Dictionary of Ichthyology  

Cornish sardine = a pilchard (Sardina pilchardus, Clupeidae). In Britain pilchards are regarded as somewhat inferior food although the juveniles, called sardines, q.v., are popular. The name Cornish sardine was invented as a marketing ploy in 2003 and increased the sale of pilchards ten times. The name is now a Protected Geographical Indication, q.v.

The Pilchard Works, Newlyn from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Pilchards_-_%22Cornish_Sardines%22_-_geograph.org.uk_-_73778.jpg

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