Effects of Dam Removal on Fish Assemblage Structure and Spatial Distributions in the Baraboo River, Wisconsin

Abstract

We evaluated the effects of dam removal on fish assemblage structure and spatial distributions after four low-head dam removals in the Baraboo River, Wisconsin, using data collected at 35 study sites over 7 years. After dam removal, biotic integrity scores (possible range = 0–100) increased by 35–50 points at three of the four former impoundments as a result of decreases in percent tolerant species, increases in the number of intolerant species, and in some cases, increases in species richness. Fish assemblage shifts were muted at a fourth, lower-gradient impoundment site, indicating that responses differ among dam sites within a river system. In tailwater areas, postremoval assemblage shifts were transient; biotic integrity and species richness declined initially but then recovered at two of the three sites within 2 years after dam removal. An analysis of spatial distributions before dam removal revealed 11 fish species that were found below, but rarely or never above, the downstream-most dam. After dam removal, 10 of the 11 species were collected at new sites upstream from the dam, indicating that recolonization of reconnected upstream sites had occurred. Some species recolonized rapidly and in large numbers. For example, emerald shiners Notropis atherinoides recolonized 16 upstream sites and were collected 123 km upstream from the dam within the first year after removal. One of the nine recolonizing species, the spotted sucker Minytrema melanops, was only detected during spring, suggesting that these fish recolonized seasonally, perhaps for spawning. Recolonizing species were generally lacustrine or large-river fishes known to undergo overwintering and spawning migrations. Our study suggests that dam removal is a viable option for restoring lotic fish communities, but further study is needed on recovery patterns as they relate to channel morphology, hydrologic characteristics, impoundment sediment storage capacity, and the distance to source populations of recolonizing taxa.