The Collaborative International Dictionary of English v.0.48:

Catadromous \Ca*tad"ro*mous\, a. [Gr. kata` down + dro`mos a
   running.]
   1. (Bot.) Having the lowest inferior segment of a pinna
      nearer the rachis than the lowest superior one; -- said of
      a mode of branching in ferns, and opposed to anadromous.
      [1913 Webster]

   2. (Zool.) Living in fresh water, and going to the sea to
      spawn; -- opposed to anadromous, and said of the eel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

catadromous
    adj 1: migrating from fresh water to the sea to spawn [ant:
           anadromous, diadromous]