Parasite fauna of Etheostoma nigrum (Percidae: Etheostomatinae) in localities of varying pollution stress in the St. Lawrence River, Quebec, Canada.

Parasite communities were examined in johnny darters (Etheostoma nigrum) collected from five localities in the St. Lawrence River in southwestern Quebec: two reference localities, one polluted locality upstream of the Island of Montreal and downstream of industrial and agricultural activity, and two polluted localities downstream of the Island of Montreal in the plume from the wastewater treatment facility. Twenty-four helminth species were found. Fish from the upstream polluted locality had the highest parasite species richness and total parasite numbers, and fish from the downstream polluted localities the lowest. Nonmetric multivariate analyses were conducted using square-root-transformed Bray-Curtis dissimilarity index. An analysis of similarity, dendrogram of centroids, and a permutational multivariate analysis of variance with contrasts all showed that fish from the reference localities had different parasite community composition than those from the polluted localities, and fish from the upstream polluted locality had different parasite communities than fish from the downstream polluted localities. Differences between reference and polluted localities were mainly due to higher abundances of the brain-encysting trematode, Ornithodiplostomum sp., at the reference localities. Differences between upstream and downstream polluted localities were mainly due to a higher diversity and abundance of trematodes in fish at the upstream locality.