IN-SITU EXPERIMENTS ON PREDATORY REGULATION OF A BIVALVE MOLLUSK (DREISSENA-POLYMORPHA) IN THE MISSISSIPPI AND OHIO RIVERS

1. In situ exclosure experiments in the Mississippi and Ohio Rivers de termined the importance of fish predation in regulating zebra mussels (Dreissena polymorpha) an increasingly important constituent of the be nthic invertebrate assemblages in both rivers. 2. We evaluated the eff ects of predatory fish on the density, biomass and size distribution o f zebra mussels in a floodplain reach of the upper Mississippi River a nd in a naturally constrained reach of the Ohio River. Fifty, six-side d, predator-exclusion cages and fifty 'partial' cages (mesh at the ups tream end only) were deployed, with half the cages containing willow s nags and half clay tiles suspended 12-16 cm above the bottom. A single snag or tile sample unit was removed from each cage at approximately monthly intervals from July to October 1994. Types and relative abunda nces of molluscivorous fish were evaluated by electrofishing near the cages in both rivers. Actual and potential recruitment of young zebra mussels on to the substrata were measured using benthic samples in bot h rivers and estimated (Ohio River only) from counts of planktonic vel igers. 3. Zebra mussels were consumed by at least three fish species i n the upper Mississippi River (mostly carp, Cyprinus carpio, and redho rse suckers, Moxostoma sp.) and five species in the Ohio River (primar ily smallmouth buffalo, Ictiobus bubalus, and channel catfish, Ictalur us punctatus) but potential recruitment seemed adequate to replace con sumed mussels, at least in the Ohio River. The number of juvenile bent hic mussels showed no apparent link with the density of veligers soon after initiation of reproduction. Recruitment of juveniles on snags an d tiles was not affected by cage type (thus eliminating a potentially confounding 'cage effect'). 4. Fish significantly influenced mussel po pulations, but the impact was often greatest among low density populat ions in the upper Mississippi. Density and biomass differed in both ri vers for cage type (higher inside cages), substratum (greater on tiles ), and date (increased over time). Presumed size-selective predation w as present in the Mississippi (greater on larger size classes) but was not evident in the Ohio. We hypothesize that fish in the Mississippi can more easily select larger prey from the low density populations; w hereas size-selective predation on tightly packed zebra mussels in the Ohio would be difficult. 5. Although fish can reduce numbers of Dreis sena polymorpha in the two rivers, current levels of fish predation se em insufficient to regulate zebra mussel densities because of its grea t reproductive capacity. The recent invasion of zebra mussels, however , could lead to larger fish populations while promoting greater carbon retention and overall ecosystem secondary production.