Why does any Archbishop get to decide if alligator is okay for Lent?

NPR ran an interesting piece about [the saga of whether or not Catholics could eat alligator on Fridays during Lent](http://www.npr.org/blogs/thesalt/2013/03/27/175058833/forget-fish-fridays-in-louisiana-gator-is-on-the-lenten-menu) . According to the story, an alligator merchant by the name of Jim Piculas advocated in a letter to [Archibishop Gregory Aymond](http://www.arch-no.org/aymond) for permission to consider alligator as seafood, and thus exempt from the prohibition on Friday meat consumption. Now, aside from the giggles on the matter, what authority does the Archbishop of New Orleans have to set out the orthopractic model of Catholic belief in New Orleans? I understand that the Bishop of Rome has the authority, when speaking ex cathedra, to promulugate doctrine - but I've never heard that other Bishops have this right. In declaring a reptile equivalent to a fish, the Archbishop seems to have made a fairly significant theological interpretation. What gives him the authority to do so?