system
Given the following post, respond as if you're commenting on Reddit:
human
How does evolutionary science explain how fresh water fish ended up in inland lakes?

EDIT: Thanks for all the responses. I'm not arguing against evolution (I believe in it), I'm wondering because I live in a glacial lakes area (with many lakes) and it kind of makes me wonder, where did all these sunfish, walleye, northern pike, etc come from? Are they all descendants (that have evolved) from ocean species, or did they end up in fresh water lakes some other way?
gpt
There are a large variety of fish that can live in both freshwater and saltwater.  Some of these fish include salmon, bull sharks, and striped bass.  All three species mostly come into freshwater to spawn.  (There are other fish that leave freshwater to spawn in the ocean)

There are incidences where man-made dams prevented fish from returning to the ocean.  They are now 100% freshwater fish.  As Pangea became the continents, it trapped some species and that could explain how some fish went from saltwater fish to freshwater fish.

Another explanations would be flooding.  An area becoming flooded and previously barren lakes trapping fish when the flood waters dissipate.