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I9B3 GRANATH & ESCH: MOSQUITOFISH SURVIVORSHIP 321 dependent, parasite-induced host mortality is operative for the mosquitofish popula- tion in Belews Lake, we are currently unable to characterize the mortality rate or to compare it to the Net Reproductive Rate (Ro) of the hosts. Increased mortality of in- fected fish in the laboratory does not necessarily mean that it is occurring in the field, nor that it is having a significant impact on mosquitofish populations in Belews Lake. Testing such a hypothesis by direct means was not practicable. Live-box techniques and mark-release-recapture procedures were not feasible in the present situation. An indirect approach was therefore employed for comparing the demographic characteristics of mosquitofish populations in Belews Lake and Charlie's Pond. It was, in effect, also a test of the Krumholz (1963) predation hypothesis. Significant differences were expected in several mosquitofish population characteristics among the various collecting sites, since Gambusia affinis in Charlie's Pond are subject to predation pressure from piscivorous fish while those in Belews Lake are not. Krumholz (1963) examined the relationship between predation and demographic characteristics of G. manni in the Bahamas and found that in a predator- free system, male G. manni at sexual maturity were larger and the sex ratio was 1:1 as compared to a system having predators where both males and females were smaller and the male/female ratio was about 1:2. Krumholz believed that the overall survivorship of G. manni increased in the predator-free system and that males increased in abun- dance in the absence of selective predation on this sex. Subsequently, Sohn (1977) ex- perimentally demonstrated that the size at maturity of male G. manni is directly related to the presence of adult males in the population and that maturation is delayed until a juvenile exceeds the adult male in size. Thus, the evidence presented by both Krumholz (1963) and Sohn (1977) suggests that predation is a prime factor in TwaI,E 6. -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 Date Ambient site X t sE Charlie's Pond X t sE Thermally altered site X~ sE F (df) 3/81 28.0 t 1.4 31.6 t 1.2 30.6 t 2.2 4.73"s (2, 58) 4/81 28.7 f 0.8 29.2 t 2.2 30.2 t 4.2 3.36"s (2,73) 5/81 27.1 t 6.5 32.3 f 1.3 30.3 t 2.8 5.37^' (2,62) 6/81 30.2 t 0.9 21.8 t 3.8 23.7 t 1.7 60.03 * (2,80) 7/81 19.7 f 1.7 23.7 f 1.8 25.4 t 1.9 15.52* (2,72) 8/81 25.1 t 1.7 23.1 t 2.7 22.1 t 3.9 2.69^s (2,58) 9/81 25.8 t 1.7 29.3 f 3.1 25.9 ~ 2.3 4.26"s (2, 76) 10/81 25.9 t 2.4 26.7 t 3.2 27.2 f 2.4 0.88"' (2,84) 11/81 30.2 f 1.3 28.8 f 2.4 29.3 t 2.0 1.74^s (2,83) 12/81 27.2 t 3.2 28.2 t 3.7 30.0 t 1.6 4.50"s (2,101) 1/82 26.7 t 2.8 29.9 t 3.5 29.6 t 1.6 5.23"' (2,96) 2/82 27.8 t 3.0 29.6 t 2.2 29.2 t 1.9 1.70"s (2, 76) F 12.88* 18.04* 17.36* (df) (11,346) (11,306) (11,329) * -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