I searched the forums and saw that no one else had brought this up before, so I would like to propose the following:
If a region's economy includes fishing, part of the region's ability to produce food should not be affected by season. Instead, the region should be able to constantly produce food by fishing. (Obviously, if the region also relies on farming, overall food production will still be vulnerable to season.) To balance this, fishing will also not get a season bonus -- and the region's ability to produce food will, of course, still be affected by other factors like population, production level, and looting.
This change will make coastal areas somewhat more desirable due to the ability to produce a steady supply of food in scarce times; however, I believe the change would not be overpowered because fishing would get no seasonal bonuses and therefore produce no more food in the course of the year than farming would, on average.
Disclosure: I am a lord in a region that includes fishing as part of its economy.
