positively than mussels to improved water quality (Figure 11-15). The first comprehensive fish population study on the Ohio River was done by ORSANCO in 1957, and the study has continued almost yearly since then. The study reports fish data according to section of the river—upper, middle, and lower. Louisville and Cincinnati are located in the middle section of the river. Changes in fish diversity since the study began have been most dramatic in the upper river, where a 40 percent increase has been measured, but diversity has increased by 13 percent in the middle section as well (ORSANCO, 1982). Numbers of species and overall fish biomass are still increasing in the middle section of the river, though they have not returned to their original levels. ORSANCO attributes the improvements to increased DO concentrations and pH, and to decreased levels of toxic materials in the river (ORSANCO, 1982).
Other studies also indicate continuing improvements in the quality of the Ohio River habitat. Studies by Geo-Marine conducted in the early 1980s near North Bend, Ohio (about 30 miles downstream of Cincinnati), found increasing numbers of species of larval fish, a life stage generally sensitive to DO levels (Geo-Marine, 1986). A trend toward a more even distribution of the numbers of individuals among the spe-
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