Destruction and Re‐establishment of Stream Fish and Invertebrates Affected by Drought

In 1953 and 1954 a severe drought virtually destroyed the fish and invertebrate populations in Smiths Branch, a small warm‐water stream in Vermilion County, Illinois, and presented an opportunity to study the adaptations, survival, and repopulation of aquatic organisms exposed to the critical conditions associated with low water levels. The fish population in Smiths Branch was first studied in 1951. In the fall of 1953, the flow was discontinuous, most of the pools dried up, and the few remaining pools were treated with rotenone to eliminate the surviving fish. After that time, the fish population was studied through the use of an electric seine, minnow seine, rotenone, and night observation. The invertebrate population was studied in a series of bottom samples. Discontinuous flow reduced the aquatic habitat and exposed the fish and invertebrates to desiccation, stagnation, and predation. Stagnation was most detrimental during early fall months, in association with leaf accumulations and drastic temperature fluctuations, or during the winter when ice covered the pools. Most of the fishes withstood the extreme drought conditions in at least a few parts of the stream. Re‐establishment of the populations began as soon as the stream resumed its flow. The upstream ingression of fish was limited to the first few pools above the stream mouth until the full flow was resumed, and then, during the following 2 weeks, 21 of the 29 regularly occurring species moved into most of the stream course. Twenty‐five of the regular species of fish had entered Smiths Branch by the end of the first summer after the population was destroyed in 1953. Only three species were significantly absent: The longear sunfish was not taken until the fall of 1955, and the black‐stripe topminnow and hornyhead chub had not repopulated the stream when the final collections were made in September, 1957. Most of the adult fish that entered Smiths Branch after the resumption of flow in 1954 reproduced successfully soon after entering the stream, so that young‐of‐the‐year fish dominated the new population. The adults of some species apparently left the stream after spawning. Fish moved over very shallow riffle areas during two brief periods of slight flow which occurred during a time that the stream was generally discontinuous. Invertebrates displayed remarkable adaptations to drought conditions and repopulated Smiths Branch soon after the flow was resumed. The insects that were at first most abundant apparently were from adults that had flown in from other streams. The invertebrate population became re‐established soon enough to serve adequately as food for ingressing fish. The versatility of the stream organisms, adaptations and movements associated with environment and life cycles, may account for the rapid recovery of aquatic populations in Smiths Branch.