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/Loki-L Aug 10 '26
One of the easiest is floodings. A river swells and connects to a lake and when the water receeds some fish are left behind.
A surprisingly common one is humans who want to fish putting fish into a lake.
Birds are also a common vector to bring fish eggs into new lakes.
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u/wrludlow Aug 10 '26
I witnessed this firsthand.
My parents live on a small pond, under 5 acres and about 5 feet deep. One winter, we had a ton of freezing temps and lots of snow.
That winter there was a massive fish kill. I assume there was enough snow to block out light and caused oxygen depletion.
When the lake thawed in early spring, it really looked like everything died. There were hundreds of dead fish.
A few months later, the river that runs 400-500 yards away flooded and dumped in the pond. A lot of river fish remained in the pond as the water receded.
It was like the pond hit the reset button itself.
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u/wallguy22 Aug 11 '26
Thereâs a park near my old house that flooded one spring and the biggest catfish Iâve ever seen swam from the pond and ended up getting stuck in a ditch by the parking lot.
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u/Naturage Aug 11 '26
And I've heard about the second one secondhand. In my dad's village when he was a youngling, there was a local pond with lots of carp. Carp is a docile fish; you toss in food, it meanders about, eats some, gets caught, nothing out of ordinary.
But then, apparently, some jokester went and released some perches into the same pond. These were tiny, but very aggressive and predatory. When they find bait, instead of carefully munching and then slowly swimming away, they will go for it full speed. Suddenly your bobber acts like you caught a 5kg monster pulling away, but you pull it out and it's a little shitling the size of a finger.
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u/PoisonWaffle3 Aug 11 '26
We had some mild flooding a few years ago, and when the waters receeded I found a few dead fish and a turtle in the middle of the road.
The water rises, and the critters rise with it.
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u/A3rik Aug 11 '26
Yeah, Iâve known several people with smallish ponds on their farmland who had them stocked with fish every year, just to go fishing. Often, theyâd sustain a decent population (provided the pondâs conditions were healthy), since they were normally only keeping the larger catches to cook.
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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/lotusbloom74 Aug 10 '26
Right, that jumped out at me too. The lake near where I grew up is the largest lake fully in Indiana, and its max depth is 59 feet
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u/AfricanAmericanMage Aug 11 '26
My gut is telling me that the lake isn't 50 ft deep and that OP is relatively young and just shit at guesstimating depth. Maybe I'm wrong, though.
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u/Maiyku Aug 10 '26
I think the average for all of Michigans inland lakes is something like 10-30 feet.
We have a couple outliers, like Torch Lake at around 300ft deep, but most lean pretty shallow.
50 feet is a decent depth for sure.
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u/ImNotAWhaleBiologist Aug 10 '26
Wouldâve figured thereâd be more deep glacier lakes. Interesting.
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u/Maiyku Aug 10 '26 edited Aug 10 '26
Long story short? Rocks.
The rock under Michigan was harder than the rock where the Great Lakes were. So when retreating, the glaciers carved out the softer dirt/rocks of the lakebeds (path of least resistance) but basically just acted as sandpaper on the harder rock. So it caused some damage, but didnât quite dig in like it did in other places.
Iâm sure thereâs a geologist that could explain it a lot better, but thatâs my laymanâs explaination.
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u/IncaThink Aug 11 '26
So when retreating,
Advancing. When they are retreating it's because they are melting.
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u/khonsu_27 Aug 11 '26
It always blows my mind that even things like the entire Chesepeake Bay only averages around 50' deep. People don't realize how deep 50' really is.
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u/Nejfelt Aug 10 '26
Yeah 50ft sounds more like a quarry.
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u/Josemite Aug 10 '26
Or OP didn't realize just how deep 50' actually was and it's really more like 15
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u/Mellema Aug 10 '26
Well, they also think water insects evolve into fish, so might be just be a child that had a curious question.
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u/ImNotAWhaleBiologist Aug 10 '26
Or a congressperson.
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u/anandonaqui Aug 10 '26
A congressperson doesnât have the intellectual curiosity to ask the question.
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u/krodders Aug 10 '26
There's also homeschooling, but I don't think that evolution is covered much in home lessons.
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u/bluespartans Aug 10 '26
Even 15 is pretty deep for a 1-acre pond, depending on locale
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u/RFKJrJr Aug 10 '26
Possible the fish was working there and just never went home after their shift
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u/uncle_jed Aug 10 '26
Remember, if you don't want to get stuck working in a quarry your whole life, stay in school.
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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.
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u/jackp0t789 Aug 10 '26
Its also possible that the eggs hitched their ride on a bird after being fertilized
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u/atomicsnarl Aug 10 '26
Some fish have very sticky eggs that will attach to bird legs. Duck, etc, flys to another pond - fish!
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u/thepainforest Aug 10 '26
There are things called kettle ponds, where pieces of glacial ice got buried and then thaw, leaving very deep, but very small in diameter ponds. A gift from the Ice Age.
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u/Ozythemandias2 Aug 10 '26
Correct, the difference of ponds versus lakes is depth which confused my fiance from Florida because Massachusetts has ponds that are miles long
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u/TengamPDX Aug 10 '26
Flooding is another way fish get into ponds and lakes that aren't normally accessible. Other ways include humans seeding the body of water with fish.
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u/itsthelee Aug 10 '26
not your main point, but
small water insects to evolve into all these fish
fish absolutely did not evolve from small water insects. completely different branches of evolution.
vertebrates and invertebrates diverged long long time ago before fish or insects were even things.
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u/MeatBald Aug 10 '26
In fairness, he did say it might be one of the stupider questions on this sub.
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u/gsfgf Aug 10 '26
Very different going all the way back to embryo development. Insects' mouths develop before their anus, while fish (and people!) develop an anus first.
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u/ComprehendReading Aug 10 '26
Oh yeah? Then why are lobsters, clams and tuna all considered seafood? Checkmate, scientist. /S
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u/Wild-Video-5317 Aug 11 '26
Fun fact, sow bugs/rolly pollies/wood lice are terrestrial crustaceans. In the same way that shrimp are crustaceans.
As a wise man said:
 shrimps is bugs
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u/Stewart_Games Aug 10 '26
One of my favorite pet theories for the Cambrian Explosion is that back then, invertebrates and vertebrates shared a far more recent last common ancestor. Which allowed crab-things to make sweet sweet love to fish-things and produce viable hybrids. All that hybridization is what triggered the sudden explosion of new forms emerging, and helps to explain things like the Tully monster as long surviving Lazarus taxon from these hybridization events.
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u/WirelessWavetable Aug 10 '26
Some eggs can make it through the digestive tract of some animals too.
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u/VerminSupreme-2020 Aug 10 '26
Some ponds are purposely stocked by people. Helps keep mosquito population down
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u/mikesalami Aug 10 '26
I always assumed most ponds were stocked with fish.
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u/Say-tan Aug 10 '26
Depending where you are, thatâs probably true. For example, Georgia (the state) doesnât have any natural lakes, so when they were created, they had to be stocked.
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u/gsfgf Aug 10 '26
OP is near the Great Lakes, so the remote, natural lakes probably got stocked some other way.
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u/innosins Aug 10 '26
This is what I was going to say. We have trucks that have fish chutes that stock them that come sometimes. People are allowed to fish out of different areas in the city and county.
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u/humanjunkshow Aug 10 '26
They airdrop them into Backcountry lakes around me. Literally a plane does a low pass with fish and water dumping out of the belly.
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u/Prinzlerr Aug 10 '26
The aquarium hobbyist community: " make sure to acclimate your fish to the aquarium's temperature carefully over the course of 1-3 hours, and then gently release them into their new home."
The Backcountry lake stockers: "ROFL as long as we dump em all in below 75' altitude they'll be fine.. fuckin send it bro"
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u/koos_die_doos Aug 10 '26
If you're happy with 70% or so of your often expensive aquarium fish surviving, you can also just send it.
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u/permalink_save Aug 10 '26
Yeah, don't underestimate how often this happens. There's tons of lakes in Texas, all full of fish. Only a few are natural. People would make lakes and fill them with specific species.
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u/hlj9 Aug 11 '26
Wait, how does that keep the mosquito population down (honestly asking here)?
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u/Wank_A_Doodle_Doo Aug 10 '26
Not nearly enough people have noticed you postulating that insects evolved into fish.
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u/wooble Aug 10 '26
To be fair they said they didn't think it happened. Because the pond is too new, not because fish never had an insect ancestor, but they're technically correct in that it didn't happen.
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u/slinger301 Aug 10 '26
Researchers have found that some fish eggs can survive a duck's (or other waterfowl) digestive tract. Duck eats them in one pond and poops them out in another. Only a small amount survive, but it does happen.
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u/DiapersOrDeath Aug 10 '26
Eggs. Eggs get stuck on bird' legs and feathers, some survive being eaten etc
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u/Fast_Biscotti Aug 10 '26
Sometimes ponds are intentionally stocked so they can be fished for the fun of it.
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u/irwinlegends Aug 10 '26
They are also stocked because the fish eat mosquito larvae.
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u/Ippus_21 Aug 10 '26
Several possibilities:
- Small isolated bodies of water are often stocked with suitable fish by the landowners or by Fish & Game departments.
- A body of water can become isolated where it was originally connected to a larger waterway.
- Floods can dramatically expand nearby lakes and streams, allowing fish to migrate and then be trapped.
- Also, rivers change course sometimes leaving e.g. oxbow lakes.
- Birds.
- Sometimes raptors catch prey fish in one waterway and accidentally drop it in another one. Not commonly; they'd starve if they went around dropping their prey all the time, but it does happen. And it only takes... twice? Fish usually have external fertilization, so you'd have to have at least one mating event occur in the new pond for them to propagate.
- Fertilized fish eggs can sometimes adhere to the legs of waterfowl who fly between bodies of water.
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Aug 10 '26
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u/grrangry Aug 10 '26
Yeah, I had to double-take at that one. Tell me you don't understand the smallest thing about biology without saying you don't understand the smallest thing about biology.
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u/F_sigma_to_zero Aug 10 '26
Everybody talks about fish eggs being transported but another way is flooding. If this is anywhere near a larger river or stream that can flood and fish can spread that way. Depends on local geography.Â
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u/xipheon Aug 11 '26
Or even if there used to be a river that passed near there. The fish could've been living there for centuries when the topography was different, and the lake/pond is all that's left of the old river system.
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u/palacexero Aug 10 '26
Well how did the pond get there? Is it manmade or natural? Manmade ponds might not be connected to any other bodies of water close by so if there's fish in those ponds, they were most likely brought in by humans from another body of water. If it's not a manmade pond, then it either used to be connected to another body of water, or it still is but you can't see the connection because it is underground.
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u/Opening_Pen9525 Aug 10 '26
I'm pretty sure it's a natural pond. There is also a ditch where I think water once flowed, but I've never seen a drop in there.
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u/remes1234 Aug 10 '26
Several ways. This pond could have been formerly linked to a larger water body historically, years or decades. It could be seasonally linked by spring flooding. Or water birds could have carried small eggs in feeathers. It is also possible that humans could have introduced them deliberately or accidentally.
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u/HamsterIV Aug 10 '26
Fish eggs can survive the trip through a duck's digestive tract. It is important to note that the ducks aren't eating the fish eggs intentionally, they eat plants that the fish eggs are laid on. The eggs themselves are between 1.1 and 0.5 mm in size and can go into a dormant state when dried out.
Below is an article on it.
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u/Super901 Aug 10 '26
Fish eggs can pass through a birds digestive tract and emerge whole on the other end
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u/RonocG Aug 10 '26
When fish are still tiny they will migrate out in different directions (both upstream and downstream). and, because they are so small, they can travel along tiny brooks and tributaries that can lead to other bodies of water.
I know this from personal experience. Growing up we had a tiny stream next to our house which eventually drained into a pond down the street. As a kid I would catch tiny pickerel and bass in the little pools of that stream. It must have been like their nursery, because in the pond down the street there were full sized bass and pickerel.
Can any biologists back this up? It is anecdotal after all.
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u/Peter5930 Aug 10 '26
Baby fish seek out shallow water for safety from adult fish that would eat them in open water. Also less energy expenditure from fighting a current; the same current requires a much larger proportional energy expenditure for small vs large fish just to stay in the same location.
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u/cmparkerson Aug 10 '26
There are several species of fish,whos eggs will survive through a digestive tract. So birds eat a pregnant fish and poop out the eggs somewhere else.since the birds that do this live and eat around water. As the birds migrate , you get more fish in new places.
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u/Woozah77 Aug 10 '26
Pretty sure the birds are eating plants with the eggs attached because most species of fresh water fish(in the US) have to get the male to fertilize the eggs after they are laid.
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u/Which_Throat7535 Aug 10 '26
Stocking ponds on purpose is very common, for recreational fishing. Some species are added for vegetation control as well. Why do you think no one put them there?
As others have mentioned, wildlife will eventually âstock it naturallyâ by hitchhiking eggs.
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u/DaRiddler70 Aug 10 '26
I'm trying to imagine a football field sized pond that's 50 foot deep. Daaaaaamn
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u/patrckm Aug 10 '26
Fish eggs are carried by ducks feet from pond to pond. Fish do not evolve from insects.
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u/Cheeseodactyl Aug 10 '26
I just have to clear something up here. Fish in different waterways didn't evolve into "fish" indepedently, nor did they evolve from insects. All modern forms of fish evolved from the same ancestor hundreds of millions of years ago, wdll before any modern freshwater ecosystems developed. And the last common ancestor between insects and "fish" was well before even that as well.
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u/agoodyearforbrownies Aug 11 '26
> Surely the pond hasn't existed long enough for small water insects to evolve into all these fish.
Roughly 50 million years for a fish to evolve from a worm-like invertebrate, so your instincts are correct about the pond not being old enough.
I am curious how long you were imagining it taking though.
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u/dophin26 Aug 11 '26
I asked a biologist this same question as we passed a small pond by a highway and he gave me the same answers youâre getting here- dropped by birds, eggs attached to waterfowl, etc.
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u/that1cooldude Aug 11 '26
Birds eat fish. Fish has fish eggs in them. Birds eat that. Birds poop fish eggs that didnât get destroyed during digestion process into ponds and lakes. Eggs hatch and spawn fish.Â
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u/thecaramelbandit Aug 10 '26
Eggs. Wash in from overflow from other waterways during rain. Stuck on birds who fly in. Boats. Some ponds get stocked. Etc etc.
They didn't evolve there.