The circle around the building shows where the fishermen will fish - the more of this that falls in water (lakes and rivers), the more fish will be produced. Overlapping this circle with another fishing dock's circle will decrease fish production for both docks. Placing water-based buildings (e.g. trading posts, bridges) in their circle will also lower their production.
This building can store up to 50 fish, before the fisherman takes a trip to the nearest non-full storage barn.
Fishermen are greatly affected by tools, as almost all of their time is spent casting, which is affected by tools.