Every book I've read about Hokkaido (necessarily in English) has sounded as though it's talking around a few things carefully, as though each writer were trying not to drop/swallow glass marbles while enjoying a bite of a meal. :/ That includes the few written by Ainu--I can see some of the gaps and silences, but not how someone might fill them. It's lovely to have a bit of a glimpse via your post.

May I ask--the mackerel sushi, was it saba, aji, or something else?