Fish Assemblage Changes in an Ozark River after Impoundment: A Long-Term Perspective

—We conducted an intensive fish survey in the tailwater reach of a large Ozark river 30 years after its impoundment and compared the recent fish assemblage with those prior to impoundment and shortly (4 years) after impoundment. Our primary objective was to assess whether relatively short-term monitoring following dam construction can adequately quantify the long-term effects of impoundment on downstream riverine fishes. The preimpoundment survey (1962–1963) described a fish assemblage composed of warmwater fish species, predominantly Cyprinidae, Ictaluridae, Centrarchidae, and Percidae. Yoke darter Etheostoma juliae (34%), central stoneroller Campostoma anomalum (24%), and Ozark madtom Noturus albater (7%) were the most abundant species. The postimpoundment surveys of 1965–1966 and 1968 documented immediate changes in the fish assemblage. No Ozark madtoms and only four yoke darters were collected shortly after impoundment. Central stonerollers accounted for 45–50% of the fish collected, and both short-term postimpoundment surveys collected five species of darters (Percidae) that accounted for 41–42% of the fish collected. Thirty years after impoundment, we found that the tailwater fish assemblage was composed almost entirely of coldwater species. Ozark sculpin Cottus hypselurus and four species of introduced trout (Salmonidae) accounted for 98% of the fish assemblage by number during the 1995–1997 surveys. The rank abundance of species was negatively correlated between our survey and the preimpoundment survey but not between our survey and the short-term postimpoundment surveys. Many species that we collected (54%) are habitat generalists, and we did not collect 77% of the fluvial-specialist species that were present in historical collections. All postimpoundment surveys documented dramatically reduced species richness and diversity. We conclude that short-term monitoring following impoundment is inadequate to determine the impact of dams on lotic fish assemblages and suggest long-term postimpoundment monitoring to determine when a fish assemblage has stabilized. Understanding how dramatic changes to the environment affect community structure is a major focus of aquatic ecology (Matthews 1998). The North American fish fauna has been subjected to a wide array of human-induced disturbances. Among the most widespread and drastic of disturbances has been the damming of rivers for hydroelectric power generation, flood control, recreational uses, and water storage (Brooker 1981; Benke 1990; Collier et al. 1996). Extensive liter* Corresponding author: [email protected] 1 Present address: Arkansas Game and Fish Commission, 2 Natural Resources Drive, Little Rock, Arkansas 72205, USA. 2 Present address: U.S. Geological Survey, North Carolina Cooperative Fish and Wildlife Research Unit, Box 7617, North Carolina State University, Raleigh, North Carolina 27695, USA. 3 The Unit is jointly supported by the Arkansas Game and Fish Commission, University of Arkansas, U.S. Geological Survey, and Wildlife Management Institute. Received November 19, 2001; accepted July 12, 2002 ature reviews have been compiled on the effects of river impoundment on downstream ecological communities (e.g., Brooker 1981; Walburg et al. 1981; Cushman 1985). However, the effects of river impoundment and regulation on downstream riverine fish assemblages have been poorly documented considering the extent to which North American rivers have been dammed (Benke 1990). The White River of northern Arkansas and southern Missouri has been substantially modified by several large hydropower dams. The upper White River fish assemblage was sampled extensively by several early investigators (Jordan and Gilbert 1886; Black 1940; Knapp 1958; Keith 1964; B. Crawford and O. D. Brown, Arkansas Game and Fish Commission, unpublished report), and their collections now serve as a historical record of the fishes that occurred before Beaver Dam was constructed. Two short-term studies were completed after Beaver Dam became operational in 1965, and they were the first to document the fishes of Beaver Dam tailwater (sampling during 1965–1966, Brown et al. 1968; sampling during 111 RIVER IMPOUNDMENT EFFECTS ON FISHES 1968, Bacon et al. 1969). Thirty years later, we thoroughly sampled the fish assemblage of Beaver Dam tailwater from 1995 to 1997 in conjunction with an evaluation of instream and riparian habitat rehabilitation (Quinn 1998; Quinn and Kwak 2000). During our survey, we noticed that the fish assemblage had changed considerably since river impoundment and the earlier short-term studies. Therefore, we present a long-term perspective of the fish assemblage changes that occurred in the tailwater after construction of Beaver Dam, and we assess the importance of long-term monitoring to quantify the effects of impoundment on downstream riverine fishes.