Studies on Populations of Helminth Parasites in Brown Trout (Salmo trutta L.)

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Abstract
Six species of helminths occurring commonly in the trout were considered: Discocotyle sagittata and Phyllodistomum simile in the gills and urinary bladder respectively, Dacnitis truttae and Crepidostomum metoecus chiefly in the pyloric caeca region and C. farionis and Neoechinorhynchus rutili mainly in the alimentary canal behind the caeca. There was a tendency for the proportions of N. rutili in the pyloric caeca to decrease with increase in the age of the host. Certain pairs of helminth species were associated to a greater degree than would be expected by chance alone. These positive associations can be explained on the grounds that the infective stages of these helminths show overlapping distributional patterns. Statistically significant negative correlations occurred between other species of helminths, particularly those frequenting the same region of the alimentary canal. The possible causes for these inverse relationships including interspecific competition for food or space and the action of these parasites in adversely affecting the biochemical environment either directly or through the medium of host reaction are discussed. In most cases, however, there were no negative correlations thus giving support of the general contention that interspecific competition between animals is rarely encountered in nature.