I don’t know if it has anything to do with oxidation at all vs the quality of fish you get from the average takeout place.
- The block of fish is sitting there exposed to oxygen for hours sometimes even days at the top sushi places. There is no vacuum sealing of fish that I’ve seen anywhere.
- Some fish are deliberately aged to enhance flavor or texture.
http://www.sushiyasuda.com/restaurant.html
" While Sushi Yasuda brings in fish from all around
the world and much from Japan, the chefs highly value outstanding local
and regional varieties. Mitsu and Tatsu select the fish one by one,
evaluating each for freshness, size and its “spirit” or “energy.” They
then carefully begin the comprehensive process of cleaning, preparing
and storing. Paper-thin sheets of delicate Japanese cedar-wood line many
of the boxes, and different varieties of fish are stored in separate
boxes. The chefs carefully control the aging process of the fish—an
essential part of making sushi. “Just-caught” fish is not always ideal for being eaten immediately as sushi, and different fish require different methods of refrigeration and storage for ideal preservation and taste."