Effects of a Selectively Reduced Gizzard Shad Population on Trophic Interactions and Age-0 Fishes in Walker County Lake, Alabama

Within 2 years of a December 1995 reduction in a population of gizzard shad Dorosoma cepedianum in a 66-ha public fishing reservoir, their abundance rapidly returned to high levels. During 1996, age-0 gizzard shad density was low, and they experienced rapid growth as larvae and juveniles, achieving average lengths (±SE) of 236 ± 7 mm total length (TL) by 13 November 1996. In contrast, during 1997, larval gizzard shad density was higher and growth was slower, with age-0 fish achieving first-year lengths of only 142 ± 2 mm TL by 16 December 1997. Rapid age-0 growth during 1996 afforded fish from that year-class reduced vulnerability to gape-limited predators, such as largemouth bass Micropterus salmoides, and potentially allowed them to spawn during the following spring at age 1. During 1997, concurrent with higher age-0 gizzard shad densities, densities of large zooplankton taxa (Daphnia, cyclopoid copepods, and Chaoborus) were lower than during 1996. Despite increased gizzard shad density and reduced density of large zooplankton taxa in 1997, density and survival of larval sunfish (of the genus Lepomis) were not negatively affected. Density of juvenile bluegills L. macrochirus and their availability to age-0 largemouth bass were similar between years. However, slower growth and greater abundance of age-0 gizzard shad in 1997 provided more prey for largemouth bass than in 1996. The effects of this gizzard shad population reduction appeared to affect multiple trophic levels; however, effects were short-lived and within 1–2 years the population returned to the level existing before reduction.