Ok so time to over complicate
So my thoughts are this:
1. Having a specialty ship is right on. A fishing ship with a net will always catch more fish than a Howkers crew with poles, and why would you use a SOTL?
2. I would say look at current fishing regs. You must get a permit and fleets can only take as much as the quota offered by the governing entity. (like the amount of available land in plantations per port...hmmmm??)
3. When i first started I had 10 fleets fishing to make some coin, which is different than if I did the same now.
4. Why shouldn't those who want to fish be able to research for more production?
My suggestion steals a little from everyone.
1. New fishing ship
2. one tech tree to increase yield (maybe upgrading nets, cargo hold, salting practices)
2. Low level players may have (x) fleets per port till level (x) for free
3. Those who want to fish a ports waters get a licence for ($ x) (lasts 30 days, doesn't need approval, goes right to tax income and auto renews till unchecked) and cost is per fleet. Dont get the licence, your poaching, and go on port gossip and you gain disfavor with nation every day, hit (x) disfavor and go on plunder boards as a poacher
4. With that licence, comes a monthly quota, exceeding that quota decreases the amount of money you can sell your extra take for. ie: half price (makes you balance your fleets)
5. You can "bribe" the port or controlling country to double or triple quota. (that will take approval and $$$ and must be done manually every thirty days).
So were is the risk?
1. If you go over quota, and have not acquired extra papers, you go on port gossip and start getting disfavor with nation everyday, hit (x) disfavor and go on plunder boards as a poacher
2. There could be natural events that hamper or boost production like, red tide -50% haul or Stormy seas stir up food +25% haul or you managed to find a school of swordfish, goods sold at 125% or you nets ripped on the sunken hull of "random active players name's" ship, no income today while you repair.
Remember...
You can sheer a sheep many times, but only skin it once!