NATURAL REHABILITATION OF STREAM FISH POPULATIONS IN AN ILLINOIS CATCHMENT

1. An extreme drought in 1988 dried first- and second-order streams in the Salt Fork basin (1190 km 2) in east-central Illinois. This event provided a natural experiment in which the natural rehabilitation of f ish populations could be measured in the whole catchment following res umption of stream flows. 2. Fish were sampled before, during, and afte r the drought (1987-1990) throughout the basin (eighty-eight samples i n July-September) using methods of known efficiency. Analyses of covar iance [using log (distance from source) as the covariate] indicated no significant differences (P > 0.2) of biomass or species richness betw een pre-drought and post-drought samples from sites desiccated during 1988 or among pre-drought, drought, and post-drought years among sampl es from perennial streams. Therefore, recovery occurred within 1 year, but there was no indication of increased biomass or species richness in permanent streams resulting from fish moving down or remaining down stream during the drought. 3. Fish biomass per unit length of stream s egment was modelled as a power function of distance of the segment fro m the source of each stream to estimate changes in biomass of the whol e catchment. Before and subsequent to the drought, 90.0 t of fish occu pied the total 729 km of stream length in the basin, compared with 74. 6 t in the drought year. Although.the drought affected 80% of total st ream length, fish biomass was reduced by only 17% in the drought year because only the lower-order streams were desiccated and their normal biomass density was lower than in perennial streams. 4. In conclusion, it is expected that no expenditure on stocking fish would be necessar y in restoration projects on these lower-order, surface runoff streams , providing that they are connected to permanently flowing streams tha t contain a full complement of species.