Rotterdam is about to join other Dutch cities including Amsterdam and Leiden in regulating what you can feed the ducks in lakes and canals in the cities, mandating that only appropriate food can be fed.
The problem is largely that many people enjoy feeding the wildfowl on discarded bread, which is said to be unhealthy for birds because of its high salt content and because it's often moldy by the time it become duck food. The mold can be fatal. Other complaints include an increase in rats, mice and seagulls who are attracted by the bread.
For those who still want to feed the birds, conservation organizations have recommended corn, peas or salad greens, or duck food pellets available in pet shops. That's not quite far enough for some: the Dutch Animal Rights Party has introduced a resolution banning feeding of wild animals altogether.