It seems that the Tuna are caught wild, then fattened in the cages out there, Mussels are usually grown on ropes.

Not sure there is any connection to dead fish in L'Ampolla, I have fished for Palometa there at the harbour entance as the fishermen clean out their catches and throw out the undesized fish. Some boats near here got caught for undersized catches recently, but there is still plenty blatantly on sale. One of the new Aquatium/Fishmongers in Tortosa recently sold a friend 4 different bags, all illegal/undersized/protected species. One night fishing near L'Ampolla last year, you could clearly hear a banging that was Poachers using an illegal method to drive fish into nightime nets...... all Forestals safely in bar/bed, they carried on safely.