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 ¼ 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.
A multiyear tag and recapture study was conducted to determine whether channel catfish Ictalurus punctatus were migratory and if they had strong homing tendencies. Over 10,000 channel catfish were tagged from the lower Wisconsin River and adjacent waters of the upper Mississippi River during the 3‐year sampling period. Data on movements were obtained from study recaptures and through tag returns and harvest information provided by sport anglers and commercial fishers. Channel catfish occupied relatively small home ranges during summer, migrated downstream to the upper Mississippi River in autumn, then migrated back up the Wisconsin River in spring to spawn and to occupy the same summer home sites they had used in previous summers. Fish size was a factor in the degree of fidelity to summer home sites, with larger fish showing greater fidelity.
We examined changes in populations of largemouth bass Micropterus salmoides and bluegill Lepomis macrochirus associated with mechanical plant harvesting in a lake heavily infested with Eurasian watermilfoil Myriophyllum spicatum. In Aug 1994, 18% of the total plant biomass in Fish Lake, Dane County, Wisconsin, was removed in a radial pattern of 2‐m‐wide channels. Largemouth bass and bluegill abundance, survival, growth, and length frequency were compared between pretreatment (1992–1993) and posttreatment (1995–1996) years. Following vegetation removal, mean abundance of largemouth bass and bluegill did not change significantly, but growth increased for age‐2–4 largemouth bass and declined for age‐5 largemouth bass and age‐4–5 bluegills. Survival increased for age‐2, −3, and −5 largemouth bass and age‐4–6 bluegills. Population size structure increased for both species. We conclude that removal of Eurasian watermilfoil may increase growth and survival of some age‐groups of centrarchid populations without altering the abundance of either species. However, this study lacked replication and control that is needed to separate effects of plant harvesting from effects of other biotic and abiotic factors.
We examined changes in angling effort, catch and catch rates, and fishing and natural mortality of bluegills Lepomis macrochirus associated with mechanical plant harvesting in a lake heavily infested with Eurasian watermilfoil Myriophyllum spicatum. In August 1994, 18% of the total plant biomass was removed in a radial pattern of 2‐m‐wide channels in Fish Lake, Dane County, Wisconsin. After the vegetation removal, winter angler hours increased significantly, whereas summer angler hours did not. The number of boat trips in summer increased significantly, but the number of shore trips did not. Catch rates and total catch of bluegills did not change significantly, but the size structure of harvested bluegills increased. Catches of bluegills by shore anglers decreased in summer, whereas catches of bluegills by boat anglers increased. Natural mortality declined for bluegills of ages 4–6, but fishing mortality more than doubled for bluegills of the same ages. We conclude that removal of Eurasian watermilfoil may increase winter angler effort, decrease natural mortality of bluegills, and increase fishing mortality of bluegills. We also conclude that removing vegetation may improve the quality of the bluegill fishery, as evidenced by the increase in the number of larger bluegills harvested. However, this study lacked the replication and controls needed to separate the effects of plant harvesting from those of other biotic and abiotic factors.
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