IMPACTS OF THE NILE TILAPIA(OREOCHROMIS NILOTICUS) INTRODUCTION ON THE TROPHIC STRUCTURE OF THE AQUATIC EOSYSTEMS OF THE CAATINGA BIOME.
Keywords:biological invasions, biodiversity, freshwater fishes, trophic cascades, omnivory, reservoirs, semi-arid tropics.
The Nile tilapia, Oreochromis niloticus (Linnaeus 1757), is an omnivorous fish which has been introduced into the aquatic ecosystems of the Caatinga Biome since the 70Â´s to improve fisheries and aquaculture. However, the environmental impacts of this species introduction are still poorly known. The aim of this work was to experimentally investigate possible mechanisms by which this species might negatively affect other fish species. Two field experiments were conducted where the abundance of tilapia was manipulated and the phyto- and zooplankton dynamics were monitored together with the variation in water transparency. We tested the hypothesis that the Nile tilapia would decrease the abundance of zooplankton, increase the abundance of phytoplankton and consequently decrease water transparency. Results from both experiments confirm the hypothesis but suggest that only larger zooplankton is negatively affected while only smaller algae are enhanced by tilapias. Even though the Nile tilapia is an omnivorous species feeding on both zooplankton and phytoplankton, its effects on plankton communities were very similar to the effects of strictly zooplanktivorous fish. Results also show a strong negative effect of tilapias on water transparency due to its bioturbation or ressuspension of sediments. Therefore, through its negative effects on zooplankton abundance and water transparency, the Nile tilapia may inhibit the recruitment of other fish species that feed mainly on zooplankton and are visually oriented to locate and capture their prey, at least in earlier stages of their lives.