Size-dependent feeding of omnivorous Nile tilapia in a macrophyte-dominated lake: implications for lake management

Some species or size classes of omnivorous/herbivorous fish in (sub) tropical lakes feed on periphyton and phytoplankton, potentially promoting growth of macrophytes via reduced shading. Others feed on macrophytes, thereby enhancing the risk of having a turbid phytoplankton-dominated system in shallow lakes. Nile tilapia (Oreochromis niloticus), an abundant species in many warm lakes, feed on periphyton, phytoplankton, and macrophytes depending on their size and may therefore have a size-dependent effect on lake ecosystems. We conducted a C-N stable isotope analysis on different size-classes of tilapia and their potential prey to determine their food sources in a tropical eutrophic shallow lake dominated by submerged macrophytes. The shares of the dominant macrophyte, Vallisneria natans, periphyton, and seston in the diet of fish averaged approx. 54, 26, and 20%, respectively. Large-bodied specimens fed mainly on macrophytes, while small-bodied specimens were more dependent on periphyton, seston, or detritus. The results were confirmed by subsequent stomach analysis. We conclude that small-sized tilapia promote macrophyte growth by removing periphyton and seston, whereas large-bodied individuals may restrict macrophyte development. Selective removal of large-bodied tilapia might, therefore, stimulate the development of submerged macrophytes and promote or maintain a clear water state in eutrophic shallow lakes with abundant tilapia.