EFFECTS OF TENCH AND PERCH ON INTERACTIONS IN A FRESH-WATER, BENTHIC FOOD-CHAIN

The purpose of this study was to determine the importance of cascading trophic interactions in a freshwater, benthic food chain and if the strength of the indirect interactions depends on the degree of specialization in the predator. The effects of predation by two benthivorous fishes, tench (Tinca tinca) and Eurasian perch (Perca fluviatilis), on benthic macroinvertebrates, epiphytic algae, and submerged macrophytes were studied in a field experiment, using cages (2 m x 3 m x 0.8 m) placed in a eutrophic pond in southern Sweden. Cages were assigned to four different treatments: fishless controls, tench, perch, and tench + perch. Core samples were taken to assess the density of benthic macroinvertebrates and submerged macrophytes, and artificial substrates were used to study the dynamics of periphyton. Nonmolluscan benthic macroinvertebrates were not greatly affected by the presence of fish, whereas predation by tench dramatically reduced the biomass of snails and bivalves. Tench had an indirect, positive effect on the biomass of periphyton through a reduction of grazing pressure by snails, whereas perch had no demonstrable direct effect on molluscs or indirect effect on periphyton. Further, in the cages with low snail and high periphytic biomass (tench and tench + perch cages), growth of the dominant submerged macrophyte (Elodea canadensis) was reduced, probably due to shading by periphyton. This experiment confirms that a predator can have profound effects on interactions in benthic food chains and that the strength of the indirect interactions is dependent on the strength of the direct interactions. The strong direct effect of tench on snails indirectly affected periphyton and submerged macrophytes, whereas perch had no direct effect on benthic macroinvertebrates and,