Survivorship and Parasite-induced Host Mortality among Mosquitofish in a Predator-free, North Carolina Cooling Reservoir

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. Survivorship of infected and uninfected fish was also significantly reduced as water temperatures were increased to 25 and 30 C. When mosquitofish were maintained at 25 C, larger individuals with lower parasite densities survive longer than smaller individuals with higher densities of the cestode. Pearson correlation and multiple regression 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 predation pressure from piscivorous fishes on the other.