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

Predation and competition act to structure trophic interactions in natural north temperate lakes in relatively linear top-down or bottom-up interactio ns. Conversely, food web interactions in reservoirs are more complex. Omniv orous 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 conduct ed an in-lake enclosure experiment. Gizzard shad predation reduced crustace an zooplankton. In turn, reduced crustacean zooplankton permitted rotifer p opulations to increase such that no overall difference in total zooplankton occurred during the experiment. Age-0 gizzard shad growth was negatively r elated to age-0 gizzard shad density, but positively related to the density of crustacean zooplankton. Gizzard shad exerted strong predatory impacts o n crustacean zooplankton and permitted a competitive shift toward smaller t axa in the zooplankton assemblage. Gizzard shad growth was affected by intr aspecific competition and by food quality. Complex predatory and competitiv e impacts allow gizzard shad to structure reservoir zooplankton assemblages .