Quantifying Responses to Hybrid Striped Bass Predation across Multiple Trophic Levels: Implications for Reservoir Biomanipulation

Top-down effects from piscivores to phytoplankton have been documented in north temperate lakes, thus permitting managers to regulate these food webs. From our review of the literature, reservoir trophic interactions appear less amenable to biomanipulation owing to the presence of a fast-growing, omnivorous planktivore, gizzard shad Dorosoma cepedianum. If reservoir zooplankton can be enhanced by reducing gizzard shad through biomanipulation, this could increase survival of food-limited early life stages of sport fishes dependent on zooplankton and could improve water clarity by reducing phytoplankton. To quantify this potential, we evaluated responses of age-0 gizzard shad, crustacean zooplankton, and phytoplankton to the addition of hybrid striped bass Morone saxatilis × M. chrysops (22 kg/ha) to four of eight 0.4-ha ponds. Hybrid striped bass nearly eliminated age-0 gizzard shad from ponds within 10 d after being stocked, which permitted zooplankton density and size to increase but had no effect on phytoplankton. These pond results independently confirmed the relationships among zooplankton density, zooplankton production, and age-0 gizzard shad density that had been developed previously in 1-m3 enclosures. Only reservoirs with fewer than 10 age-0 gizzard shad/m3 and daily zooplankton production greater than 220 mg/m3 may be amenable to biomanipulation; these conditions occurred in our ponds due to hybrid striped bass piscivory and high zooplankton productivity. About 5% of Ohio reservoirs possess these two characteristics, which makes the potential for improving sport fish recruitment by reducing age-0 gizzard shad abundance a limited option for reservoir managers.