Long-Term Studies on the Population Biology of Diplostomulum scheuringi in a Thermally Altered Reservoir

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Abstract
The prevalence and mean infrapopulation density of Diplostomulum metacercariae were monitored for 53 mo in populations of mosquitofish, Gambusia affinis, located in ambient and thermally altered locations in Par Pond, a 1,012-ha cooling reservoir located on the Savannah River Plant near Aiken, South Carolina. Except for six monthly sampling periods, prevalence of D. scheuringi remained > 75% at the ambient temperature station; peak prevalences occurred in late summer and early fall, then declined in late winter and spring. Mean densities at the ambient station varied significantly over the course of the study; seasonal peaks occurred in late summer and fall, and lowest densities in spring. Seasonal prevalence at the heated site paralleled that which occurred at the ambient station during the initial 28 mo, but over the last 25 mo, prevalence varied erratically. Mean densities of D. scheuringi fluctuated significantly at the heated site during the 53 mo, but a distinct seasonal pattern was not apparent. A significant linear decrease in mean density was observed during the final 30 mo of the study. During 33 of the 53 mo, densities of metacercariae were not significantly different at the two sampling stations. During 17 of the 20 mo when they were significantly different, densities were highest at the ambient temperature site. The decline in prevalence and density at the thermally altered station during the last 30 mo may have resulted from changes in parasite transmission dynamics that were caused by temperatures fluctuating rapidly over wide ranges, or by changes in foraging or nesting behavior of the definitive host, which would reduce the numbers of infective agents being dispersed in the habitat. The prevalence and density of D. scheuringi were compared in four, arbitrarily chosen, size classes of mosquitofish in the two areas of the reservoir over one 12-mo period. Both density and prevalence were affected most in the largest (⩾ 36 mm) and smallest (15-25 mm) size classes in the ambient location during late winter and early spring; prevalence declines among all size classes of mosquitofish in the thermally altered station during winter and spring. The various seasonal changes in parasite population dynamics at both stations were attributed to a combination of seasonal variation in cercarial shedding from infected snails, recruitment of metacercariae, and to seasonal patterns of birth and mortality of the short-lived mosquitofish. At the heated station, periods of cercarial shedding and metacercarial recruitment were altered by thermal effluent that also reduced the normal life expectancy of mosquitofish, thereby further modifying the population biology of the parasite.