I’m with you on this. Found some tasty water? Swim towards it. It gets tastier the further we go? Keep going.

So how do they find the river outlet into the ocean? There surely is some bird compass thing involved. I am only half joking when I write that Venus guides them.

That nature works at all is astonishing.

It's all chemoception, the same as with single cell organisms. They swim towards a saline gradient ( which they can taste, for sure ) and follow it up into fresher and fresher water.

Word gets around? Animals probably have way better communication than we think. One crab says to a friendly eel "hey dont tell those damn Salmon but this estuary is good again" and before you know it, everyone's favourite restaurant is booked out till March.

I used to go to this estuary until it became too crowded.

Yeah, no need to make this complicated.

Thanks, I was bang my head on this one, until you suggested a nice simple solution