Larval Gizzard Shad Success, Juvenile Effects, and Reservoir Productivity: Toward a Framework for Multi-System Management

Abstract

Recruitment success of larval fishes can determine subsequent effects of older life stages on ecosystem structure and function. Because size and abundance of juvenile gizzard shad Dorosoma cepedianum determines, in large part, their effects on zooplankton abundance and sportfish recruitment, we investigated recruitment of larval gizzard shad to the juvenile stage. We hypothesized that larval survival would correlate positively with availability of small (<0.40 mm) zooplankton and reservoir productivity and would dictate abundance of individuals reaching the juvenile stage. In downstream and upstream areas of four Ohio reservoirs ranging from mesotrophic to hypereutrophic, we evaluated larval gizzard shad success by comparing hatch, growth, and survival of weekly cohorts of larvae with availability of small zooplankton, and then we determined whether larval success could predict juvenile gizzard shad size and abundance. Total larval hatch abundance increased with reservoir productivity and was higher upstream than downstream. Weekly cohorts that produced abundant survivors reflected not only high hatch but also high survival in upstream areas during periods of abundant small zooplankton. Overall, larval hatch and survival predicted juvenile abundance but not juvenile size. Juvenile abundance increased with reservoir productivity, particularly in upstream areas, whereas juvenile size reflected the combined influence of juvenile abundance and reservoir productivity. Across mesotrophic to eutrophic conditions, juvenile gizzard shad size decreased with increasing abundance; but across eutrophic to hypereutrophic conditions, juvenile gizzard shad size increased despite continued increasing juvenile abundance. A conceptual framework for management, which incorporates reservoir productivity and anticipated effects of juvenile gizzard shad, should improve our success in attaining fisheries objectives across disparate systems.