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OF WASHINGTON, VOLUME 50, NUMBER 2, JULY 1983 ' 207 \. ~ L'!.J ,oCJ l.J SCALE: FEET -"- , ell a ~ ~ ~ ~ ~I~ ~i~ Figure 1. Map of Belews Lake showing main collecting sites in ambient (M 1) and thermally altered (HA 1) areas. J collected from the ambient (M 1) and thermally altered (HA 1) areas of the lake (Fig. 1), Each sample was comprised of 60 mosquitofish, with 20 fish in each of three arbitrarily established size classes (:::; 30 mm, 31-40 mm, and::::: 41 mm). The fish were transported to the laboratory, where they were weighed, measured for total and standard lengths, sexed, and necropsied with the aid of a binocular dissecting microscope. During necropsy, the numbers of worms were counted. In order to estimate the prevalence and density of B, acheilognathi in the natural population of mosquitofish, the following computations were made, The per- centage of a randomly collected sample (N = 60) of fish in each of the three size classes was determined for each monthly collection (these fish represented a col- lection different from the fish examined for parasites), This percentage was mul- tiplied by the mean density or prevalence of the parasite in the size class, By summing the results for all three size classes, and repeating the procedure for each monthly sample at each collecting site, an estimate ofthe overall prevalence and density of the parasite in the natural population of mosquito fish was determined. Estimates of prevalence and density were computed because there was no a priori indication that the entire Belews Lake mosquito fish population consisted equally of individuals in each of the three size classes, or that prevalence and density are the same in all size classes all year long, Recruitment of B. acheilognathi byG, affinis was monitored monthly from March 1980 through February 1982. Plankton were collected monthly in the littoral zone at sites M 1 and HA 1 with several (20-30) horizontal tows ofa #20- mesh Wisconsin plankton net. The plankton were returned live to the laboratory, where they were placed into 4-liter aquaria containing uninfected G, affinis; water temperatures were maintained at 220C. Four days after the fish were exposed to