Lover is a type of Person. Girlfriend is a type of Lover. 
Fisherman is related to Fish. Fish is related to Animal. Animal is a different concept from Human. you are likely to find Human in/at/on Boat. 
Person <--IsA-- Lover <--IsA-- Girlfriend 
Fisherman <--RelatedTo--> Fish <--RelatedTo--> Animal <--DistinctFrom--> Human --AtLocation--> Boat 