The thing is, its pretty hard to pull of sashimis or others with cheap fish. Youre gonna get frozen food no matter where you go, where just too far off pacific salmon or tuna to get it fresh. You should be able to recognize sushi grade fishes with a little bit of self education.
Dont eat cod, haddock and hake or any fresh water fish as there a lot of chances of parasites in them. But anyway, they shouldnt serve this... I believe.
Salmon belly is salmon belly, youre not gonna get the super buttery taste of some, but still...
Coincidentally enough, I asked my dad who is a chef, earlier this week why you don't see tartars using white fish and he said that basically.