diet is considered desirable for best growth 

 and breeding. The fish were fed all they could 

 eat within a five minute period in the mornings, 

 and excess food was removed to prevent clouding 

 of the water. The fish were acclimatized to 

 laboratory conditions for a minimum of 7 days 

 before toxicity tests. No guppies were taJcen 

 from tanks where losses were occurring. 



To facilitate exchange of gases such as 

 loss of carbon dioxide and uptake of oxygen at 

 the surface, an electric air pump operated con- 

 stantly, allowing air to enter each tank by means 

 of glass tubing. The bubbling, though sufficient 

 to keep the water slowly circulating, was 

 limited to less than eight bubbles a minute in 

 order to prevent spraying of water and conse- 

 quent loss of DDT onto the glass covers and 

 sides of the tanks. The allotted volume of 

 water per fish was 700 cc/guppy. To prevent 

 competition for oxygen, Turner (1937), Farris 

 (1950), and Silliman and Gutsell(1958) recom- 

 mended about 9 adult guppies per 6 liters or 

 660 cc/fish. The frequent removal of wastes 

 was imperative as the presence of wastes in 

 the water would increase the carbon dioxide 

 level and chlorinated hydrocarbons are likely to 

 adsorb to organic material. Therefore, two 

 liters of water were siphoned from each bio- 

 assay test aquarium and filtered weekly. 



One to 14 -day TL/m determinations for 

 adult guppies: -- To determine what concen- 

 trations of DDT to use in the two week test 

 series, a wide range of concentrations -- 1.0, 

 .56, .18, and .10 ppm (milligrams per liter) 

 DDT were tested for a 24 -hour period by placing 

 two fish in 1400 cc of each concentration in wide 

 mouth jars of 3-1/2 liter capacity. As one of the 

 two survived in the . 10 ppm for 24 hours and none 

 survived the other concentrations, . 10 ppm was 

 the highest concentration tested in the 2 -week 

 tests . 



Ten fish has been considered an adequate 

 number for bioassays by Hart_et al_. , (1945) and 

 10 guppies, 6 female and 4 male, were used. 

 EXiplicate samples of 5 fish were placed into 

 3-1/2 liter duplicate samples of DDT solutions, 

 in museum jars 5-3/4" high and 8-1/2" in diam- 

 eter. Concentrations of . 10, .056, .032, .018, 

 .010, .0056, and .0032 ppm of DDT had been 



added from the stock DDT solution within 30 

 minutes prior to the test. Records of percent 

 survival were kept and dead fish removed as 

 soon as possible. To determine the TL/m for 

 one, 4, 7, and 14 days the recommended method 

 of straight line graphical interpretation was used, 

 plotting the percent survival of fish against the 

 logarithmic concentration (fig. 1, form recom- 

 mended by Doudoroff^al_., 1951). As Hart 

 noted, there are generally not enough points for 

 a sigmoid or s -shaped curve and the straight 

 line graphic interpolation is sufficient for prac- 

 tical purposes. The median is not influenced by 

 extreme variance and 100 percent mortality 

 criteria would not be a measure of tolerance . 



.OlS ppn 



30 40 50 60 



PBR CQlTSUHTIVil 



Figure 1: 



-Fourteen day TL/m determination 

 for adult guppies exposed to DDT 

 (data from table 1) . 



Resistance to .032 ppm DDT of two 

 different strains: -- Even though the guppies 

 were acclimatized for at least 7 days before 

 being tested in DDT, it seemed possible that 

 strains from varied backgrounds might differ 

 in tolerance to the toxicant and a duplicate set 

 of 5 fish from a different strain was exposed 

 to .032 ppm DDT under constant conditions to 

 compare results with the 14-day bioassay. 



Resistance to DDT of fish exposed to 

 sublethal dosages: --To determine if a primary 



