"Europe", as a whole, probably has a large enough variety of fish to fish all year long, but any particular region won't. The seasons will not be the same, but there are definitely seasons to fishing.
Whales are migratory too, so there is a whaling season.
I think any region with fishing listed as part of its economy has, by definition, enough fish present throughout the year to justify its impact on the region's economy. In many places of the world (and some in Europe), fish were a staple food consumed year-round. I wouldn't mind giving fishing on and off seasons (200% in one season, 25% in another, etc.), but balance-wise I think this particular feature would work better if fishing stayed at a constant 100% throughout the year. Let me explain:
| Spring | Summer | Autumn | Winter |
| Farming | 75 | 100 | 200 | 25 |
| Fishing | 100 | 100 | 100 | 100 |
This table shows that, with each harvest in a particular season, all other things being the same, 50% of food production will come from fishing over the length of a year if the region has both farming and fishing. Lords of these regions will have to stockpile food in warehouses throughout the year, not just in particular seasons, making them vulnerable to looting year-round -- although they will also be able to recover faster than regions that depend on farming. While it's true that regions with fishing might be able to support inland populations during a time of famine, it will take careful planning and a cooperative lord; uncooperative ones might use it to their advantage.
How would you like to set the seasons for fishing? Remember, too, that these settings will be used in
every region that has fishing, regardless of their climate or geography.
