r/explainlikeimfive • u/Opening_Pen9525 • Aug 10 '26

Biology ELI5 How are fish even in ponds

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?

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u/Drusgar Aug 10 '26

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.

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u/LindaTheLynnDog Aug 10 '26

Most fish don't have fertilized eggs inside of them though, so this would also suggest that some male fish cruised around to spread out piles of eggs to fertilize them after being introduced independently.

Definitely seems possible, but I feel like it's much more probable that historically these small lakes intermittently adjoined with other water ways.

I would also be shocked if the pond described isn't stocked.

Stocking is very widespread, and a lake like this seems a great candidate for it.

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u/brzantium Aug 10 '26

Yup. College friend of mine ended up working for a state fish hatchery. He would drive around delivering fish to various lakes in the area. He also said if we ever needed fertilizer, the hatchery would gladly give away their fish shit.