| Summary: | The trypanorhynch cestode Grillotia angeli Dollfus, 1969, infects mackerel Scomber scombrus L. up to the age of two years and survives thereafter as an encvsted plerocercus for as long as the mackerel lives. In the eastern North Atlantic infection can only take ilace in and to the south of ICES sub-area VII, ie to the south and west of Britain and Ireland. In year classes 1977 and earlier, 13.8% of mackerel which originated from nursery grounds in sub-area VII were infected with -G. angeli; in yLar classes 1978 and later, prevalence decreased to 1.2%. Data from year elasses 1977 and earlier indicate that: (1) mackerel caught off the north coast of Spain (sub-area VIII) and off Portugal (sub-area IX) were of dif ferent nursery origin from those caught further north; (2) mackerel caught in the southern North Sea (Division IVc) were predominately of southwestern origin; (3) to the northwest of Scotland (Division VIa north of 58ON) the proportions of mackerel of southwestern origin decreased from approximately 50% in September to 10% in January-February, then increased to >50% in March-April; (4) approximately 30% of all rnackerel caught in the Norwegian Sea (Division IIa) and the northern North Sea (Division IVa) were of southwestern origin. Brevalence data from different areas suggest that mackerel of year classes 1978 and later from southwestern nursery grounds have been migrating into northern areas in greater numbers than earlier year classes.
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