Why herring and white bread, every year since 1574? Because that's what the boats carried — salt fish keeps, and the loaves were baked somewhere else and rowed in across the flooded fields; in the painting they're handing them down out of baskets right there in the street. Hutspot is the other half, roots and onions in one pot, the cheapest hot thing a city with nothing can make, and that's been the supper for four hundred and fifty-odd years.
Rosa — cutting the dikes salted the polders for years after. The grain wouldn't come back for a while, but the brackish water pulled in waders: avocets, godwits, the sort of birds that like a drowned field better than a dry one. Farmers lost a harvest and the counts went up.
The reward came the next year, 1575 — William of Orange founded the university there, and the story that's always told is that the city was offered a choice between that and years off its taxes, and took the books. Probably tidied up in the telling, but Leiden has kept the version it likes.