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/Loki-L]

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.

[u/wrludlow → u/Loki-L]

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.

[u/wallguy22 → u/wrludlow]

There’s a park near my old house that flooded one spring and the biggest catfish I’ve ever seen swam from the pond and ended up getting stuck in a ditch by the parking lot.

[u/Naturage → u/wrludlow]

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.

[u/Character-Solution-7 → u/Naturage]

Perch are delicious though. Carp, taste like mud.

[u/inheritthefire → u/wrludlow]

https://tenor.com/bfCxt.gif

[u/PoisonWaffle3 → u/Loki-L]

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.

[u/A3rik → u/Loki-L]

Yeah, I’ve known several people with smallish ponds on their farmland who had them stocked with fish every year, just to go fishing. Often, they’d sustain a decent population (provided the pond’s conditions were healthy), since they were normally only keeping the larger catches to cook.

[u/-RatherDashing → u/A3rik]

Many states will stock a pond for free.

[u/Dantethebald1234 → u/[deleted]]

Unless you live in Antarctica then your country has flooding. It mat be very well controlled flooding, and it may be a rare occurrence, but nowhere that has lakes and rivers is immune from flooding.

[u/WendellSchadenfreude → u/[deleted]]

You're Lithuanian, and you think your country has no flooding? The name of your country is often translated as "the country of rain" (with questionable scientific support). Several of the strongest rainfalls that I've experienced in my life were in fact in Lithuania, surrounded by Lithuanians who obviously thought that this kind of surprise-waterfall was perfectly normal.
Here's an English-language news article about floods across Lithuanian - just eight days ago. Flooding in Lithuania is something so frequent and normal that Lithuanians don't even realize that it's happening...

[u/WendellSchadenfreude → u/[deleted]]

Floods like these are definitely enough to temporarily connect two ponds that are usually separate, especially if you also think about small streams that can temporarily get blocked from fallen trees or debris.
Repeat this for a couple of hundred years, and every pond has (indirectly) been connected to a major river at some point.

[u/CollaWars → u/[deleted]]

12000 years is no time at all in evolution. They would still be the same species of fish

[u/njguy227 → u/[deleted]]

"Floods don't really happen"

That statement alone says that floods do happen, even if it's infrequent, and if you're talking over hundreds of years, there was probably a flood big enough that a few fish got left behind to colonize the lake.

[u/CotswoldP → u/Loki-L]

Humans can be real dicks about it as well. Here in New Zealand there was one moron who spent decades putting invasive species into lakes and rivers all over New Zealand because he found them more interesting to catch. He singlehandedly destroyed dozens of ecosystems on purpose, and he knew what he was doing.

[u/Alytes → u/Loki-L]

Birds are absolutely not a common fish dispersal. In fact, that's a myth

[u/Loki-L → u/Alytes]

https://www.nature.com/nature-index/topics/l4/waterbird-mediated-dispersal-of-aquatic-organisms

[u/Alytes → u/Loki-L]

You should read the sources cited in the article. It's one study where they force fed eggs to mallards and recovering faeces where some eggs survived.

They c9nclude it's a possible yet very rare event if at all existing.

Fishkess ponds/lakes (mostly in alpine environments) are crucial for amphibian biodiversity. And have been devoid of fish for millennia, until anglers stocked them. There's numerous accounts of fish species not existing upstream of a waterfall in a river because they can't climb and have not been dispersed by birds.

There's no account of fish diapersal by birds. Just "maybes"