They were used to keep tuna nets in place. It's known as the cemetry of anchors. The location is Barril Beach, near Fuseta, Algarve. My mate Mr Google definitely knows more about the history than I care about.
Edit: There are hundreds of them, weighing about 200kg each. 1841 is a significant date for some reason.
There is an interesting collection of various anchors that have been used for moorings at service depots of many harbours. Dartmouth Hoodown, for one. Probably awaiting scrap metal prices to increase.