Environmental effects of marine fish pond culture in the Ria Formosa (Southern Portugal) Franc¸ois Noe¨l Hubert 1, *, Marc Pellaud 2 & Sofia Gamito 3 1 IMAR, presently in CCMAR, Faculdade de Cie ˆncias do Mar e do Ambiente, Universidade do Algarve, Portugal 2 Faculte ´ des Sciences, Universite´ de Gene`ve, Switzerland 3 IMAR, Faculdade de Cie ˆncias do Mar e do Ambiente, Universidade do Algarve, Portugal (*Author for correspondence: Tel.: +351 289800900, Fax: +351 289818353; E-mail: fhubert@ualg.pt) Key words: lagoon system, semi-intensive fish farms, environmental impact 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 sus- pended 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. Introduction The Ria Formosa is a coastal lagoon situated in the Algarve region of Southern Portugal and comprises barrier islands, barrier platforms, back barrier lagoons, inlet deltas and shoreface (Pilkey et al., 1989). This lagoon system covers a maxi- mum area of about 63 km 2 and is characterized by a strong water renewal between tides (Sprung, 1994). Aquaculture in the Ria Formosa is a traditional activity of great socio-economical importance. Mollusc extensive culture (mainly of Ruditapes decussata, Linnaeus 1758) produces up to 80% of Portugal’s bivalves (Rosado & Bruxelas, 1995). Extensive multi-species fish farming is also a traditional activity being undertaken in ancient saltpan reservoirs. Semi-intensive fish farming of sea bass (Dicentrarchus labrax, Linnaeus 1758) and gildhead sea bream (Sparus aurata, Linnaeus 1758) has been developing for the past 20 years. This type of production is also undertaken in ancient salt- pans or in concrete tanks. The fish density is high and the water is renewed every day through tidal gates and water pumping. The fish diet consists mostly of commercial food pellets. Effluents from aquaculture facilities can be a major source of pollution in marine environment (Gowen et al., 1990; Wu et al., 1994; Hargreaves, 1998) and fish farming usually discharges impor- tant quantities of organic wastes in the surrounding environment (Folke & Kautsky, 1989). However, the severity and the extent of the impact depend on the kind of farming activity and on the water and sediment characteristics of the receiving system (Pillay, 1991; Wu, 1995). So far, extensive farming Hydrobiologia (2006) 555:289–297 Ó Springer 2006 H. Queiroga, M.R. Cunha, A. Cunha, M.H. Moreira, V. Quintino, A.M. Rodrigues, J. Seroˆ dio & R.M. Warwick (eds), Marine Biodiversity: Patterns and Processes, Assessment, Threats, Management and Conservation DOI 10.1007/s10750-005-1125-1