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32O THE AMERICAN MIDLAND NATURALIST 110(2) vivorship of both infected and uninfected mosquitofish declined significantly when water temperatures were raised to 25 or 30 C. Moreover, at higher temperatures, in- fected fish died sooner than uninfected mosquitofish. Laboratory experiments also demonstrated that mortality caused by Bothriocephalus acheilognalhi is a function of parasite density and host size. Thus, when Gambusia affinis harboring only nonsegmented worms were exposed to temperatures which stimulated parasite growth and development, smaller, heavily infected hosts died before more lightly infected, but larger, fish. It would appear that as water temperature in Belews Lake increases in spring, not only would some cestodes be eliminated by intraspecific competition, as suggested by Granath and Esch (1983b), but more heavily infected hosts would also be killed. This may also partly explain why the density of B. acheilognalhi decreased during spring when water temperatures rose and then increased in autumn as water temperature fell (at a time when maturation of the cestode could not occur). Anderson and May (1978) and May and Anderson (1978) proposed that a series of factors are involved in creating conditions which will promote mutual regulatory in- teractions between host and parasite populations. Among these are 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 faster than linearly with parasite density. In the present case, we know that Bothriocephalus acheilognalhi is contagiously distributed within Belews Lake mosquitofish. A previous study (Granath and Esch, 1983b) suggests that parasite mortality is density- dependent, possibly as a consequence of intraspecific exploitative competition. Finally, although our experimental results strongly suggest that a mechanism of density- TABLE 5. -Comparison of the length (mm) of female mosquitofish collected from the ambient site, Charlie's Pond, and the thermally altered site, from March 1981 to February 1982 Ambient Charlie's Thermally site Pond altered site Date X t sE X t sE X f sE F (df) 3/81 31.7 t 6.0 33.2 t 4.2 38.8 t 3.1 11.54* (2,116) 4/81 35.114.2 33.1 f 5.6 39.410.8 8.22* (2,101) 5/81 35.3 t 5.0 39.2 t 2.1 43.8 t 1.8 13.37* (2,112) 6/81 37.2 t 4.5 21.3 f 5.6 23.3 t 3.5 126.60* (2,94) 7/81 19.6 t 2.6 26.4 f 4.4 22.5 t 2.3 4.38"s (2,102) 8/81 26.0 t 4.6 25.7 f 7.4 24.8 t 4.7 0.09"' (2,116) 9/81 29.7 t 5.7 29.4 t 6.6 29.0 t 3.3 0.02"s (2 98) 10/81 27.4f 2.7 24.7 f 4.0 31.015.4 5.53"' (2,90) 11/81 31.414.7 29.2 f 4.0 24.915.4 4.86"s (2,91) 12/81 25.0 t 4.5 28.4 t 6.0 33.3 f 4.9 2.58tt9 (2, 73) 1/82 32.0 t 8.5 33.8 t 6.0 35.0 f 6.1 1.67"s (2 78) 2/82 30.8 f 6.7 29.7 t 6.6 36.6 t 4.1 13.39* (2,98) F 35.68* 20.75* 47.71* (df) (11,350) (11,390) (11,350) * -significant at P < 0.005 ns -not significant at P > 0.005 Underscored means are not significantly different as determined by Duncan's multiple range test