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1983 GRANATH & ESCH: MOSQUITOFISH SURVIVORSHIP 317 Definitions. - In this paper, mean density refers to the total number of parasites in a sample divided by the total number of infected and uninfected fish in a sample. An in- frapopulation is defined as the total number of Bothriocephalus acheilognathi, at any stage of development, within a single host (for a complete discussion of this concept, see Esch et al., 1975). Mean infrapopulation density refers to the total number of parasites in a sample divlded by the number of infected hosts in the sample RESULTS Survivorship of infected and uninfected mosquitofish. -The first set of studies was under- taken to assess the impact of Bothriocephalus acheilognathi on the survivorship of mos- quitofish under a variety of temperature regimes. At temperatures of 20, 25 and 30 C, survivorship of infected mosquitofish (Fig. 1) was significantly reduced as compared to uninfected individuals held at the same temperatures (Breslow and Mantel-Cox tests, P < 0.001). At 20 C, maximum survivorship of infected Gambusia affinis was reduced by approximately 15% (Fig. la), at 25 C, by 27% (Fig. 1b) and at 30 C, by 55% (Fig. lc). Moreover, survivorship of both infected and uninfected G. affinis declined precipitously with increasing temperature. Host mortality, as a function of parasite density and host size, was measured in the laboratory at 25 C. As has been previously shown (Granath and Esch, 1983b), this temperature will stimulate growth and development of nonsegmented Bothriocephalus acheilognathi. Larger fish with lower parasite densities survive longer than smaller fish with high densities of the parasite (Table 1). Pearson correlation and multiple regres- B C y O fd LL O W m 2 oars Fig. 1. -Survivorship curves for infected and uninfected mosquitofish maintained in the laboratory at 20 C (A), 25 C (B) and 30 C (C)