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322 THE AMERICAN MIDLAND NATURALIST 110(2) regulating the population dynamics of G. manni and that, in the absence of predation, several demographic features of the fish populations were altered. Comparison of sex ratios and size differences of mosquitofish in Belews Lake and Charlie's Pond does not conform to the Krumholz (1963) predation hypothesis. In ac- tuality, the demographic features of mosquitofish in Belews Lake are quite similar to those of Gambusia affinis in Charlie's Pond where piscine predators occur. It appears that despite the loss of piscine predators from Belews Lake, certain of the demographic characteristics of the mosquitofish population have remained relatively unchanged, in marked contrast to what occurred in similar, previously described systems (Krumholz, 1963). We believe that the cestode, Bothriocephalus acheilognathi, has "buffered" the Belews Lake system by removal of a portion (or all) of the hosts which would have otherwise fallen prey to piscine predators. This contention is supported in part by laboratory observations which showed that infected fish died before noninfected fish, thus establishing a direct relationship be- tween parasitism and host mortality. In addition, when mosquitofish harboring only nonsegmented worms were exposed to temperatures which stimulated cestode growth and development, the smaller, more heavily infected individuals died before larger and lightly infected mosquitofish. Since male Gambusia affinis do not grow once they reach sexual maturity (Krumholz, 1948), these individuals would be most susceptible to parasite-induced host mortality. Thus, the ratio of one male to two females at the predator-free site can be explained by differential, sex-related host mortality. Finally, such asex-specific - or perhaps more precisely asize-specific -relationship may help to explain the lower densities of Bothriocebhalus acheilognathi in larger fish (Granath and Esch, 1983b). It has also been shown that larger G. affinis feed .less extensively on copepod intermediate hosts (Granath and Esch, 1983a; M. R. Riggs, pers. comm.). Such foraging behavior would contribute to limiting densities of the cestode in larger hosts which, in turn, would also reduce the probability of parasite-induced mortality in larger fish. The present study, and those of Granath and Esch (1983a, 1983b), indicate that several factors combine to control the population dynamics of Bothriocephalus " acheilognathi and Gambusia affinis in Belews Lake. Thus, temperature along with host feeding behavior controls the time when the cestode will be transmitted and when it will develop to an egg-shedding adult. However, as development proceeds, intraspecific competition may limit the number of gravid worms which can be supported within a given infrapopulation. Moreover, the survival of infected fish, regardless of water temperature, is significantly reduced (increased mortality of infected fish at lower temperatures cannot be currently explained). Increasing water temperature stimulates growth and development of the cestode and, in turn, determines the composition of the infrapopulation. Since parasite-induced mortality is also a function of host size, male mosquitofish are presumably more vulnerable to mortality caused by B. acheilognathi than females. It is therefore proposed that temperature-dependent growth and develop- ment, intraspecific competition, a contagiously distributed parasite population and size-related, parasite-induced host mortality all combine to effectively regulate parasite S population density and composition while simultaneously manipulating certain of the demographic features of the mosquitofish populations in Belews Lake. Ackrzowledgments. - We want to thank Mike Riggs for his generous assistance with some of the statistical analyses and Mike Riggs and Dennis Lemly for their lively discussions regarding the biology of Belews Lake. This research was supported in part by a grant from the Wake Forest University Research and Publication Fund. LITERATURE CITED ANDERSON, R. M. 1978. The regulation of host growth by parasitic species. Parasitology, 76:119-157.