Simply referred to as Castro Marim. A large protected wetland reserve (2,089 ha) lying opposite Spain on the west side of the rio Guadiana. The reserve was established in 1975 to protect the natural environment and landscape. This is also an internationally important Ramsar Site. Habitat variety is good and includes productive and abandoned saltpans (salinas), salt marsh (sapal), tidal creeks, seasonally flooded pastureland and the tidal shore of the Guadiana. Enclosing and bisecting the wetlands are grassy hills, dry scrubland, farmland and orchards that significantly increase the biodiversity of the reserve. This is an under-watched locality with little published information - much awaits discovery.

Throughout the year, birds provide a memorable backdrop but are often wary and a telescope is an advantage. The Information and Exhibition Centre (Sede) viewing facilities are disappointing. Holidaymakers in safari vehicles expecting to see flamingos account for most visitors. Non-breeding Greater Flamingos occur throughout the year, often in good numbers. Egrets, herons, storks and spoonbills are present and in winter, good numbers of cormorants, duck and a handful of geese join them. White Storks nest conspicuously and bill clapping is frequently heard. The variety and number of shorebirds is good and includes locally breeding Avocets, Black-winged Stilts and Kentish Plovers. Audouin's Gulls, Yellow-legged Gulls and Little Terns breed and Slender-billed Gulls regularly visit. Black Terns are common migrants and non-breeding Caspian and Sandwich Terns are often seen. Marsh Harriers and Ravens breed and Common Buzzards, Common Kestrels and Peregrine Falcons visit. In winter, a large flock of Little Bustards and an even larger number of Stone Curlews reside on the farmland and grassy hills. The sapal attracts breeding Crested Larks, Greater and Lesser Short-toed Larks, Spectacled Warblers and Yellow Wagtails. Wintering passerines include Water Pipits, Dartford Warblers, Spanish Sparrows and Common Starlings (uncommon in the Algarve). Unexpected species occur and Glossy Ibis, Great Egret, Squacco Heron and Temminck's Stint are examples. Iberian Hares are common.

The small, friendly town of Castro Marim is embraced by the reserve and has an imposing setting and a ruined castle. Castro Marim train station at the southern edge of the reserve is a convenient point from which to explore the Salinas do Cerro do Bufo but trains stop infrequently. The Information and Exhibition Centre in the northeastern part of the reserve provides an explanatory and access leaflet in several languages. Land in the reserve is private but access on foot along proper tracks is tolerated.

Hunting is not a problem but illegal bird trapping occurs. Both local and Spanish eel fishermen occasionally disturb the sanctuary-like inner areas. Loose dogs chase hares and disturb ground dwelling birds but avoid humans. More of a problem is the use of off-road vehicles, quad bikes, trail bikes and even microlight aircraft. Nevertheless, in the early morning and for much of the year the reserve is a haven of peace and quiet.