Impacts of aquaculture in cages on benthic community of Guarapiranga reservoir

Abstract

Nile tilapia, Oreochromis niloticus, was raised in cages, without feed, between 1997 and 1998 at Guarapiranga reservoir, in order to identify the impacts on benthic community. Sediment samples were collected monthly with an Eckman-Birge grab sampler at three different sampling sites: upstream (M), downstream (J) and below the cages (TR). Density and richness of benthic community, besides abundance of Oligochaeta, were used as possible indicators of environmental changes. It was observed enrichment with organic matter and high densities of benthic organisms: the average density at TR was 1.7 times greater than at the other sites. However, the richness did not show significant differences between the three situations. Oligochaeta was the group best represented at TR. The results show that the benthic fauna is efficient to indicate changes in the substrate texture and composition. At M and J, the sediment, that was constituted of compact clay turf, was not favorable to developing benthic fauna. Fish excrements increase the amount of organic matter in the reservoir sediments. However, this culture system, utilizing cages, does not seem to promote impact in the water quality, because it does not involve imports of allochthonous material to the system. The addition of organic material to the sediment favors the increase of benthic population, what benefits the environment in many aspects, for example, enlarging the food area to the fishes in their natural environment.