A coastal fish weir comprises two artificial walls of stone and/or wood to which are attached nets and/or a trap to catch fish. The weir is located in coastal waters, either on a gently shelving coastline or in a river estuary. The walls are between 100m and 200m in length and form a substantial enclosure with a V- or L-shaped plan. The narrow point is on the seaward side.
Coastal fish weirs are recognised as field monuments by the arrangement of their main components, especially the large V-shaped weir. Positive identification usually requires intensive survey because the weirs and traps are often positioned below the water level at high tide. Coastal fish weirs may also be spread over a considerable length of coastline and are best located by aerial photography. Documentary sources also allow the identification of coastal fish weirs, usually through legal documents, place-names, and being depicted on old maps.
Coastal fish weirs and the individual components that comprise them may sometimes be confused with other classes of monuments, especially those which also involve water and/or fish management, such as oyster beds, eel traps, and river fisheries. This is especially the case where the ephemeral components have become degraded or destroyed. They may also be confused with other classes of coastal monuments such as sea defences. In all cases, the classification of the monument as a coastal fish weir relies on the careful examination of the arrangement of the main components represented as field evidence and, where possible, the examination of relevant historical records.
Specifically excluded from this definition of coastal fish weirs are all other kinds of coastal ponds and water-filled features connected with the exploitation of marine life. These include oyster beds, eel traps and other forms of coastal fishing. Also excluded from the definition of coastal fish weirs are river fisheries which use the same basic principles but are located on rivers and are concerned only with migratory fish.
Coastal fish weirs were constructed in medieval and post medieval times to catch sea-water fish in order to provide a constant and sustainable supply of food. As such, the ownership of a coastal fish weir was a very valuable resource.