Environmental Assessment and Fish Populations of the Wabash River and its Tributaries

Monitored since 1967, the fish community of the Wabash River main stem has experienced increases in most species populations since 1983. Some species populations expanded into previously unoccupied areas of the river. The average size of an individual has also increased for many species, suggesting the possibilities of greater longevity and faster growth. The density of carp remained unchanged and gizzard shad declined because of increased predator pressure. This improvement probably resulted from a combination of long-term, 50% reduction in BOD loading through improved point-source waste treatment and low flow summers in 1983,1988, and 1991, which facilitated good reproduction and survival through the first year. A 25% reduction in agricultural loadings to the river may have occurred during the 1983 Payment-in-Kind program, which paid farmers not to grow crops. Continued improvement may depend upon limiting nutrient delivery and reducing diatom densities in the river. Many smaller tributaries lost darters, sunf ish, smallmouth bass, and sensitive minnows during the 1970s through a combination of agricultural impacts. However, three summers of drought since 1983 reduced nonpoint agricultural pollution and permitted the fish communities of many streams to recover, especially in stream systems in which less disturbed tributaries served as refugia.