a. 



I 



M 

 6-1 



O 



O 



.056 r 



,032 

 .027 



♦0195 

 .018 



.010 



7 

 DAYS 



14 



-J 

 50 



Figure 2: -- Time concentration curve of tolerance to predict a TL/a value of DDT 

 sublethal to adult guppies under extended exposure based oh two, 4, 

 7> and 14 day TL/m values. 



in resistance to DDT. The first strain had been 

 shipped from Florida, while the second strain 

 came from a local source. The 14-day TL/m 

 values for the two strains were .018 and slightly 

 more than .032 ppm, respectively. 



Resistance to DDT of fish exposed to 

 sublethal dosages: --The' fish acquired increas- 

 ed resistance to DDT upon exposure to sublethal 

 concentrations. On the basis of the limited 

 number of fish tested (table 2), the fish in con- 

 centrations ranging from .0032 to .018 ppm for 

 two weeks were more resistant to .032 ppm than 

 unexposed fish. This concentration had proven 

 lethal to normal fish of the same strain within 

 three days, but appeared to have little effect on 



Table 2: — SxrrviTal of guppies transferred from 



sublethal concentrations to .032 ppm DDT. 



Previous Number 

 concentration of 



in ppm 



fish 



Percent survival in .032 ppm 

 3 days 7 days IS days 30 days 



the DDT-exposed fish by this time, as some 

 fish from each original concentration were still 

 alive after three days in .032 ppm. Fish from 

 .010 ppm and .0056 ppm were most resistant to 

 the stronger dose of DDT since 80 percent and 

 60 percent, respectively, were still alive after 

 30 days. Twenty-two percent of those originally 

 in .0032 ppm were alive after 30 days, while 

 none from .018 ppm survived this long. 



Thirty-day exposures to sublethal dosages 

 of DDT with some observations on growth and 

 reproduction: - As predicted by extrapolation of 

 a time concentration curve of the 14-day TL/m 

 values, two test series indicated that concen- 

 trations below .018 (.010, .0056, .0018, and 

 .0010 ppm) were sublethal to 50 percent of the 

 guppies, even over a 30-day period of exposure. 

 Changes in weight were irregular and probably 

 insignificant in the two tests. There was no 

 marked loss or gain of average weight of fish in 

 any of the tanks. In the first 30-day test, the 

 total number of deaths in all the tanks was 6 out 

 of 60 or 10 percent and occurred in 4 of the 6 

 tanks including the control. The death of these 

 fish was attributed to the change of environment 

 at the beginning of the experiment rather than 

 from DDT poisoning. 



