You are twisting what I am saying to make them appear in your mind as ramblings. The salmon did not always swim upstream to spawn, this behaviour developed over millions and millions of years. There must have been a point when the salmon only swam halfway upstream "to spawn." Since I omitted the purpose of the upstream swim and you could not divine this purpose since it was so covertly hidden, I mention it now. The salmon in question is not just out for an afternoon swim, it's on a mission.
Why did the chicken cross the road?
What was the moon before it was our moon?
Suppose you wish to walk across the room. First, of course, you must cover half the distance. Then, you must cover half of the remaining distance. Then you must again cover half of the remaining distance ... and so on ad infinitum. Logically, then, it is impossible for you to walk across the room, or even to move at all.