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<br />212 <br /> <br />, PROCEEDINGS OF THE HELMINTHOLOGICAL SOCIETY <br /> <br />40 <br /> <br />WATER TEMPERATURE (OC) <br /> <br />30 <br />20 <br /> <br /> <br />10 <br /> <br />80 <br /> <br />ESTIMATED <br />PREVALENCE (%) <br /> <br />I <br /> <br />100 <br /> <br />60 <br /> <br /> <br />40 <br /> <br />20 <br />o <br /> <br />':~ <br /> <br />ESTIMATED X DENSITY <br /> <br /> <br />I <br />J J <br />1981 <br /> <br />I I I I I <br />SONDJF <br />1982 <br /> <br />I I I I I I I <br />M A J J A SON D J F <br />1980 <br /> <br />Figure 3. Monthly changes in temperature, and estimated prevalence and density of Bothrioceph- <br />alus acheilognathi within Gambusia affinis from the ambient site (M 1). The horizontal bar indicates <br />when recruitment of the cestode occurred. <br /> <br />thermally altered site are given in Figure 5, The estimates suggest a spring increase <br />in prevalence during the first year, but not in the second, Lowest prevalences were <br />observed in the summer of both years, with rises during the fall of both years. <br />Estimated changes in densities of B, acheilognathi appeared to follow the pattern <br />seen for prevalences. Thus, lowest densities occurred in the summer of both years, <br />with a rise in the spring of the first year, but not the second. <br /> <br />Discussion <br /> <br />Seasonal changes in the dynamics of most helminth parasites in aquatic eco- <br />systems may be affected by a wide range of factors, of both biotic and abiotic <br />origin (Chubb, 1980). Perhaps the most significant are normal temperature vari- <br />ations (Aho et aI., 1982; Camp et aI., 1982; reviewed by Esch, 1983) and factors <br />associated with host reproductive and foraging strategies (Smith, 1973; Anderson, <br />1976; reviewed by Esch, 1983). <br />In both ambient and thermal locations, lowest densities and prevalence of B, <br />acheilognathi were observed when water temperatures were highest, whereas peak <br />densities and prevalences were recorded as water temperatures declined, especially <br />during late fall. Reproduction by G, affinis in the ambient areas of Belews Lake <br />was seasonal, beginning in April and continuing through October of both years, <br />In the heated location, eyed embryos (indicating reproductive activity) were pres- <br />ent in all months of the year. Because of continuous host reproductive activity <br />in both areas during the summer months, some of the decline in density and <br />