Sustainable management of a coastal lagoonal system (Ria Formosa, Portugal): An ecological model for extensive aquaculture |
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| Authors: | S Gamito |
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| Institution: | (1) UCTRA, University of Algarve, Campus de Gambelas, 8000 Faro, Portugal |
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| Abstract: | The Ria Formosa lies in southern Portugal, extending for about 55 km. It is a true barrier island system, comprising mainland,
backbarrier lagoons, inlet deltas, barrier islands, barrier platforms and shoreface. Of the Ria Formosa system, which covers
a total area of 163 km2, 20 km2 are occupied by salinas and aquaculture ponds. The main water reservoir of thesalinas and the extensive aquaculture ponds behave like small lagoons where there are one or more openings to a tidal channel. These
small lagoons have the advantage of being easy to study and to model.
An ecological model was developed to estimate the potential production of the gilthead seabream in the Ria Formosa, in an
extensive aquaculture regime. This model was based on information concerning chemical and physical factors (forcing functions)
and secondary production estimates from four sites in the Ria Formosa with different environmental conditions, where extensive
aquaculture is practised. Published information on optimal growth parameters of the gilthead was used to build the model,
which was based on some assumptions concerning the detrimental effect of an excessive increase of salinity and other environmental
factors both on gilthead growth (one state variable) and on prey production (divided into two state variables), which is essentially
benthic macrofauna.
Reservoirs with low water renewal undergo large environmental fluctuations. The growth of gilthead or other fish can be difficult,
not only because of the adverse environmental conditions but also because of the low secondary production. The maximum yield
of fish is predicted to be less than 6 g m−2. In reservoirs where the water exchanges on almost every tide, the maximum yield of gilthead can be in excess of 22–25 g
m−2. Nevertheless, the high productivity of these sites in terms of secondary production of benthos and fish, as well as primary
production, can lead to oxygen depletion during the night or at day-break. An attempt to further improve fish production by
fertilisation or by adding food could provoke a rapid deterioration of the water quality and endanger all production. |
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| Keywords: | aquaculture ecosystem modelling fish production |
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