Environmental effects of marine fish pond culture in the Ria Formosa (Southern Portugal)

Abstract

Macrobenthic organisms, sediment and water characteristics were analysed in two fish farms in the Ria Formosa at control sites, near the outflows and inside the decantation ponds, in winter and summer 2001. Both farms operate under semi-intensive and intensive regimes. Statistical analysis reveals a very localised effect of fish farms near the outflow of the semi-intensive ponds, with an increase of density of small-sized organisms. The fish farms released high concentrations of nutrients and relatively low quantities of suspended solids. The effects of the effluents on the sediment were significant in the immediate vicinity of discharge point-source. The effects were localized mainly because all farms operate near the sea inlets, where water renewal rates are higher to ensure good fish production, thus permitting a important flushing of waste water. The effects of the effluents were more pronounced in summer because of the farms’ higher productivity at that time of the year. The decantation ponds, obligatory for the intensive regime, were apparently efficient in removing part of the solid waste. However, semi-intensive fish farms represent important sources of dissolved nutrients in the Ria Formosa and the joint effect of several fish farms in a semi-enclosed coastal system may be considerable.