This feature is similar in size and form to NPRN 519355 (Ynys Castell Fish Trap) but appears more ephemeral on aerial photographic imagery and is not depicted on any OS map. The feature may be contemporary with the modern house on the adjacent plot of land, which does not appear on the 3rd edition OS map (1914-25). The feature is approximately 150m from end to end, and begins on the shoreline, adjacent to what appears to be a cleared area leading from the house. The feature extends in a southeasterly direction, parallel to Ynys Castell fish trap, approximately 200m southwest. The feature is also parallel to a modern footbridge leading to a landing stage (approximately 70m northeast), and a modern causeway (approximately 120m southwest). If this was a fish trap it may have worked by channeling fish between Ynys y Big and the shoreline.

Sources include:
National Assembly of Wales, 2009, vertical AP
Ordnance Survey, 1886-7, 1st edition Landmark
Ordnance Survey, 1899, 2nd edition Landmark
Ordnance Survey, 1914-25, 3rd edition Landmark

Maritime Officer, RCAHMW, January 2011.