It's quite common actually. It's the presence of cysts just beneth the skin caused by trematode parasites.
http://www.glaucus.org.uk/gunnel.htm has an image of a butterfish with the disease.
Cryptocotyle lingua seems to be the prime suspect as it can encyst on lots and lots of different fish species. The actual worm is only trying to get into sea birds and mammals by firstly living in periwinkles then fish. I'm not an expert by any means but back in uni i'll try to find out for sure. :?
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