PREVALENCE AND ABUNDANCE OF HELMINTH-PARASITES IN AN INTENSIVELY FISHED POPULATION OF BROOK TROUT (SALVELINUS-FONTINALIS) AT A SMALL SUB-ARCTIC LAKE

Brook trout from a 5-ha lake near Schefferville, Quebec, were intensively sampled by small-mesh gill nets during 2 successive years to study patterns of helminth parasite occurrence in relation to changes in the age and size class structure of the fish population. Nearly 1800 brook trout were removed from the lake, of which all were measured and weighed, 568 were aged by otolith readings, and 361 were examined for parasites. The digeneans Crepidostomum farionis, Phyllodistomum umblae, and Diplostomum sp. and the cestode Eubothrium salvelini were present in more than 50% of the necropsied fish, and the prevalence and (or) abundance of all species except C. farionis increased from the first to the second year of the study. The fish catch in the first year was dominated by the 3 + year class and older fish, whereas by the second year 1 + and 2 + fish were proportionately more abundant, and the importance of older cohorts declined. This change in brook trout population structure, resulting from gill net selectivity, appears to have had the effect of improving per capita food availability for members of the residual stock. After the initial year of intensive fishing, the fish that remained fed more upon both benthic and planktonic invertebrates and their acquisition of parasites was intensified.