In the year 1289, King Philip IV of France was worried about fish. “Each and every watershed of our realm,” he proclaimed, “large and small, yields nothing due to the evil of fishers.”
(The Atlantic (2019)
The medieval period is much too long to provide a definitive answer to your question. Besides, there often wasn't such a thing as 'a single fisher'. Often enough, you'd see fishing villages, villages that exported fish. Or fishing guilds. Fishing families. Especially seaside when requiring a boat to fish. You don't operate a fishing boat by yourself. When setting fishing traps and weirs with a group of people, how much was caught by whom?
When you consider river fish, you probably won't catch the same fish year-round. During the summer months, salmon moves up the river to spawn. The more watermills were built, the less salmon was caught.
Some of what we know about medieval fishery comes from the area of Colchester (UK), near the river Colne and relatively near the Channel. There has been fishery in all kinds of forms since at least 800 AD, years later they even had their own oysters. In (and after) the late medieval period, small wars have been fought about fishing rights and over-fishing there. If you're looking for historical records, I'd think that area to be a good start. Perhaps the Domesday Book has usable information for you as well.
How much fish they'd actually catch would depend on many factors, among which the amount of organization, their tools used (hooks, nets and traps were already used quite extensively in some places during the middle medieval period) and the local fish-population (which would vary quite a bit).
In the end, it matters not. You ask:
But I don't know how many people could be fed by a single fisher?
Not many. If fish would be all they ate, they'd eventually die of malnutrition. They would require bread as well. So you'd need a farmer, and hunter/gatherers both for hunting mammals/fowl and gathering fruit/vegetables.