These shoals are attracted by common features, inlet streams, sewer outfalls and eddies and slacks. These will be target areas for catfish. Below is a picture taken of an inlet stream, it shoals the water is black with 1lb mullet most of whom are swimming in a few inches of water. Catfish will attack the fish in these areas on a hit and run raid, this can happen in the day but if you sit around at dusk catfish attacks will be quite relentless on these areas. The mullet will eventually scatter and head for deeper water, usually the carnage is all over by midnight.
Sewers are the greatest catfish hotspots of all.
Sewage is not the only item pumped into the Ebro, outlets from abattoirs and animal processing is also pumped straight out into the river. To us in the UK this sounds abhorrent, but the fish life seems to thrive on this kind of pollution. So find an outlet and fish downstream of it, cats will be sure to be around. The picture below shows a greasy slick along the right bank, this is an outlet from a chicken processing factory and is a known hotspot on the Ebro with many cats to over 100lb being produced from this mark. Note the two white floats trotting down in the flow.