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TABLE 1.-Regression statistics for stress studies. m = slope; b = Y•intercept; T = PeaTum product-moment correlation coefficient; F, = sample statistic of F distribution; P = le:.el ,.t "!A significance for test of relationship between dependent and independent variable. Regresu,,., comparisons = tests of difference between two analogous regression lines (e.g., tab sympairu „ lab allopatric ovary weight). • = statistically significant relationship, Pvramriers _ _ m b r f, P romps,;,,,,,, LAB L Sympatric (n=)2) Somatic weight .042 -.800 .910 4810 <,001' P<,05- Ovary weight .002 °,094 3x9 1.2 <15 2>P>.I Fertility .512 --6.75 .416 2.1 <25 P<.01• 11. Allopatric (n=8) Somatic weight .087 -2.05 968 88.7 <.001+ Ovary weight 025 -.687 .820 12.4 <.025' - Fertility 1.90 -50.6 .809 11.4 <025• - LAB 11 L Sympatric (n=10) Somatic weight .086 -1.88 .983 226.1 <001• P>.I Ovary weight .002 -0.05 .452 2.1 <25 P<.05• Fertility .178 -4.44 .359 1.2 <.50 P<05• 11. Allopatric (n=9) Somatic weight .097 -2.14 .994 572.4 <001' - Ovary weight .014 -.366 .771 10.3 <025• - Fertility 1.07 -23.4 .829 15,4 <.01• - FIELD I. Sympatric (n=20) Somatic weight .102 -2.46 .939 140.3 <.001* P>.9 Ovary weight .035 -.979 .863 52.3 <.001• P>.9 Fertility 2.53 -62.8 .896 32.1 <.001' P<05• U. Allopatric (n=20) Somatic weight .060 -1.15 .938 133.0 <001• - Ovary weight .009 -.145 .425 4.0 .05<P<.1 - Fertility 1.38 -27.0 .822 15.4 <.005• - could not be demonstrated in nature, this may reflect the artifact of topminnows unable to escape harassment in the laboratory. The observation that P. occidentalis stops feeding in the presence of G. affinis, retreats to a hiding place, and dies (Schoenherr, 1981), was not supported here. In laboratory aquaria, little species segregation was found, and topminnows always fed despite frequent attacks from mosquitofish. Surface and underwater field observations confirmed the lack of segregation, anti quantitative (Schoenherr, 1981) and qualitative (Meffe, unpubl. data) gut content analyses argue against cessation of feeding. However, a stres-S response demonstrated in the laboratory indicates at least a potential for contributing to species replacement in spatially restricted habitats in nature, such as small, isolated pools. Further support for the predation hypothesis is provided by qualitative observations at Bylas. This formerly "pure topminnow" locality was colonized by mosquitofish in winter 1978-79, and numbers of the native poeciliid declined precipitously over the next two years (Meffe et al., 1983; Meffe, 1983a). In March 1982, with topminnows nearly extirpated, a live stock of P occidentalis wa d d h 1,8 1:6 1.4 1.2 I O w 0.8 3 0.6 0.4 0,2 B 30 2 y 2 J 1 W LL 1 i s remove an t e spring was poisoned with the 26 28 30 32 34 36 38 40 26 28 30 32 34 36 38 40 STANDARD LENGTH (mm) Fie. 4.-Results of laboratory stress analysis if. Symbols as in Figure 3.