How do fresh-water fish populate different basins?

[Colibri]

@Pardel-Lux

I wonder if anyone knows how fresh-water fish species populate separated river basins. They cannot take the sea route and they cannot jump the mountains in between, do they? How do they manage? I guess some were introduced by humans, like carps, trouts or koi, but that cannot be the answer for the species that are not bred for human consumption.

Different kinds of freshwater fish have different levels of tolerance to salt water. What are known as “secondary freshwater fish” have some saltwater tolerance and can pass between mouths of separate rivers or cross narrow ocean barriers. Primary freshwater fish have little or no tolerance for saltwater and their dispersal is by other means.

@Darren_Garrison

Tiny eggs sticking to bird legs.

That may happen in a few species, but it is not the general means of dispersal of primary freshwater fish between different basins. In particular, this is not a likely method of transfer for fish that live in mountain streams.

@Chronos

Also, what are separate basins now weren’t necessarily separate basins in the past. Rivers can change their course and even direction, and landforms can change their shape.

The main method of the spread of primary freshwater fish between different river basins is the process of stream capture. In particular, in steep mountain terrain sometimes erosion can cut into the watershed of the next river over, resulting in what’s known as headwater capture. This allows fish to spread between basins, but it’s a pretty slow process. Fish of South American origin have gradually spread northward through the Isthmus of Panama since its closure about 3 million years ago, but most haven’t spread that far even over that length of time.

@Pardel-Lux

Yeah, I read somewhere that the Amazon once started in what is now the Sahara and flowed in the opposite direction into the Pacific, but since then evolution has made the species in the Niger and the Amazon diverge. Those are different species now, I was wondering about the same species.

There’s some geographical confusion here. You’ve gotten the Amazon and Congo mixed up in some fashion. As far as I known the Congo never originated in the Sahara, or was connected to the Niger.

The Amazon on the other hand once did flow into the Pacific. The rise of the Andes redirected it to take its present course to the Atlantic.

Let me ask the question differently: imagine Antarctica melted and got a climate where rivers flowed most of the year. Would fresh-water fish get there? How?

Primary and secondary freshwater fish would be unlikely to colonize Antarctica. It’s just too far.

Would there be separate species in different parts of Antarctica, would they diverge or would they converge? Or would it be more likely that a sea-water species evolved to colonize the new fresh-water habitat? How long might that take?

There are plenty of saltwater fish that have tolerance for freshwater. No doubt some would specialize for the newly available habitat. Some colonization would take place immediately. It might take tens to hundreds of thousands of years before speciation took place.