Definition of "finnock"

Previous to their descent, I caught in all, not far from the tideway, about half-a-dozen finnocks, on two or three different occasions.

1866, Thomas Tod Stoddart, An Angler's Rambles and Angling Songs, page 46

Have you ever had a day's finnock-fishing? If you have not, and you happen to be a keen disciple of Izaak Walton, take my advice and seize the first opportunity of doing so.

1893, J Moray Brown, Stray Sport

Emigrating smolts were captured by fyke net in 1998 and early 1999. […] The returns to date suggest that the sea survival of stocked fish returning as finnock is consistently lower than that of wild smolts and that the survival of both groups to mature sea trout is almost non-existent at present.

2006, Graeme Harris, Nigel Milner, editors, Sea Trout