The use of parasites as biological tags in population studies of herring (Clupea harengus L.) in the North Sea and to the north and west of Scotland

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Abstract
The metacercariae of the digeneans Cercaria pythionike Rothschild and Cercaria doricha Rothschild and the plerocerus of the cestode Lacistorhynchus sp. were used to trace recruitment migrations of herring (Clupea harengus L.). The two digeneans have similar life cycles, with the gastropod mollusc Turritella communis Lamarck as first intermediate host, teleost fish as second intermediate hosts, and piscivorous sea birds as definitive hosts. Lacistorhynchus has calanoid copepods as first intermediate hosts, teleost fish as second intermediate hosts, and elasmobranch fish as definitive hosts. All three parasite species occur in the visceral cavities of herring, mainly around the pyloric caeca. Most, if not all, infection of herring with the digeneans occurs in the first year of life but infection with Lacistorhynchus can occur in older herring. All three parasite species have long life spans, probably equal to those of the infected herring. The Northeast Atlantic study area extended from Shetland in the north to the Irish Sea and central North Sea in the south, and from northwest of Ireland in the west to the Kattegat in the east. More than 23 000 herring, aged from 0 to 12 winter rings, and predominantly autumn-spawned, were examined from 1973 to 1982. Examinations of juvenile herring caught on nursery grounds showed that each parasite species infected herring only in certain areas. Identification of the infective area or areas for each species enabled conclusions to be drawn as to the probable origin of different adult herring populations. It was estimated that herring of eastern North Sea-Skagerrak-Kattegat origin in the Minch increased from an estimated 55% of the 2-ring population to 85% of age groups 5 rings and older. Most of the remainder were probably recruited from inshore nursery grounds along the east coast of Scotland and possibly the northeast coast of England. There is evidence that a large proportion of the adult herring caught to the northwest of Ireland were recruited from nursery grounds in the Minch. Adult herring caught in the area to the north and west of the Outer Hebrides had parasite prevalences closest to those in juvenile herring from the eastern North Sea north of 56°N and in the Skagerrak and Kattegat. Parasite prevalences indicated the eastern North Sea-Skagerrak-Kattegat area as the source of recruits to adult herring populations in the northwestern North Sea apart from a component, estimated at 20%, of the 2-ring herring caught in the Orkney—Fair Isle—Clyth Ness area. This component probably recruited from nursery grounds along the east coast of Scotland and possibly the northeast coast of England. There was evidence of an interchange of autumn-spawned herring between populations in the Firth of Clyde and the Irish Sea.

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