Answer:
Geographical isolation.
Explanation:
Due to Geographical isolation, there are unique species present in each pond because they can't inter-breed with one another due to no way of movement to one another location. These unique fish species are geographical isolated from one another which is also responsible for its uniqueness. If they are not Geographical isolated so these fishes mate with each other and their uniqueness will disappear so we can conclude that geographical isolation is responsible for the uniqueness of fish species in each pond.
