Discoloration of Porgy's backend

Discoloration on the backend of this porgy had us questioning what type of fish it was for a brief moment. Although, the question still arises - why the discoloration? Fungus? Parasite? Pigmentation issue? Whatever it is, we thought it was interesting enough to share. What are your thoughts?

Видео Discoloration of Porgy's backend канала City of Deerfield Beach