A year ago I spoke with a woman who knows all about koi and that pond. The City should have let professionals make sure the pond stayed aerated and habitable for koi and other fresh water critters. I know one company offered to maintain the pond for free.
The koi are gone because the pumps (aerating the pond) broke down one day and no one noticed until it was too late. The koi went to that great pond in the sky.
As for feeding bread: I think everyone of us who sees people doing that should man up and tell those people to knock it off. Nicely, of course. There must have been two dozen mallards (looked to be mostly female/young males) on the grassy bank, begging, just two days ago.
Free here ducky ducky handouts and the pond has turned to muck. Not only does the sinking bread pollute the pond but then the ducks are pooping out all that unhealthy for them white flour and accompanying preservatives.