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The ecological status of Hout River Dam was evaluated in 2011 by assessing fish health, based on condition factor (K), health assessment index (HAI) and inverted parasite index (IPI), which provided a simple and rapid means of establishing the health of fish. Surveys were conducted during summer, winter and spring 2011. To verify results, water quality was analysed during each survey.Clarias gariepinus,Oreochromis mossambicusandCyprinus carpiowere collected using conventional angling gear and gillnets. MeanKvalues of 0.6, 2.0 and 1.4, and HAI scores of 38, 41 and 35, were recorded for each species, respectively. The metrics that correlated best with fish health scores were haematocrit values and ecto- and endoparasite indices. Differences inK, HAI, and HAI using IPI determinations, were insignificant (p> 0.05) between species, while water contaminants, with the exception of ammonium (NH4), nitrate (NO3) and aluminium (Al) concentrations in summer, fell within the levels stipulated by the South African target water quality range guidelines for aquatic ecosystems. It was thus inferred that the dam was in a good ecological state. [ABSTRACT FROM PUBLISHER]

Dams have been shown to impact freshwater mussels. We examined how mussels respond to differently sized dams (18 vs. 4 m) on the St. Croix River and its tributary, the Sunrise River. We hypothesized that: mussel density and growth rate would be greater downstream of the smaller dam due to the relatively greater food subsidies and temperature effects of the reservoir above it; and the effects of the small dam would moderate downstream as the localized impacts of the dam were reduced. We quantitatively sampled mussels upstream and downstream of the dams. For a common species, Actinonaias ligamentina, we ascertained growth rates by measuring successive growth rings. The highest mussel richness and diversity were upstream and downstream of the large dam. Higher mussel density was found immediately below the small dam but declined downstream. A. ligamentina downstream of the small dam grew faster and were larger than individuals in other reaches. Food availability and temperature appeared to influence mussel density and growth rate for A. ligamentina downstream of the small dam. This study provides information that may help managers decide whether to remove small dams or to maintain them because of the unique mussel habitats below these structures. [ABSTRACT FROM AUTHOR]

Ecosystems are fragmented by natural and anthropogenic processes that affect organism movement and ecosystem dynamics. When a fragmentation restricts predator but not prey movement, then the prey produced on one side of an ecosystem edge can subsidize predators on the other side. When prey flux is high, predator density on the receiving side increases above that possible by in situ prey productivity, and when low, the formerly subsidized predators can impose strong top-down control of in situ prey-in situ prey experience apparent competition from the subsidy. If predators feed on some evolutionary clades of in situ prey over others, then subsidy-derived apparent competition will induce phylogenetic structure in prey composition. Dams fragment the serial nature of river ecosystems by prohibiting movement of organisms and restricting flowing water. In the river tailwater just below a large central Mexican dam, fish density was high and fish gorged on reservoir-derived zooplankton. When the dam was closed, water flow and the zooplankton subsidy ceased, densely packed pools of fish formed, fish switched to feed on in situ prey, and the tailwater macroinvertebrate community was phylogenetic structured. We derived expectations of structure from trait-based community assembly models based on macroinvertebrate body size, tolerance to anthropogenic disturbance, and fish-diet selectivity. The diet-selectivity model best fit the observed tailwater phylogenetic structure. Thus, apparent competition from subsidies phylogenetically structures prey communities, and serial variation in phylogenetic community structure can be indicative of fragmentation in formerly continuous ecosystems. [ABSTRACT FROM AUTHOR]

The article discusses potential environmental effects of the planned hydropower dams for the Mekong River, with a focus on the Tonlé Sap Lake in Cambodia. The lake floods every year, but dams built on the upper Mekong will likely change the river's flow, disrupting the human, plant, and animal life that depends on the floods. Additionally, a dam is planned for the lower Mekong in Laos to generate electricity. Members of the Mekong River Commission (MRC), a supervisory board made of delegates from Laos, Cambodia, Thailand, and Vietnam, think the dam could be devastating. INSET: The Legacy of the Three Gorges Dam.

Hydropower is the largest renewable energy resource in the United States and the world. However, hydropower dams have adverse ecological impacts because migrating fish may be injured or killed when they pass through hydroturbines. In the Columbia and Snake River basins, dam operators and engineers are required to make those hydroelectric facilities more fish-friendly through changes in hydroturbine design and operation after fish population declines and the subsequent listing of several species of Pacific salmon under the Endangered Species Act of 1973. Public Utility District No. 2 of Grant County, Washington, requested authorization from the Federal Energy Regulatory Commission to replace the ten turbines at Wanapum Dam with advanced hydropower turbines designed to improve survival for fish passing through the turbines while improving operation efficiency and increasing power generation. As an additional measure to the primary metric of direct injury and mortality rates of juvenile Chinook salmon using balloon tag-recapture methodology, this study used an autonomous sensor device-the Sensor Fish-to provide insight into the specific hydraulic conditions and physical stresses experienced by the fish as well as the specific causes of fish biological response. We found that the new hydroturbine blade shape and the corresponding reduction of turbulence in the advanced hydropower turbine were effective in meeting the objectives of improving fish survival while enhancing operational efficiency of the dam. The frequency of severe events based on Sensor Fish pressure and acceleration measurements showed trends similar to those of fish survival determined by the balloon tag-recapture methodology. In addition, the new turbine provided a better pressure and rate of pressure change environment for fish passage. Overall, the Sensor Fish data indicated that the advanced hydroturbine design improved passage of juvenile salmon at Wanapum Dam. [ABSTRACT FROM AUTHOR]

Comments on the plans to remove 63 dams in 15 U.S. states and the District of Columbia. Effect of the nation's aging dam infrastructure, combined with a growing understanding of the negative ecological impacts of dams; Problems presented by dams, including drowning valuable habitat under reservoirs and blocking annual migrations of fish; Benefits of dam removal, including better water quality and revitalized fisheries.