i think this is intended:
In dark age and feudal age, fishing is an incredible source of food. Not requiring villagers is a huge advantage over hunting/farming. Later drops off fairly quickly as fishing spots become further and further away from the docks. fish traps are meant as a last resort, so fishing ships have something to do in the end game. They are not meant to be as good as farms.
Like so many things in this game this is kind of trade off makes decision making interesting: Do i invest in a few more fishing ships for a short term gain, or in farms for long term