How did fish repopulate lakes after the Ice Age?

While I don't know how natural fish repopulation works, I can answer the second question. Crater Lake formed in the caldera remnant of Mt. Mazama, a large volcano that erupted ~7,700 years ago. The last ice age ended about 4,000 years before that eruption. read more

Lakes which are devoid of fish must, naturally, be repopulated by having new fish transported there. The chief mechanism is flooding, which is common enough occurrence. Animal transport is another way. Fish eggs can hitch a ride on birds and, less commonly, on other animals. Unless, of course, by other animals one means humans. read more

Some freshwater lakes have been seeded with fish, but many now unconnected lakes were at one time connected. At the end of the most recent glacial age, they were due to the massive influx of melted glacial water which has now receded. Even today we find sea life in unexpected places. read more