Green’s Dictionary of Slang

a herring; when smoked it is known as a Digby duck.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1897		N. Platte Semi-wkly Trib. (NE) 14 May 3/3: ‘Digby chickens,’ ‘Glasgow magistrates,’ ‘Gourock hams,’ ‘Dunbar wethers,’ ‘Norfolk capons’ all meaning red herrings.
1901		Minneapolis Jrnl (MN) 13 Apr. 8/3: A herring is called in different localities in England a ‘Digby chicken,’ a ‘Norfolk capon,’ a ‘Dunbar wether,’ or a ‘Gourock ham’.
1947		Exeter & Plymouth Gaz. 24 Dec. 4/2: Herrings served as ‘Norfolk capons,’ ‘Digby chickens,’ ‘Dunbar wethers,’ or ‘Gowrock hams’.