I’ve not found any studies whatsoever on instability of DHA/EPA at low temperatures, only marked structural changes (including unhealthy ones) at high temperatures, including prolonged temperature exposures.
Frankly, the mere question alone would prompt my own investigation if I had a laboratory since I’ve never cooked fish in that manner. My only concern is the lengthy cook time. Poaching achieves the same result with, IMHO, minimal loss. Likewise with low temperature baking (filets, rather than steaks) until ‘just done’.
But those results are solely anecdotal based upon my personal experience: The years-long course of health recovery mandated that I become a literal lab rat, one result being repeatable with tangible health benefits via consumption as-described vs. supplements & prior ignorant methods.
A quick perusal of the Googleverse (did I just coin ANOTHER word?!) indicates no adverse nutritional effects from flash freezing.
When I was a teenager, my buddy and I would trek down to Punta San Miguel near Ensenada, Mexico, where we would gather driftwood to build a fire among the large rocks lining the shore. Then we would head out into the kelp beds to spear our meal. Preparation consisted of gutting the fish at the water’s edge followed by a quick salt water rinse, then onto the spear shaft and over the fire. Our fingers were our utensils. Fresher fish you will never find.
Baring that experience, I always buy flash frozen fish from the store freezers, NEVER thawed from the display. (Who knows how long it’s been sitting there losing its freshness?)
Most of the time we thaw the fish in our very cold refrigerator right before we cook it, but with sous vide, you can toss it on frozen and add another 15 minutes to the 30 minutes required for thawed fish.
We also have been known to bread and fry the cod according to a recipe from the 70s we found when preparing abalone. We did switch panko for breadcrumbs, though.