Survival of fish impinged on a rotary disk screen

An impingement survival study was conducted to determine 48-h survival of fish impinged on a modified rotary disk screen equipped with fish protection features. The rotary disk screen was installed for a technology evaluation at the cooling-water intake structure of the North Omaha Station located on the Missouri River in Omaha, Nebraska. Hatchery-raised fish and native fish collected from the Missouri River were released in batches into the rotary disk screen bay and collected with a system that was constructed to recover fish from the screen\u27s vacuum system. That system was designed to remove fish from the rotary disk screen and return them to the river. Screen performance was assessed in April and August, representing spring and summer environmental conditions. The 48-h survival rates of hatchery-raised fathead minnow Pimephales promelas, channel catfish Ictalurus punctatus, and bluegills Lepomis macrochirus and a group of mixed native species approached 100%. Survival rates were not statistically different between test groups and controls, indicating that impingement did not contribute to the observed mortality. High survival rates of impinged fish removed from the screen indicated that the rotary disk screen would reduce impingement losses at the North Omaha Station, where losses due to impingement on the existing vertical traveling screens are assumed by the U.S. Environmental Protection Agency to approach 100% because the screens lack fish protection features. Our study results suggest that the rotary disk screen tested could be considered an alternative technology under section 316(b) of the Clean Water Act, which requires power plants to install the best technology available to reduce impingement. Use of hatchery-reared fish and native fish collected from the river assured that an adequate number of fish were tested to provide statistically reliable results and allowed the use of controls to account for mortality due to handling stresses experienced by test fish