I lost all my fish over the winter.
A large critter (possibly an opossum or maybe a raccoon) must have chosen the pond to die in just before it iced over. My dog was REALLY trying to tell me there was something there all winter. I had a small heater to keep an open space in the ice for gas exchange, and the critter must have been close to that. When things finally thawed this spring, the smell was god awful and everybody was dead (goldfish and red shiners).
I got away with not using a screen last year, but from now on I am going to screen the pond in the fall, even though I have resisted the idea.