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Fish host–cestode parasite stable isotope enrichment patterns in marine, estuarine and freshwater fishes from Northern Canada

Cestode parasites from freshwater (threespine stickleback, Gasterosteus aculeatus), estuarine (brook charr, Salvelinus fontinalis) and marine (Greenland cod, Gadus ogac) fish from northern Québec, Canada, were used to investigate the hypotheses that cestode parasites are 13C and 15N enriched relative to host food sources, but 15N depleted with respect to host muscle tissue as a result of differential enrichment during the assimilation of common nutrient sources. Cestode parasites and fish were generally similarly enriched in 13C with respect to common food sources and, in the case of Greenland cod, cestode parasites were 13C enriched relative to host tissue. Cestode parasites were also generally 15N enriched with respect to mean host dietary signatures, but depleted with respect to host muscle tissue. In the case of Greenland cod cestode parasites, no significant 15N enrichment relative to host dietary signature was observed. Cestode parasites appear generally to experience smaller 15N enrichment than hosts as a result of trophic transfer of common dietary sources. Differential 15N enrichment patterns in parasites and fish may be attributed to differences in parasite and host metabolism, particularly the anaerobic and aerobic natures of their respective metabolisms. Results imply that isotope enrichment paradigms developed for the study of aquatic foodwebs cannot be routinely applied to quantitatively assess the role of parasites in aquatic foodwebs and that reference to host muscle tissue measures will not allow accurate characterization of parasite foodweb position. Appropriate reference to assimilated food sources is required to accurately estimate parasite isotopic enrichment patterns and to determine parasite trophic position relative to the host.