Effects of turbidity and introduced tilapia (Oreochromis spp) on macrophytes and invertebrates in a shallow tropical lake

Abstract

Tropical freshwater wetlands are subject to multiple stressors but there is little information on which stressors cause wetland degradation. Increased turbidity is considered a major cause of degradation, but the effects of introduced fish are often overlooked. Tilapia are frequently introduced in tropical regions, especially species in the genus Oreochromis, and the ecological effects of introducing tilapia are poorly studied. We used enclosure experiments in a shallow lake in Madagascar to assess the effects of tilapia and turbidity on macrophytes and benthic invertebrates, and to test management interventions designed to increase both. Tilapia at high and low stocking densities had negative effects on survival of Charophyte algae and water lilies, but no direct effect on benthic invertebrate abundance or diversity. Invertebrate abundance was highest on submerged Charophytes, so herbivory by tilapia indirectly affected invertebrates. Turbidity affected Charophyte survival, and abundance and diversity of non-Dipteran insects. As a complication, tilapia may increase turbidity by re-suspending the lake sediment. Our results suggest that herbivory by tilapia is a plausible cause of the removal of macrophytes from the lake and an impediment to their re-establishment. Tilapia are widely introduced in tropical areas as a food resource. Our results demonstrate serious consequences to these introductions.