The Collaborative International Dictionary of English v.0.48:

Pickerel \Pick"er*el\, n. [Dim. of Pike.] [Written also
   pickerell.]
   1. A young or small pike. [Obs.]
      [1913 Webster]

            Bet [better] is, quoth he, a pike than a pickerel.
                                                  --Chaucer.
      [1913 Webster]

   2. (Zool.)
      (a) Any one of several species of freshwater fishes of the
          genus Esox, esp. the smaller species.
      (b) The glasseye, or wall-eyed pike. See Wall-eye.
          [1913 Webster]

   Note: The federation, or chain, pickerel (Esox reticulatus)
         and the brook pickerel (Esox Americanus) are the most
         common American species. They are used for food, and
         are noted for their voracity. About the Great Lakes the
         pike is called pickerel.
         [1913 Webster]

   Pickerel weed (Bot.), a blue-flowered aquatic plant
      (Pontederia cordata) having large arrow-shaped leaves.
      So called because common in slow-moving waters where
      pickerel are often found.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pickerel weed
    n 1: American plant having spikes of blue flowers and growing in
         shallow water of streams and ponds [syn: pickerelweed,
         pickerel weed, wampee, Pontederia cordata]