Efeitos de peixes onívoros planctívoros e bentívoros em lagos tropicais

Abstract

Planktivorous and benthivorous fish may promote the increase of phytoplankton biomass and turbidity by suppression of zooplankton, resuspension of sediment and recycling or translocation of nutrients. These effects are recognized and studied in shallow lakes of temperate environments since the 1960s, and have led to the development of restoration techniques based on the manipulation of ichthyofauna called biomanipulation. Such techniques have been applied in temperate lakes with relative success. However, the use of biomanipulation in tropical and subtropical lakes raises questions. The fish communities in these environments present some peculiarities, for example, the highest degree of omnivory. To evaluate how omnivorous fish affect aquatic communities and water quality in tropical systems and possible implications for biomanipulation, mesocosms experiments were carried out with two omnivorous species: Nile tilapia (Oreochromis niloticus Linnaeus 1758), an exotic species which feeds primarily on phytoplankton and zooplankton organisms, and the curimatã (Prochilodus brevis Steindachner 1875), a native species that feeds on benthic algae, debris and microinvertebrates. The results suggest that both species may contribute to increased water turbidity and increased phytoplankton production. However, they do so through distinct mechanisms: the effects of tilapia stem mainly from trophic cascade and nutrient recycling while the effects of curimatã are mediated by their interaction with sediment. The experiments also indicate a lack of synergic interaction between the effects of the two species on planktonic communities and water transparency. Although there is evidence that tilapia may negatively affect curimatã, this effect was not observed in the experiments. The results presented here suggest that the reduction of planktivorous and benthivorous fish densities with the objective of improving water quality can be a viable strategy also in tropical lakes.