system
Given the following post, respond as if you're commenting on Reddit:
human
Where did the fish in volcanic crater lakes come from?

I've seen two volcanic crater lakes in Southeast Asia - Lake Toba and Lake Taal. Both have fish, but both are isolated and cut off from other bodies of water. Since they're volcanic crater lakes wouldn't the eruption would have killed all the fish? So where did the fish in the lakes come from?
gpt
Mostly from migratory Birds. A duck that goes from Lake to Lake will often carry eggs on its feet. All it takes is a few and with them traveling so much it's about the only way that you can have a species of fish that spans multiple areas. Otherwise every Lake would end up with a unique species due to a lack of genetic mixing. Some are also intentionally introduced by fisherman or government projects.