Sustainable management of a coastal lagoonal system (Ria Formosa, Portugal): An ecological model for extensive aquaculture
Tóm tắt
The Ria Formosa lies in southernPortugal, extending for about 55 km. It is a truebarrier island system, comprising mainland,backbarrier lagoons, inlet deltas, barrier islands,barrier platforms and shoreface. Of the RiaFormosa system, which covers a total area of163 km2, 20 km2 are occupied by salinas andaquaculture ponds. The main water reservoir of thesalinas and the extensive aquaculture pondsbehave like small lagoons where there are one or moreopenings to a tidal channel. These small lagoons havethe advantage of being easy to study and to model. An ecological model was developed toestimate the potential production of the giltheadseabream in the Ria Formosa, in an extensiveaquaculture regime. This model was based oninformation concerning chemical and physical factors(forcing functions) and secondary production estimatesfrom four sites in the Ria Formosa with differentenvironmental conditions, where extensive aquacultureis practised. Published information on optimal growthparameters of the gilthead was used to build themodel, which was based on some assumptions concerningthe detrimental effect of an excessive increase ofsalinity and other environmental factors both ongilthead growth (one state variable) and on preyproduction (divided into two state variables), whichis essentially benthic macrofauna. Reservoirs with low water renewal undergo largeenvironmental fluctuations. The growth of gilthead orother fish can be difficult, not only because of theadverse environmental conditions but also because ofthe low secondary production. The maximum yield of fishis predicted to be less than 6 g m−2. In reservoirswhere the water exchanges on almost every tide, themaximum yield of gilthead can be in excess of 22–25g m−2. Nevertheless, the high productivity ofthese sites in terms of secondary production ofbenthos and fish, as well as primary production, canlead to oxygen depletion during the night or at day-break.An attempt to further improve fish production byfertilisation or by adding food could provoke a rapiddeterioration of the water quality and endanger allproduction.
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