John, my oldest son (he's 31), has been DEATHLY allergic to fish since the day he was tested for all allergies at the age of 3. If he eats any fresh water fish, his allergic reaction is the same as those allergic to peanuts................. his throat constricts and he simply cannot breathe.
However, my son is also one of THE MOST avid fly fishers I've ever had the good fortune to be associated with! Like me, he never really developed a taste for fish, so it's just that much easier for he and I to be "catch and release" fishermen. (I am NOT allergic to any fish, I just don' t care for the taste of fish, other than sea going types).
Ben handles all his own fish, and always has. The only danger he faces, is if he CONSUMES actual fresh water fish, in any form. Merely handling them and releasing them, does nothing towards his allergic reactions whatsoever.
So, there really is no "big deal" about his allergic reaction to eating fish and his love of fly fishing! There's no difference between his way of enjoying the sport and that of anyone NOT allergic to eating fish.