I myself have seen the local guys on a saturday set up on the bank with their beachcasters,beer and crib table and then just leave the poor fish lying on the bank to die.Is this a Spanish thing knowing that they are not quite as animal PC as us "caring Brits" or because they are not a native species in the Ebro.The Aussie are legally bound to kill European carp if they catch one in their rivers or billabongs and yes,the eastern europeans do eat their catch,most of the Polish lakes are devoid of fish unless they are privately owned and policed.
In answer to the questions,and I'm no biologist,if I was one of natures largest scavengers I wouldn't want to work to hard for a meal so dead(freshly killed) would be best.They are pretty much blind and find thier food using their "whiskers",a fresh drop of blood would have them banging the table with their knives and forks in anticipation.We were given a bit of inside info by a barman who suggested when  to fish a spot near a chicken processing plant where the over flow  would spill into the river,he also told when they would be cleaning it and told us when not to then fish it as the chemicals would send them away.