Mullet feed on algae and can be seen browsing near outfall pipes.  Perhaps avoided in the river, (why ?) but sea fish are OK so look at Rick Stein's and others' recepies.  A course flesh admittedly, and a little grey, but they're first class protein and eaten throughout the world.  If you can't affored salmon or gilt head (like most of the world's people) there's mullet. Cheap plentiful and well worth the eating.  Much better for you than Chinese panga, sold in most supermarkets, grown in the Mekon River and fed (allegedly) on human and animal excrement, (but note names like Panga, Blue fish, Gilthead relate to many different species).  Personally, I wouldn't eat anything caught below Flix but I have eaten many a fish from the Danube which is vastly more polluted than the Ebro.  BTW have you tasted calves' brains?  Very tastey if taken with vast quantities of rough Romanian red wine!!!!!!