I’ve never gotten the full explication on this. I hear animals bring the eggs, esp birds but that doesn’t really make sense to me. Don’t the eggs need to be fertilized in the water?
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>I’ve never gotten the full explication on this. I hear animals bring the eggs, esp birds but that doesn’t really make sense to me. Don’t the eggs need to be fertilized in the water?
Sure – but then it’s not like waterfowl specifically pick out only the *un*fertilized eggs to stick to their bodies 😉
Many remote lakes lakes were artificially stocked with fish years ago and they flourished, while some lakes have to have continued efforts to keep them stocked with fry or fingerlings over year from hatcheries.
Most lakes have natural tributaries or small streams, brooks or creeks that flow freshwater into them, along with fish.
I have seen fish dropped by prey birds. I bet that some fish populations started this way. Many bodies of water are also connected at one point or another even if its just during flooding events.
Through rivers and stream, during floods, sometimes they’re specifically stocked, people release pets that get too big for aquariums, birds may snatch & drop them…
The eggs are eaten by galliforms, ducks, swans, geese etc. and pooped out. It was thought that they could not survive the stomach acid however this has now been proven to not be the case.
Also fish that give birth to live young are less likely to occur in newly created bodies of water.
https://www.smithsonianmag.com/smart-news/these-fish-eggs-can-travel-through-swans-digestive-system-and-emerge-intact-180972426/
Pumpkins evolved along with mastodons and mammoths. Their seeds can survive the pachyderm’s digestive tract and get pooped out in a warm blanket of fertilizer.
If humans hadn’t cultivated pumpkins when the big guys went extinct pumpkins would have gone extinct, too. They aren’t really found in the wild anymore.
With that in mind, it’s not hard to imagine a bird eats some fertilized fish eggs and dive bombs the next lake on it’s annual migration.
People will talk all about fish eggs being spread on birds and things, but really a _huge_ amount of it is people. People just love to put fish in isolated bodies of water, from farm ponds to mountain lakes. Isolated bodies of water are quite often fish-free until people put fish in there, for example many mountain lakes and streams were known to have been historically fish-free. If you see trout or bass or sunfish in a pond, it was probably people who put them there. And they’ll go to huge efforts to do it. In the old days they’d lug trout in on mule-back in buckets. Nowdays sometimes they drop them from airplanes.
The other big part of it is that fish move around during flood and high water events. Many bodies of water that _seem_ isolated are in fact connected to others during high rain and runoff events.
I’m sure the occasional bird-transported (fertilized) egg or fish does happen, but if it was common, fish-free ponds would have been less common in the historical record of areas before people showed up.
In the modern day, and for a lot of human history? People. People like to fish, and they like to have fish in their local body of water. So they go to a body of water that has fish, they catch some, and they bring them to another body of water and put them in.
In modern times, there are fish farms that exist almost entirely to restock lakes and rivers with popular fish in order to attract tourists.
There are some ways this can happen naturally as well though.
For one thing, very few bodies of water are completely isolated. If they were, they’d usually dry up, or turn into a salt lake. Many lakes have temporary streams in and out during the wet season. This can be enough for fish to swim into them.
Another is birds. There’s speculation that some birds can accidentally carry fish eggs on their feathers or even pick up whole adult fish they intend to eat, and then land or drop them into an isolated lake. This probably doesn’t happen very often, but it’s a legitimate theory of how some fish can move from lake to lake.
In some species of fish (mostly ones that live in areas with seasonal lakes) the eggs can actually survive being dried out. So the fish lay the eggs, the water dries up and the eggs dry out. When the eggs get wet again, they hatch.
While humans may be responsible for a lot of fish migration today, they can’t be the *only* explanation. The same species of freshwater fish are often found on both sides of a continental divide, are present in the fossil record long before humans came on the scene, and since these fish can’t tolerate seawater, they couldn’t have arrived by sea. So whether the body of water is
isolated from the ocean or not, fish can apparently [migrate from one river system to another](https://www.nps.gov/yell/learn/nature/yellowstone-cutthroat-trout.htm) without human help.
Maybe birds might carry eggs, but this hasn’t been observed, and if they carried eggs between continental watersheds, they ought to carry those eggs to islands far from the continents too. But the only native freshwater species in [places like Hawaii](https://hilo.hawaii.edu/campuscenter/hohonu/volumes/documents/Vol02x20ColonizationofFishintoFreshwaterStreamsinHawaii.pdf) are those that live part of their lives at sea, so bird transport, if possible, isn’t easy.
To my mind the most interesting explanation involves **geology**. Stream channels shift over time. In particular, if one river erodes the land faster than another, it can cut into the other’s watershed and *steal its tributaries*. This is called [“stream capture”](https://en.wikipedia.org/wiki/Stream_capture) or “stream piracy”. When this happens, any fish that used to live in the old tributary now find themselves connected to a new river. These shifting connections between rivers can lead to species [migration](https://www.frontiersin.org/articles/10.3389/fgene.2017.00199/full), [isolation](https://www.frontiersin.org/articles/10.3389/fevo.2021.788328/full), and [re-connection](https://en.wikipedia.org/wiki/Stream_capture#Australian_freshwater_fish).
Several other factors can let fish get into surprising places. First, very few bodies of water are entirely isolated from a river network: usually there’s an intermittent stream during a rainy season or flood that provides a temporary bridge. Mountain passes that connect watersheds together are often [rather marshy places](https://www.usgs.gov/observatories/yvo/news/two-ocean-pass-place-where-fish-can-swim-over-continental-divide) where the spring meltwater stagnates before “picking a side”, wet enough for aquatic species to pass through. And finally, just because a lake is isolated at the top of a massive waterfall now, doesn’t mean it always was. Waterfalls evolve quickly and don’t last very long, on the timescale of geology and species. (Not to mention there are [fish that can climb 200-foot waterfalls.](https://www.youtube.com/watch?v=4bJ6jG8-5Xs))
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