The Collaborative International Dictionary of English v.0.48:

bladderwrack \bladderwrack\ n.
   1. similar to and found with black rockweed.

   Syn: Ascophyllum nodosum.
        [WordNet 1.5]

   2. common black rockweed used in preparing kelp and as
      manure.

   Syn: black rockweed, bladder fucus, tang, Fucus vesiculosus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bladderwrack
    n 1: similar to and found with black rockweed [syn:
         bladderwrack, Ascophyllum nodosum]
    2: a common rockweed used in preparing kelp and as manure [syn:
       bladderwrack, black rockweed, bladder fucus, tang,
       Fucus vesiculosus]