The Collaborative International Dictionary of English v.0.48:

Yellowfin \Yel"low*fin`\, n. (Zool.)
   A large squeteague.
   [1913 Webster]

WordNet (r) 3.0 (2006):

yellowfin
    n 1: may reach 400 pounds; worldwide in tropics [syn:
         yellowfin, yellowfin tuna, Thunnus albacares]