ELI5: How do fish and other pond creatures end up in ponds in the first place, without being connected to the ocean?
Aquatic biologist here:
There are a number of ways.
First, fish rarely get to ponds from the ocean. Ponds are usually freshwater, the ocean is usually saltwater, and few animals can live in both. There are important exceptions in coastal areas, but generally speaking, the animals in ponds get there from other ponds or streams or freshwater areas.
How do they get there? Well, lots of people will tell you eggs and things stick to bird legs, but I’ve looked in the scientific literature and have never found a documented example of this for fish. It does, however, definitely happen with snails, small crustaceans, various other invertebrates and protozoans, and aquatic plants. I’m personally pretty dubious it’s a major method of fish transportation. Fish eggs are generally not very tough and dry out easily. I also don’t know of fish eggs that can survive passage through a bird digestive tract, although again this is much more plausible for various invertebrates with tougher eggs.
There are occasional examples of fish getting caught up in tornadoes and windstorms and being transported away from water, but I’m also skeptical this is a major method of fish transport.
Another thing I’m skeptical of is fish transport by ospreys or other birds picking up fish and dropping them in a new pond. Birds of course pick up fish and drop them all the time, but flying all the way to a new body of water, and the fish surviving the experience, is another story. And then it has to happen twice, because most fish use external fertilization. Females don’t carry fertilized eggs, so a male and female would both have to be dropped in the same pond within a few years.
So what are some other methods? One big one is that the fish just swim there. Most ponds, even those without obvious outflows, overflow during flood events. Fish can move between ponds at these times. Particularly in flat areas, the whole landscape can be covered in water, allowing for fish dispersal. This was even more true before humans put dams everywhere.
Over longer timespans, the landscape of the earth changes in ways that let fish move between bodies of water. Stream capture lets fish move from one watershed to another as the stream they live in shifts its outflow. During the past ice age, enormous glacial lakes allowed fish to disperse, and areas that are currently dry were sometimes much wetter.
Sometimes pond animals walk (or fly, if bird or insect) to the pond. A handful of fish do this, but obviously this is an important option for birds, insects, frogs, and salamanders. Indeed many ponds do lack fish and only have insects, frogs, and salamanders. This is especially true for seasonal ponds called vernal pools. Mountain ponds and lakes, and lakes in recently glaciated areas like Maine, also often lack (or historically lacked) fish and were important habitat for amphibians.
These examples are also important to bring up because they illustrate that fish don’t always get into isolated bodies of water. This is one reason I don’t think the “fish eggs on legs” thing happens often…you would see a lot fewer bodies of water that naturally lack fish in that case. Instead you’d find fish everywhere the way you find snails and small crustaceans all over the place.
So what’s the real explanation for fish in these ponds?
I’ve gone this whole way and mostly talked about how fish don’t get to ponds. The bit about floods and stream capture is an important way for fish to get to ponds, for sure, but I expect that a different explanation accounts for most of the ponds that people in this thread are probably thinking about.
That explanation is people. People are absolutely crazy about sticking fish in every available body of water from horse troughs on up. Caught bluegill and bass in a farm pond? People almost certainly put them there. Saw goldfish in a pond in the city? People? Caught trout in some isolated lake way up in the mountains? People. In the old days people packed baby trout up into the mountains in old metal milk jugs carried by mules to stick them in all those previously fishless lakes. These days we just drop them from airplanes. Ponds are regularly stocked with bluegill and bass (and often minnows and catfish). Most common fish that people fish for have been widely introduced outside their native range. Brown trout and carp in N. America are all introduced. Rainbow trout away from the Northern Pacific coastline are all introduced. Bass and bluegill outside of the eastern half of the USA are all introduced. Goldfish outside of northeast Asia are all introduced. If you went fishing for a fish in freshwater, there’s a very good chance some person put its ancestor in that body of water (even in areas where the fish are native! For example, rainbow trout are widely stocked all across their native range in CA).
Wow this is really interesting. You definitely seem to know your stuff. Thanks alot for taking the time to help!
Yup, Buddy in Alaska has a pond WAAAAAAY up a mountain. I asked how the fish could have got there.
“Oh Frank brought up a few trout 20 years ago, now we can fish for trout here.”
Incredible answer. That video of the fish free falling out of that airplane was exactly what I needed today.
i know OP already thanked you but i wanted to say ditto; i was hoping there’d be a longer, genuinely informative comment. guess that zoology class stuck with me if i found this an entertaining read lol
Your extensive knowledge of the matter is less impressive than your storytelling skills : that was an awesome read ! You managed to write that much content, and yet everything in there is ELI5. Thank you.
Personal stocking of ponds. My brother-in-law made a deal with the DEC in NYS that if he allowed them to build a dike on his farm NYS would give him a stocking license. He went to Lake Ontario and caught large mouth bass, sun fish, crappie, and perch so the bass had food. He also caught some bullhead and stocked the pond with them. It was a sportsman’s heaven there. I fished it with friends for years. We had to throw all catches back but man, if you wanted to stroke your ego, this was the place to go.
Could the muddy boots of a farmer spread tadpoles or whatever all over his farm and ultimately into various ponds?
Birds like ducks fly from body of water to body of water and while they swim fisheggs can get stuck on their feet. So when they land in another lake they bring fish babies with them!
Humans do the the same thing intentionally with eggs or small live fish so you can fish in the lake/pond
In addition, nearly all bodies of water are being fed by some sort of stream/creek/river and they flow out into another stream/creek/river etc that makes a chain to the ocean. After enough time life finds it’s way along that chain up to the top.
Really, this is the top comment? Hands down, flowing water is the biggest way fish get from place to place. The main source, outside the normal means like rivers, is flooding.
But there would need to be a minimum number of eggs to create a viable population, wouldn’t there? A couple wouldn’t be enough?
Ranked by Size