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?
In: 7
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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