| Infection rates of Discocotyle sagittata on juvenile Atlantic salmon (Salmo salar) were assessed in relation to a mine site leaching metals (zinc, copper) into the Northwest Miramichi River (New Brunswick, Canada). Discocotyle sagittata infection rates were significantly reduced at sites downstream of the mine compared to reference and upstream sites, suggesting that metal effluent was responsible for the parasite's absence. This decrease in parasite prevalence could not be attributed to any other biological or physical variables (fish community, salmon population structure, depth, current, flow regime and substrate type), and no other biological effects were observed at sites downstream of the mine. Subsequent experimental work indicated that although D. sagittata hatching and development were unaffected except at high exposure levels, the survival and fecundity of adult parasites were significantly reduced at zinc and copper exposure levels similar to those at field sites downstream of the mine. These field and laboratory observations indicated that D. sagittata was highly sensitive to metals. In an effort to assess the susceptibility of monogeneans relative to other major parasite taxa, these results were incorporated into a larger meta-analysis of standardized effects sizes (as Cohen's d) of responses of each taxon to selected types of pollution (pesticides, hydrocarbons, PCBs, pulp effluent, metals, municipal sewage, eutrophication, and acidification). Several taxa were generally not affected by pollution,while others were negatively (digeneans, monogeneans) or positively (microparasites) affected. Digeneans and monogeneans were the most sensitive parasite taxa, while metals, eutrophication and acidification elicited the largest effect sizes among pollutant types. Further meta-analysis indicated that the monoxenous taxa are more susceptible than the heteroxenous, and that ectoparasites and the free-living transmission stages are more susceptible than endoparasites. Furthermore, studies conducted in freshwater environments yield more consistent results than marine-based studies. The thesis concluded with a discussion of the implication of these results for the use of parasites as biological indicators of pollution. Although a number of knowledge gaps, technical challenges and philosophical hurdles remain, the development of sensitive parasite taxa such as monogeneans and digeneans as biological indicators of pollution in aquatic ecosystems shows considerable promise. |
