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1983 GRANATH & ESCH: MOSQUITOFISH SURVIVORSHIP 315 delays in the development of transmission stages of the parasite. In most natural com- munities, they argued, host.-parasite populations are maintained at equilibrium den- sities through a balanced interaction of these stabilizing and destabilizing forces. In- deed, May and Anderson (1979) later stated that "parasites (broadly defined) are prob- ably at least as important as the more usually studied predators. and insect parasitoids in regulating natural populations." One of the problems in supporting the contentions of May and Anderson (1979) is that there are too few laboratory and field studies which combine to focus on the dynamics of a single host-parasite system. Most studies have been in the field or "the laboratory, but not both. Recently completed investigations (Granath and Esch, 1983a, 1983b) on several aspects of the population biology of B~othriocephalus acheilognathi in the mosquitofish Gambusia affinis provide an excellent opportunity for a combination of laboratory and field experiments to determine the potential for mutual regulatory in- teraction for this host-parasite system. Specifically, the present study was designed to assess the effect of B. acheilog;nathi on the mortality and survivor;>hip of mosquitofish and to evaluate the impact of the parasite on certain demographic features of the mos- quitofish population. The Bothriocephalus acheingnathi-Gambusia affinis system offered several distinct ad- vantages for such an approach. First, the life cycle of the parasite is relatively simple, with only two hosts required for completion. Second, the mosquitofish is relatively short-lived, surviving a maximum of 15-18 months under normal conditions. Third, the habitat in which the study was conducted is free of piscivorous fish species which meant that predation was not important in regulating the dynalics of the mosquitofish population. And, finally, since Gambusia affinis is a hardy species, laboratory manipula- tion was easy. MATERIALS AND METHODS Study area. -Belews Lake is a 1563-ha cooling reservoir located in the northern Piedmont area of North Carolina. It was constructed in 1970 and was designed to pro- vide cooling water for two 1 ]140-Mw turbine generators. Cooling water is drawn from the epilimnion of the heated arm and discharged into the W arm at 5-10 C above am- bient temperature. Water ri°turns to the main arm via a cormecting canal. Surface temperature in most of the main arm is ambient. A complete description of the study areas is given by Granath and Esch (1983a, 1983b). Some of the intake water :is used to transport fly ash to a 142-ha settling basin. After particulates have settled, effluent water is returned to the rese°rvoir via a controlling discharge facility. This feature of the steam station operations has created serious prob- lems for biota in the main a~°m (excluding the headwater areas which have remained normal) and heated locations of the lake. Thus, in 1975 the standing crop for all species of fish was 90 kg ha '; during the present study, it was virtually nil. In 1975, 29 species of fish were present, quite representative of what would be considered as typical for other impoundments in the area. Currently, only five species of~ fish remain. These in- clude mosquitofish Gambusia affinis, a fathead minnow Pimephcsles promelas, red shiner Notropis lutrensis, a few carp Cyprinus car~iio and a few channel caitfish Ictalurus punctatus. In effect, the main arm and heated locations of Belews Lake were free of piscivorous fishes during the course of the study. The cause of the collapse of the fishery was the accumulation in the lake of selenium from the fly ash basin. Bio~magnification of selenium through the food chain was ultimately responsible for the: decline in fish production (Cumbie, 1978). Immediately adjacent to 1:he reservoir is a small pond (Charlie's Pond) of approx- imately 4 ha. It possesses a typical diversity of fish species, with largemouth bass Micropterus salmoides and green sunfish Lelbomis cyanellus as the top predators. Gambusia affinis is also present. Charlie's Pond was originally part of Belews Lake; it became per- manently, and completely, separated when a service road was constructed. Because of