Fish are a considered raw material used in supplying a city. It can also be sold on the market for a base price of $100/unit.
Amount/Unit depends on demand AVG at a city.
Crops and Meats affect population at 5x normal and Water which affects overall population at 10x the typical rates. The surplus or lack of food and water in a city carries much more weight than other products when calculating population growth.
To make fish, you need an ACP and a tile that has pastures as a resource. Click on the ACP page for more info about production levels and upgrades.