The population dynamics of the monogenean gill parasiteDiscocotyle sagittataLeuckart on Windermere trout,Salmo trutta, L.

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Abstract
Relatively little is known of the way in which the populations of monogenean parasites are related to the behaviour and physiology of their hosts or how such populations vary in composition over the year. Bychowsky (1957, trans. Hargis, 1961, p. 108) has described the results of studies upon six different fish hosts and parasites, while Llewellyn (1962) has studied two other species of Monogenea living on one host species. In all these investigations, the various ecological factors which influence the life cycles of the parasites have been determined.Dogiel, Petrushevski & Polyanski (1961, p. 14) referred to a previous population study ofDiscocotyle sagittataon a hostCoregonus lavaretusin Lake Ladoga, Russia. The parasite was said to breed in July and August, while in September broods of young and old parasites overlapped. The large individuals disappeared in October and November and this ‘death of the older generation’ was taken to indicate that the parasites had a life span of only one year.The only relevant work available when the present study was commenced was the population study of the monogeneansGastrocotyle trachuriandPseudaxine trachuriby Llewellyn and as this concerned a marine host, it was of obvious interest to investigate the population dynamics of a parasite of a freshwater fish.Discocotyle sagittatawas chosen because the biology of the British host, the brown trout (a different host from that in the Russian study ofDiscocotyle), has been intensively studied for many years. It was thought that this would be of advantage in attempting to relate any seasonal fluctuations in the parasite population to the biology of the host.