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Survivorship and Parasite-induced Host Mortality among Mosquitofish in a Predator-free, North Carolina Cooling Reservoir WILLARD O. GRANATH, JR. t and GERALD W. ESCH Department of Biology, Wake Forest Unioersity, Winston-Salem, N. C. 27109 AssTRncT: Survivorship of mosquitofish infected with the pseudophyllidean tapeworm Bothriocephalus acheilognathi was significantly reduced as compared with uninfected mosquitofish when fish were held at temperatures of 20, 25 and 30 C. Sur- vivorship of infected and uninfected fish was also significantly reduced as water temperatures were increased to 25 and 30 C. When mosqultofish were maintained at 25 C, larger individuals with lower parasite densities survive longer than smaller in- dividuals with higher densities of the cestode. Pearson correlation and multiple regres- sion analyses of survival time of Gambusia affinis on density of Bothriocephalus acheilognathi and total host length indicate that survivorship is a function of parasite density and host size. Certain demographic characteristics of mosquitofish populations in a system devoid of piscivorous predators (but with B, acheilognathi) were compared with those in a system having piscivorous predators (but without B. acheilognathi). Random samples of mosquitofish taken monthly from the two systems revealed no significant differences in either sex ratios or in mean lengths of male or female mosquitofish. The Krumholz (1963) predation hypothesis would predict otherwise, to wit, that sex ratios in the predator-free system should be 1:1 (with predators, sex ratios should be approximately 1 male: 2 females) and that both males and females should be significantly larger than males and females in a system possessing a full complement of piscivorous fishes. It is hypothesized that similarities among demographic features of mosquitofish populations in the two systems investigated in the present study are the result of parasite-induced, size-selective host mortality in the predator-free system on the one hand and of preda- tion pressure from piscivorous fishes on the other. INTRODUCTION The concept of mutual regulatory interaction between host and parasite popula- tions was quantitatively formalized by Crofton (1971 a, 1971b). He asserted that equilibrium levels between host and parasite populations are a function of the density of parasites necessary to kill an individual host, the extent of contagion exhibited by parasites within a host population, and the value of the so-called "achievement factor in relation to the reproductive rate of the host population." Subsequently, Anderson (1978), Anderson and May (1978) and May and Ander- son (1978) modified Crofton's (1971a, 1971b) approach for considering the dynamic aspects of host-parasite interacttons. They identified three factors which contribute to the stability of host-parasite interactions at the population level. These included a clumped distribution of parasites within a host population, density dependence relative to parasite mortality or reproduction, and parasite-induced host mortality which in- creases exponentially with respect to parasite density. A second set of factors was said to have a destabilizing effect on the host-parasite interaction. These included parasite- induced reduction in host fecundity, direct parasite reproduction in the host and time ' Present address: Department of Zoology, University of Oklahoma, Norman 73019. 314