Evidence for zooplankton compensation and reduced fish growth in response to increased juvenile fish density

Abstract

Predation and competition act to structure trophic interactions in natural north temperate lakes in relatively linear top-down or bottom-up interactions. Conversely, food web interactions in reservoirs are more complex. Omnivorous gizzard shad (Dorosoma cepedianum) regulate reservoir food webs from an intermediate trophic position through a complex series of predation and competition interactions. To evaluate the potential impact of predation on zooplankton and intraspecific competition by age-0 gizzard shad, we conducted an in-lake enclosure experiment. Gizzard shad predation reduced crustacean zooplankton. In turn, reduced crustacean zooplankton permitted rotifer populations to increase such that no overall difference in total zooplankton occurred during the experiment. Age-0 gizzard shad growth was negatively related to age-0 gizzard shad density, but positively related to the density of crustacean zooplankton. Gizzard shad exerted strong predatory impacts on crustacean zooplankton and permitted a competitive shift toward smaller taxa in the zooplankton assemblage. Gizzard shad growth was affected by intraspecific competition and by food quality. Complex predatory and competitive impacts allow gizzard shad to structure reservoir zooplankton assemblages.