Estimates of mackerel stock composition in different areas and reasons as indicated by a cestone parasite

Recently collected data support earlier indications of the value of the plerocercus of the trypanorhynch oestone Grillotia angeli as a biological tag for estimating the size of the Western stock component in populations of adult mackerel, Scomber scombrus L., of mixed stock composition. During 1981, three samples of mackerel were examined from the northern North Sea, one from the Norwegian Sea and one from off the north coast of Scotland. The proportion of Western stock mackerel in each of these samples was estimated by comparing the prevalence rate of G[?]angeli in each sample with the mean prevalence rate in mackerel samples from the western English Channel. The results indicate that some mackerel samples of Western stock origin do not return to Western stock spawning grounds but remain in the North Sea to spawn. The sample form the Norwegian Sea was estimated to consist of about 90 % North Sea stock, a result contrasts strongly with calculation of stock composition in ICES Division Ila based on returns of “mechanical” tags