I'm allergic to something

1

I noticed that whenever I eat sushi, I get rashes / hives on some parts of my skin. I confirmed this last night and surely enough I was right. Except I don’t know what exactly I’m allergic to. I ate all kinds of fish made in all kinds of ways. How do I find out? I’d hate to go to a sushi restaurant only to order one kind of fish and eat and wait for a few hours, and repeat.