It is a bit strange to think of it as a study of fish at first sight; I was thinking about cooking fish for a second.
study of fish/fish science = ichthyology (English) = ictiologia (Portuguese), however I'm sure people don't say that terminology in daily life either.
@Paulenrique, sorry, I'm confused by your reply ... what did you meant? Did you mean:
if I want to say fish is what I can cook better, I say "Peixe é minha especialidade" without "a"?
if I want to say I'm specialised in fish science, I say "Peixe é a minha especialidade" with "a"?
But it doesn't make sense to me that the article will a difference though... Thanks. 10/03/2017