Size-dependent Effects Of Nile Tilapia (oreochromis Niloticus) On Vallisneria Natans And Periphyton And Its Implication For The Eutrophic Lake Restoration

Submerged macrophytes, playing a key role in maintaining clear-water state inshallow lakes, might be reduced by phytoplankton and periphyton shading, andomnivorous/herbivorous fish grazing, which enhancing the risk of a shift to a turbidphytoplankton-dominated state in shallow lakes. Tilapia (Oreochromis niloticus)belongs to omnivorous group, mainly feeds on algae and plant material, and has itsfeeding habits shift as it grows which may have different effects on submergedmacrophytes in lakes depending on its size. Hence, we studied the diet sources of Niletilapia in Huizhou West Lake by stable isotope analysis and the effects of twodifferent sized-group tilapia (with total length=9.8±0.3㎝and3.4±0.4㎝,respectively) on the submerged macrophyte Vallisneria natans and periphyton by tankexperiment. Results are below:1, In Huizhou West Lake, both the stable isotope data and the gut contentanalysis suggest that large-bodied tilapia fed mainly on macrophytes, whilesmall-bodied specimens were more dependent on/preferred periphyton, seston ordetritus. We thus conclude that small-sized tilapia promote macrophyte growth byremoving periphyton and seston, whereas large-bodied tilapia may hampermacrophyte development. Selective removal of large-bodied tilapia would be a usefulmanagement strategy for stimulating the development of submerged macrophytes and,via this, promote and maintain a clear water state in eutrophic shallow lakes located inwarm climate regions.2, In macrophytes dominated water, less abundant tilapia have no significanteffect on water nutrient (nitrogen and phosphorus) concentrations. Larger-sized fishsubstantially reduced the biomass of Vallisneria natans and had no significant effecton phytoplankton biomass but stimulated periphyton’s growth; while smaller-sizedfish had no significant effects on Vallisneria natan and phytoplankton biomass. Wethus suggest that removal of tilapia or avoid the introduction of tilapia would be a useful strategy to maintaining the clear-water state in shallow lakes. We conclude thatomnivorous fish has to be controlled while re-establishing submerged macrophytecommunities to restore eutrophic shallow lakes.