Initially, wouldn't you be fishing for salmon? Adding new fish suggests adding new recipes (grilled rainbow trout) or a new quantity to harvest from caught fish.
Would it be a fixed time increment with a possibility of catching fish (i.e., when you cast your line, x time passes and you may or may not get a fish) or would you always catch something but the time vary by a few seconds?
Would the size of your catch depend on the quality of your fishing rod and line? (Can't catch larger fish without the right gear.)