How common are fish allergies?

About 1 in 200. A 2004 study by scott sicherer et al pegged an overall rate of seafood allergy in the us at a little over 2% - that's one in 50. Most of that allergy was reported for shellfish. The rate of fish allergy was 0.6% - less than 1 in a 100 and closer to 1 in 200 persons.