Six Borland fish passes were built since 1970 on dams in the rivers Douro and Tejo. Although fish passes have been built, fish migrations are still obstructed and the loss of fish passage is important in both rivers. The migratory species such as Alosa alosa, A. fallax, Petromyzon marinus, and Anguilla anguilla, still continue to arrive at Crestuma-Lever and Belver dams being caught every year by fishermen. Belver and Crestuma-Lever fish passes were studied in order to identify the main reasons for unsuccessful operation. Design and operation conditions of both fish passes are described. Monitoring with video cameras was done to count and identify all fish using these devices. Relationships concerning the number of fish and species passing in each cycle, the condition of operations of the powerstation and the water levels downstream were studied. Some failures were detected resulting in lack of attractiveness mainly concerning the lower gates and inappropriate entrance locations. Also the fish passes flow was found to be not enough for an appropriate attraction to the entrance. Modifications are suggested in order to improve the efficiency of both fish passes.