In Fishing: North Atlantic you need to use the sonar to find fish. You will need to travel around in real time, using your sonar to find good fishing spots. You can also ask for fish tips at the bar in port, or go to areas with AI boats, as they fish around fishing spots as well. There is also seagulls flying over bigger habitats, which can easily be spotted using your binoculars.
NOTE1: If you use fast travel, the sonar will not update as regularly as if you travel in real time.
NOTE2: Remeber to upgrade your sonar, so you can see fish on deeper depths. If you have a sonar that only show fish at 100m depths, you will see no fish on sonar deeper than 100m.