From what I have read, there are a lot of assumptions about where they spawn. They are assuming they return to the sargosso sea because they saw some elvers out there and the only explanation on how they got there is that they must be spawing there.
As far as I know eels have never been seen spawning.
Frankly it is hard for me to belive that A) eels swim all the way back to the SS and actually make it alive and B) the elvers drift from the SS along and are distributed along the coast without being consumed. The story is hard for me to accept esp since no one has witnessed spawing in the SS. Is it possible? Sure, I would never believe a herring makes a run back to its birthplace either but they in fact do. But what makes it hard for me to believe is that most baitfish spawn in shallow waters (ponds, bays etc) not open ocean. When was the last time you saw a school of elvers offshore swimming for beach? You find them in streams and ponds.
I think given the right conditions (which I probably will never get in my pond) they should spawn. My hope is that eventually, over time they will get horny enough and give it a go.
