“I have fish allergies. I’ve experienced anaphylactic shock from tuna and sushi a couple times. But I eat shellfish very frequently and never have a problem. Would krill oil be a safe source of Omega-3s for someone with fish (but not shellfish) allergies?”
Krill is a type of shellfish but there is always a risk of cross-contamination if finfish products are processed in the same facility.
See also: Fish oil vs. krill
That said, fish (and shellfish) allergies are generally triggered by the proteins in the fish and these should be completely absent from any high quality fish or krill oil supplement.
A small study conducted in 2008 at St. Louis University found that fish oil appears to be safe for people with confirmed fish allergies. Even so, I wouldn’t try this at home without an epi pen (and friend) at my side.