Relations between Reservoir Trophic State and Gizzard Shad Population Characteristics in Alabama Reservoirs

Abstract

Abstract We compared relative abundance, condition, length at age, and the age and size structure of gizzard shad Dorosoma cepedianum between eutrophic (≥8 mg/m3 chlorophyll a) and oligo-mesotrophic (≤7 mg/m3 chlorophyll a) Alabama reservoirs. We quantified the size structure of gizzard shad in each reservoir by calculating the index of vulnerability (IOV), defined as the percentage of the electrofishing catch that was 203 mm or less in total length. Fish were collected with experimental gill nets and electrofishing, and although gill nets tended to catch larger and older fish, the gears provided similar trends in trophic state response. Gizzard shad in eutrophic reservoirs were characterized by high relative abundance, slow growth after age 1, a low percentage of fish age 5 and older (7%), and poor condition. This resulted in a high IOV (70) for gizzard shad in eutrophic reservoirs. Conversely, in oligo-mesotrophic reservoirs, gizzard shad abundance was 80-86% lower than in eutrophic reservoirs. However,...