The plerocercus of Grillotia erinaceus as a biological tag for haddock Melanogrammus aeglefinus in the north sea and north-east Atlantic

Analyses of the incidence and intensity of infestation with Grillotia erinaceus plerocerci in 8228 Melanogrammus aeglefinus indicate two distinct groups of haddock at Faroe, and three groups in the area to the north and west of Scotland. In the North Sea the following four groups were recognized: (1) in the Moray Firth and (2) to the north and east of Shetland where, in both groups, incidence of infestation was consistently high; (3) an intervening area where incidence was considerably lower up to three years of age, after which it increased to about the same level as in groups (1) and (2); and (4) in the central North Sea where incidence was consistently low. Changes in the pattern of infestation with host age and with host length within single age-groups, when considered together with conventional tagging data, suggest that many 3+ and older haddock may migrate from Scottish inshore waters via the Shetland area to the north-eastern North Sea. The results support other studies in suggesting a northward movement of haddock along the Scottish east coast.