After researching shirasu (しらす) I learned it’s a small fish local to Shizuoka prefecture.
This blog on Shizuoka explained everything.
I thought I had eaten shirasu sushi before (at Tsukiji), but turns out it was really shirauo (しらうお).
The picture is shirasu-don.
We may need to add Shizuoka to our Japan trip plans next year.