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206 . PROCEEDINGS OF THE HELMINTHOLOGlCAL SOCIETY unusually large range of definitive hosts, having been reported from more than 40 species. An understanding of the population dynamics of many host-parasite systems is frequently clouded because of predation pressure exerted on the definitive host. However, Belews Lake offers a unique opportunity for studying the population biology of Bothriocephalus acheilognathi because piscivorous fishes are absent from the main body of the reservoir. The reservoir was constructed in 1970, and by 1975 had developed a fish community that could be considered as typical for this section of the U.S. Within 3 yr, however, virtually the entire assemblage of fish species, including all the piscivorous fishes, had been eliminated from the main body of the reservoir because of the accumulation oflethallevels of selenium from a fly-ash settling basin (Cumbie and Van Horn, 1978). The presence of an introduced cestode in a thermally altered aquatic ecosystem, devoid of piscivorous fishes in all but the headwaters of the lake, offered an excellent opportunity to develop and answer several questions. First, we wanted to compare the seasonal dynamics of B, acheilognathi in mosquitofish from am- bient and thermally altered areas of the reservoir in order to evaluate the impact of artificially elevated temperatures. Second, we were interested in determining which factors are responsible for regulating the infrapopulation densities of B, acheilognathi. Finally, we wanted to determine if B, acheilognathi could be a regulatory factor in the population biology of mosquitofish; Hoffman (1980) in- dicated that the parasite could have significant negative affects on cyprinids grown as bait fish under hatchery conditions, The present report will focus on the first of the three questions. The other two aspects of the study will be considered elsewhere (Granath and Esch, 1983a, b). ., , Materials and Methods STUDY AREA: Belews Lake (Fig. 1) is a 1,563-ha cooling reservoir located in the northern Piedmont of North Carolina. Constructed by Duke Power Company in 1970, the reservoir was designed to provide cooling water for two 1, 140-megawatt turbine generators for the Belews Creek Steam Station. Cooling water is drawn from the epilimnion of the main body and subsequently discharged at 5 to 100C above ambient temperature into the west arm of the reservoir. Water is returned to the main bOdy of the lake via a connecting canal. Some of the intake water is used to transport bottom and fly ash from the boilers and precipitators to a 142- ha settling basin. Effluent water from the basin is returned to the reservoir via a controlled-discharge facility (Weiss and Anderson, 1978). As alluded to earlier, the fish community developed rapidly and normally over the first 5 yr of operation. In 1975, the community consisted of29 species (Harrell et aI., 1978); currently, only five species remain in the main arm. These are mosquitofish (Gambusia affinis), carp (Cyprinus carpio), channel catfish (Ictalurus punctatus), fathead minnow (Pimephales promelas), and red shiner (Notropis lu- trensis). Currently, the dominant species in the reservoir are G, affinis, p, promelas, and N. lutrensis, The success of these species can be attributed to their obvious tolerance to selenium toxicity and the absence of piscivorous fishes. COLLECTING PROCEDURES: Collections of G, affinis were made with a dip net during the first and third full weeks of each month, beginning at the end of March 1980 and continuing through the beginning of March 1982, Mosquitofish were h 1t