Do you know of a dog or person who is allergic to fish?

I strongly suspect that my American Bulldog is allergic to fish. Not just shellfish, but any fish, seems to give him a bad itch and rash. I suspect that even the Omega-3 supplement I've used, which is essentially fish oil, is bad for him. (But a miracle for my pug -- go figure) I'm trying to pinpoint the cause of his skin allergies. Is it relatively common for a dog to have this allergy? I know several have turned up allergic to wheat or corn, but I haven't heard of fish. Has anyone else?