are there any fish that you can eat from the river?

At certain times of the year there is a good run of sea bass up as far as the weir

what about all the crops and rice fields they water from the ebro as it flows down to the sea

Fish caught below the dam at Flix should not be eaten due to the pollution in the river. Fish from above the dam at Riba Roja are ok to eat. Some of the more popular fish to eat are Bass and Zander. At Mequinenza and Caspe these fish are regularly caught and consumed.

Ive eaten mullet in the uk I dont think they are worth the effort

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!!!!!!