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318 TItE AMERICAN MIDLAND NATURALIST 11 ~(2) Sion analyses of survival time of Gambusia affinis on density of B. acheilognathi and total host length confirm the observation that survivorship is a function of parasite density and host size (Table 2). Comparison of mosquitofish populations in Belews Lake and Charlie's Pond. -Data were ob- tained for sex ratios and mean lengths of females and males from an ambient temperature and a thermally altered site in predator-free Belews Lake and in Charlie's Pond where mosquitofish are subject to predation pressure from piscivorous fishes (Tables 3, 4 and 5). Sex ratios varied significantly within season in both Belews Lake and in Charlie's Pond. There were, however, no significant differences between sites during the same seasons (Tables 3, 4, 5). Analysis of variance indicated that mean lengths of female mosquitofish varied significantly by season (Table 5). The smallest monthly mean was observed at the am- bient temperature location in Belews Lake during July. Smallest mean lengths of females at the heated station and in Charlie's Pond were observed in June. Duncan's multiple range test revealed significant, but small, variations in mean length of females at the three stations, mostly during spring and early summer. In only 2 months, May and June, were mean lengths different in the ambient area of Belews Lake and TABLE 1. -Relationship between mortality, mean parasite density and host size at 25 C, a temperature which is conducive to cestode development Days at Number of Density of parasite Total length of host (mtn) 25 C dead fish X f sE (range) X t sE (range) 1-6 0 7 4 13.5 t 5.6 (6-28) 26.2 t 3.3 (22-30) 8 4 12.3 f 6.9 (4-24) 23.5 f 2.7 (21-28) 9 0 10 22 16.2 t 6.1 (8-33) 26.4 t 2.7 (22-31) 11 20 9.6 t 2.6 (6-18) 26.2 f 2.3 (23-30) 12 9 7.6 t 2.7 (2-13) 29.4 t 1.7 (27-35) 13 7 4.1 f 1.9 (1-7) 32.1 f 3.4 (28-37) 14 7 4.6 t 3.3 (2-12) 33.9 t 1.5 (32-36) 15 6 3.2 t 1.9 (2-7) 35.0 t 1.1 (34-37) 16 0 17 3 3.3 t 1.2 (2-5) 38.0 t 2.9 (34-41) 18 0 19 0 20 1 4.0 39.0 TABLE 2. -Pearson correlation and multiple regression analyses of survival time of mos- quitofish on the density of Bothriocephalus acheilognathi and the total length of the host Survival Time (days) Parasite density Host length r P r P r P Survival time (days) - - - 0.661 <0.001 0.811 <0.001 Parasite density - 0.661 <0.001 - 0.516 <0.001 Host length 0.811 <0.001 - 0.516 <0.001 - - Regression model: survival time =parasite density x host length r~ = 0.738, P <0.001 (parasite density contributes 43% and host length contributes 30.8% to this value)