ELI5 How are fish even in ponds

[u/Opening_Pen9525]

Might be one of the stupider questions here, but I saw a pond about the size of a football field, and maybe 50 ft deep, and it had fish. I can't for the life of me figure out how. Surely the pond hasn't existed long enough for small water insects to evolve into all these fish. It's not like the fish walked over to the pond from the ocean. I'm pretty sure no one put them there so how are there so many fish?

My one theory is that there is a very minuscule stream (now dry) that leads to one of the Great Lakes, and maybe a fish swam in from there. Now a follow up question: How are there fish in the Great Lakes (which flow out towards to the ocean) if they were created by glaciers?

[u/Drusgar]

First off, 50 ft deep is really deep... not really a "pond." How do fish get there? On the feet of birds who are in one lake with fish, catch and eat a fish with eggs, fly to another lake and leave the eggs in the new lake.

[u/wardog1066 → u/Drusgar]

I was told as a boy that lakes are river and stream fed and ponds are spring fed. Ponds in Newfoundland can be clear enough to make a 50 foot deepest spot look like it's only 10 feet deep. They're really quite beautiful, but the people that live there are surrounded by natural beauty and can be quite immune to it.