My understanding is that ALA converts at a 15% efficiency into DHA or EPA.
I researched it in depth years ago and came to the conclusion that 3-5% was average, higher among females for reasons which still elude my understanding, but far from approaching 15%.
Admittedly I haven’t looked at it since, but I doubt the reality is much different. Food marketing is rife with blatant fraud.
And those numbers were among the healthy, meaning the real numbers are FAR lower, given the state of health of people in this country these days.
Worse, most people destroy omega 3 in fish through improper cooking.
I moved to freshwater trout after the Tohoku quake for reasons which shouldn’t need elaboration.