Consequences of an Alewife Die-off to Fish and Zooplankton in a Reservoir

Abstract

Alewife Alosa pseudoharengus was the principal pelagic forage species in Claytor Lake, Virginia, prior to a severe die-off in the winter of 1977–1978. Due to successful reproduction in 1978, abundance of adult alewives rebounded sharply in 1979. Changes in the size structure of the zooplankton community between summer 1978 (larger) and 1979 (smaller) were similar to differences reported for other waters between periods of low and high alewife abundance and are attributable to selective alewife predation. Alewife-induced shifts in zooplankton composition toward smaller forms represent a potentially significant adverse impact on cohabiting planktivores, including young-of-the-year sport fishes. Alewives seldom were eaten by littoral piscivores (black basses Micropterus spp. and crappies Pomoxis spp.) but were prevalent in the diets of the pelagic species (walleye Stizostedion vitreum vitreum, white bass Morone chrysops, and striped bass Morone saxatilis). When alewives were less abundant during spring and summer of 1978, pelagic predators ate more yellow perch Perca flavescens, crappies, bluegills Lepomis macrochirus, and crayfish Cambarus sp. than in 1979. Growth of adult alewives and young-of-the-year walleyes and white bass was significantly greater in 1978 than in the 2 years preceding the die-off, suggesting that both intra- and interspecific trophic competition operate when alewife density is high. Annual growth of age-2 walleyes and white bass did not differ substantially before and after the die-off; impact of the population collapse may have been mitigated by high reproductive success and rapid first-year growth of alewives in 1978. These findings indicate that alewife size-selective planktivory may have a more enduring and potentially severe impact on the reservoir fishery than does instability in the alewife population.