i think this will be very easy to add, and make rivers/ocean have different spices of fish depending on the river size
I also thought about it. Is interesting that there are different species of fish.
There could be different methods, fishing rod and basket with bait inside. The rod could be used in the hand or nailed in the ground. If it is nailed the fish can be stolen (with the rod, optional). The basket with fish inside could also be stolen (or only the fish, leaving the basket).
I suggest that there is a fishing limit, if it is passed the number of fish decreases until don´t catch anything and have to wait for they are repopulated.
In some rivers there could be otters that sometimes steal the fish from the rod and part of those captured inside the submerged basket.
Otters eat freshwater mussels, this molluks could be another food option that humans can obtain. These would be obtained by tamming an otter that taken them out of the bottom for you. The otter wouldn´t be in the pet house but free in the river. To domesticate it you would have to give it fish, get close enough for it see the fish deposited near on the shore but not too much so that it don´t freak out and run away.
Humans could also get freshwater mussels but they should remove the armor to get into the water. It would be necessary to wear fine clothes or leather armor that could deteriorate faster.
Another resource that could be obtained from the river is otter droppings on the shore to be used as fertilizer. And the freshwater mussel shells could be used for something.
As a vegetable food, algae, there are very nutritious.
In winter the water is colder, the possibility of finding mussels or algae by diving could be less than as a beginner. With experience it could increase resistance to cold water.