Variable Gizzard Shad Recruitment with Reservoir Productivity: Causes and Implications for Classifying Systems

Abstract

Achieving sustainable prey fish assemblages that support sport fish predator populations is a fundamental challenge to fisheries managers. Among Midwestern and Southeastern (USA) reservoirs, gizzard shad, Dorosoma cepedianum, have been widely stocked to improve predator growth. However, these stockings have yielded highly variable effects on sport fish, due in part to highly variable recruitment of gizzard shad. To determine whether reservoir productivity can be used to classify reservoirs according to recruitment of gizzard shad, we quantified gizzard shad recruitment along a mesotrophic to hypereutrophic productivity gradient. We sampled 12 reservoirs during May through June 1993, to evaluate the hypothesis that larval gizzard shad foraging success and survival increase with reservoir productivity. Both hatch abundance and survival of larval gizzard shad correlated positively with total phosphorus concentrations (TP), an indicator of reservoir productivity. Abundance of 15-mm (total length) larval gizzard shad survivors, an indicator of age-0 year class strength, increased by two orders of magnitude across TP concentrations. Larval gizzard shad foraging success increased with availability of preferred, small zooplankton prey. However, abundance of small zooplankton did not increase with reservoir TP concentrations, and larval survival did not increase with foraging success. These results provide mechanistic understanding for the relative lack of gizzard shad in mesotrophic reservoirs, and the dominance of gizzard shad in hypereutrophic reservoirs. In hypereutrophic reservoirs, negative effects of gizzard shad on sport fish may be alleviated by reducing phosphorus loading from the watershed, suggesting a watershed approach to this fishery and water quality problem.