Threshold density levels were investigated experimentally in rainbow trout, Oncorhynchus mykiss, for the acanthocephalan Pomphorhynchus laevis. Fifty small fish 120–150 mm long were infected with 50 cystacanths each every 4 weeks for 16 weeks, and samples of up to 10 fish examined at 1, 5, 9, 13 and 17 weeks after the original infection. The mean intestinal length was found to have increased significantly over this period, from a mean of 125 to 173 mm and the mean number of parasites recovered increased after each interval of 4 weeks, and significantly from 19·30 to 86·40 per fish over the 17 weeks. The distribution of the parasites in the alimentary tract of O. mykiss was unaltered, indicating that the favourable region for establishment had remained unchanged. The failure to detect any manifestations of competition is discussed, and the relevance of conventional approaches to experimental investigations of both intra- and interspecific competition in fish was queried, and contrasted to the situation in birds and mammals.
Pathological Changes of Rainbow Trout (Oncorhynchus Mykiss Walbaum, 1792) Intestines Infected with Pomphorhynchus Laevis
