That lake/reservoir is called Castelo de Bode and until they opened up Alqueva it was the biggest mass of freshwater in Portugal. It's a pig of a place to fish, but when and if you get it right, it can produce some whackers.
iberiancarpa, where on the Algarve are you?
Have you ever tried Santa Clara just over on the north side of the Monchique range? Some whackers in there too, but as with Castelo de Bode, a right pig of a place.