Long-term population dynamics of Diphyllobothrium ditremum and D. dendriticum (Cestoda: Pseudophyllidea) in their salmonid hosts following a fish removal experiment

Abstract

Parasite-host interactions are important in understanding ecosystem processes, for instance by using trophically transmitted parasites as indicators of host ecology. Parasite infections are expected to correlate with host density, habitat choice and feeding behaviour. The importance of long-term data and perturbation experiments has been emphasized for advancing our knowledge of the parasite-host relationship. Over a five-year period in the 1980s a mass removal of fish was implemented on a stunted population of Arctic charr (Salvelinus alpinus (L.)) in subarctic lake Takvatn. This culling of fish can be seen as a large ecological experiment, and the effects have been monitored with long-term analysis on the fish populations of the lake. Here, I study the long-term effects of this fish removal experiment on two component populations of the tapeworms Diphyllobothrium ditremum and D. dendriticum infecting Arctic charr. Infections are also compared between charr and brown trout (Salmo trutta (L.)). The infections of both Diphyllobothrium species in Arctic charr decreased after the fish removal, reflecting a shift in habitat and diet from pelagic zooplanktivory to littoral benthivory in the charr population, and a reduced abundance of copepods in the zooplankton community. The decrease was more drastic in the D. dendriticum population suggesting altered transmission rates to the avian final hosts. Infections of both parasite species oscillated between years, likely as a result of cycles in the population density of three-spined stickleback (Gasterosteus aculeatus (L.)). Arctic charr had higher infections of D. ditremum than brown trout, while the opposite was true for D. dendriticum infections. This suggest niche segregation between the two fish species with charr grazing more on zooplankton and trout more on fish, as D. dendriticum has been described as an indicator of piscivory. The study has highlighted how parasite infections respond to a large perturbation, and how sensitive they are to variation in ecological processes.