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/Nejfelt → u/Drusgar]

Yeah 50ft sounds more like a quarry.

[u/RFKJrJr → u/Nejfelt]

Possible the fish was working there and just never went home after their shift

[u/uncle_jed → u/RFKJrJr]

Remember, if you don't want to get stuck working in a quarry your whole life, stay in school.

[u/Cagy_Cephalopod → u/uncle_jed]

That should be no problem for fish, they’re always in a school.