RESPONSE OF MOSOUITOFISH TO THERMAL EFFLUENT 507 



Size at sexual maturity and sex ratios were generally similar 

 among all populations of mosquitofish. Few females 25 mm in length 

 contained embryos at PC2, PCS, and PP. One 23-mm female at PCI 

 was pregnant. The smallest reproductively active female from Risher 

 Pond was 33 mm, but the smallest male was 24 mm. Males from 

 other populations sampled were sexually developed at 21 mm. Sex 

 ratios for the study period were also similar; female-to-male ratios 

 were approximately 1.6 at PC2, PC3, RP, and PP, and the sex ratio at 

 PCI was 2. 



DISCUSSION 



Temperature is reportedly the primary exogenous factor regulat- 

 ing reproduction of mosquitofish (Medlen, 1951). Medlen indicated 

 that the minimum critical temperature for reproduction is about 

 16°C. On the basis of our results, however, there does not appear to 

 be an upper critical temperature threshold after which embryo 

 formation is curtailed. We found that mosquitofish collected from 

 water at 40°C were reproductively active, and Ferens and Murphy 

 (1974) reported that mosquitofish collected from 32 to 44°C water 

 were also reproductively active. Our data indicated, however, that 

 mean number of embryos was generally inversely related to water 

 temperature in areas subjected to extremely high water temperatures 

 (PClandPC2) (Table 3). 



Several investigators reported a linear relationship between the 

 number of eyed embryos and body length (Krumholz, 1948; Ferens 

 and Murphy, 1974). Our data support these studies; graphically, the 

 relationship (Fig. 3a) suggests that females from thermally unaf- 

 fected areas have a higher number of embryos per length than those 

 from affected areas. The regression coefficients were not statistically 

 different between affected and unaffected populations, however, 

 because of the variability in number of embryos per female. Ferens 

 and Murphy also found no significant difference in the number of 

 eyed embryos as a function of length between thermally affected and 

 unaffected mosquitofish populations. 



Brood size of mosquitofish varies widely among populations 

 (Krumholz, 1948). The highest mean number of embryos found in 

 this study was 27.9 at PCI. Self (1940) found an average of 56.7 

 embryos per female in Oklahoma, and Krumholz reported a range in 

 average brood size of 5.3 to 68.3 in Illinois. A mosquitofish 

 population sampled in Tennessee averaged 33.2 embryos per female 

 (Blaylock, 1969). We do not feel that differences in fecundity were 



