The water level has dropped so low here - to below 10% of the reservoir's capacity - that there is already a risk the water will be contaminated by silt. Therefore, while the fish are removed, Sau's remaining water is being emptied downstream to another reservoir.
"We are trying to transfer the water as quickly as we can, because the quality right now in the winter was good [but] in the spring it will become really, really bad, and we're trying to extract all the fish we can find there," said Samuel Reyes, director of the Catalan Water Agency (ACA).
The Sau reservoir, 100km (about 62 miles) inland from Barcelona, has been supplying water to the city and other towns in the north-eastern region of Catalonia for half a century. But in recent months it has become the most visible symbol of the worst drought this area has seen in living memory.That is because of the now-notorious sight of the 11th Century church of Sant Romà de Sau, which was submerged when the reservoir was created in 1962.
In times of abundant rain, the building - situated in the reservoir - sat below the water level, but it now stands several metres above the waterline, surrounded by parched earth.