[melaniejs (OP)]

Denis asks:

How can the same species of freshwater fish occupy different water catchment areas, each being separated by relatively high and dry watersheds?

What do you think?

[alancalverd]

I think the question has been asked before - I've certainly asked it of several biologists.

General theory is that some fish or eggs have been transferred by birds, most have been introduced by Man, and a few migratory species (trout and salmon) occasionally get lost and populate a new waterway from the sea. Where the fresh water has been cut off for a very long time, you can find particularly localised species that have evolved in, say a cave.

In northern Europe a lot of fresh waterways are actually the residue of a retreating ice sheet, so at some time they were all one melt puddle around a gigantic iceberg. This may explain the presence of arctic charr in several distinctly separate highland ponds.