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 intera ctions appear less amenable to biomanipulation owing to the presence o f a fast-growing, omnivorous planktivore, gizzard shad Dorosoma cepedi anum. If reservoir zooplankton can be enhanced by reducing gizzard sha d through biomanipulation, this could increase survival of food-limite d early life stages of sport fishes dependent on zooplankton and could improve water clarity by reducing phytoplankton. To quantify this pot ential, we evaluated responses of age-0 gizzard shad, crustacean zoopl ankton, and phytoplankton to the addition of hybrid striped bass Moron e saxatilis X M. chrysops (22 kg/ha) to four of eight 0.4-ha ponds. Hy brid striped bass nearly eliminated age-0 gizzard shad from ponds with in 10 d after being stocked, which permitted zooplankton density and s ize 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 d eveloped previously in 1-m(3) enclosures. Only reservoirs with fewer t han 10 age-0 gizzard shad/m(3) and daily zooplankton production greate r than 220 mg/m(3) may be amenable to biomanipulation; these condition s occurred in our ponds due to hybrid striped bass piscivory and high zooplankton productivity. About 5% of Ohio reservoirs possess these tw o characteristics, which makes the potential for improving sport fish recruitment by reducing age-0 gizzard shad abundance a limited option for reservoir managers.