How far?  Depends on species:  Snake River Sockeye go 900 miles inland from the Pacific to spawn.  Yukon River King salmon much further than that (2,300 miles).  Eels travel as far as 2,500 +++ miles to spawn.  So, this question has hundreds if not thousands of answers depending on the species and the system.  Spawning is about migration. Migration is about purpose and movement.  In short, every living creature, plant, insect, animal, fungi…virtually every living organism “migrates” for one purpose or another.

alewives?

Quite a ways.  In our case Stetson Pond @ 105+++ river miles.  As far as river herring go that’s quite a journey.  Put into perspective:  I weigh 250 pounds, a big alewife weighs 250-350g.  Less than half a pound.  So I weigh 500X more than an alewife.  Bio-energetics.  Would I be capable of travelling 500X as far ( 52,500 miles or twice around the earth and then some) without stopping, eating, and on foot?  No Jets, no cars or boats,  just personal motivation.

So is it reasonable to assume that if the alewives got access to lake in Dexter or Great Moose lake would they be able to get that far and get back?
Great Moose and Wasookeag were historically were accessible as were big and little Indian Ponds.  So, yes!

Nate Gray