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1983 GRANATH & ESCH: MOSQUITOFISH SURVIVORSHIP 319 Charlie's Pond. In May they were higher in Charlie's Pond and in June they were higher in Belews Lake. The pattern of seasonal change in mean length of male mosquitofish at each of the three stations paralleled that of females (Table 6). There were thus significant seasonal changes within each site, but length differences between sampling locations varied only slightly. Shortest males were observed at the ambient station in July, and at the other two sites in June. DISCUSSION When naturally infected and uninfected mosquitofish are held in the laboratory, under controlled conditions, infected fish do not survive as well as uninfected fish. Sur- TABLE 3.-Comparison of sex ratios (male/female) of mosquitofish collected from Charlie's Pond and the ambient site from March 1981 to February 1982 Charlie's Ambient XZ Date Pond site 3/81 0.40 0.32 0.0"' 4/81 0.46 0.36 0.0"' 5/81 0.71 0.30 4.8"$ 6/81 0.67 0.76 0.0"' 7/81 0.54 0.36 1.0"s 8/81 0.46 0.43 0.0^g 9/81 0.76 1.00 0.4"s 10/81 1.00 0.67 1.2^° 11/81 0.82 0.76 0.0^s 12/81 0.89 1.00 0.0M19 1/82 0.93 1.40 0~4"' 2/82 0.76 0.93 0.4"s X2 (df= 11) 29.4* 39.6* * -significant at P<0.005 ns -not significant at P> 0.005 TABLE 4. -Comparison of sex ratios (male/fe male) of mosquitofish collec ted from the ther- mally altered site and Charlie's Pond from March 1980 to February 1981 Thermally altered Charlie's Date site Pond X~ 3/80 0.39 0.32 0.2 4/80 0.50 0.58 0.0 5/80 0.30 0.62 3.2 6/80 0.46 0.67 1.0 7/80 0.15 0.09 0.6 8/80 0.15 0.11 0.2 9/80 0.25 0.22 0.0 10/80 0.40 0.25 1.0 11/80 0.54 0.39 0.6 12/80 0.32 0.82 5.2 1/81 0.25 0.93 10.5 2/81 0.46 0.43 0.0 X~(df= 11) 19.2* 55.2* * -significant at P < 0.005 ns -not significant at P < 0.005