If you want to make endless fish, then do it only like in Age of Empires III. The fact is that whales there were not a separate animal, but a static “deposit” of gold. But squid and ordinary fish were not an endless source of food. Another nuance is that not only fishing boats in the Age of Empires III could fish, but also some military ones. You can, of course, make two types of fish “deposits”, one of which could end, and the other - a large one - slow production and an endless one, for which it makes sense to fight.
There is another more complex option - to leave the sources of fish limited, but they will periodically appear in a random place in the water area and their number, in order to avoid frame drops, must be tied to a certain number, different for each map and for different map scales. The second option is to let the fish, after exhaustion, go on cooldown for a few minutes and resume only after some time.
The Age of Empires II solved this problem in the form of a separate building, accessible to a fishing boat - fishing nets, which were like a farm, only for water.