Oh! many are the Fin-Backs, and many are the Dericks, my friend.
As an isolated quote, this won't make sense. In Context, a Fin-Black is a whale sometimes mistaken for a sperm whale which isn't easily catchable. Derick seems to be an unskilled captain, in part because he recently had to beg for lamp oil from the Pequod. Here the inexperienced captain is chasing a whale he won't be able to catch.
