Abstract

Pollution can have significant effects on the parasite communities and behaviour of freshwater fish. This study compares the helminth communities and predator avoidance behaviour of johnny darters ( Etheostoma nigrum ) collected from two reference localities and three polluted localities in the St. Lawrence River in southwestern Quebec, Canada. Both reference localities and one locality impacted with industrial and agricultural contaminants were located upstream of Montreal; two sites impacted with municipal sewage and urban effluent were located downstream of the city. Overall, darters were infected with 24 species of helminths, 16 of which were larval stages. Fish from the upstream polluted locality (industrial and agricultural) had a higher mean species richness than the two reference localities, which had higher richness than the two downstream localities (sewage). Fish from the upstream and reference localities had higher total parasite numbers than the two downstream localities. A non-parametric, permutational multivariate ANOVA (PERMANOVA) using Bray-Curtis dissimilarities between communities of individual fish revealed that the parasite communities differed by locality, pollution status of locality and type of pollution (upstream versus downstream). A capture time experiment and a flight distance experiment were performed, to test the effects of parasitism and pollution on susceptibility to predation. A PERMANOVA demonstrated that abundance of a brain-encysting parasite, Ornithodiplostomum sp. and locality explained capture time, but failed to detect a relationship between pollution status and antipredator behaviour. Fish with high intensity infections of the brain encysting parasite, Ornithodiplostomum sp., were more difficult to capture, reflecting increased activity of infected individuals. Abundances of Ornithodiplsotomum sp. metacercariae were greater at reference localities than at impacted ones, possibly reflecting sensitivity to pollution of transmission stages or snail hosts of this parasite. Pollution may have an indirect effect on johnny darter antipredator behaviour, by decreasing abundance of a behaviour-modifying parasite at polluted localities.