The fishermen's requirements overlap with each other. For example, the "100 Spring Fish" and "100 Fall Fish" requirements. If you are fishing and you catch 50 Horse Makerel, then you have 50 fish for both fishermen (and the "100 Ocean Fish" fisherman too) because the Horse Makerel appears all year long in the Ocean. If you then catch 50 Salmon you'll unlock the "100 Fall Fish" fisherman but not the "100 Spring Fish" fisherman, since Salmon only appear during the Summer and Fall seasons.