as often as every two weeks. Though Mounti' 

 advised a continual flow of the treated water, 

 this procedure would not provide data repre- 

 senting the results of one dosage of a specific' 

 concentration on the test fish. Although the 

 cause of the deaths might be a highly contagious 

 disease, this seems unlikely, particularly since 

 sterilized equipment was used in the second 

 experiment. It seems most likely that the deaths 

 resulted from an increasingly high concentration 

 of dissolved nitrogenous wastes which were not 

 removed by filtering. 



Though relatively few guppies were born 

 in the test aquaria, the survival rate was some- 

 what reduced for tnose fish born in DDT solu- 

 tions. These DDT-exposed fish were generally 

 less mature at the time of birth as indicated by 

 the distended yolk sacs. The correlation of 

 failure of yolk resorption and hypoactivity of 

 the adrenal gland will be discussed later. The 

 sli^t, but perhaps significant, difference in 

 length between the fry born in DDT water and 

 those bom in untreated water is attributed to a 

 failure of the previously DDT-exposed fry to re- 

 cover completely in a 40 -day period. The only 

 literature on growth effects is that of Linduska 

 and Surber (1948) who reported no apparent 

 inhibition of growth of oysters in DDT treated 

 beds. 



6/ 

 Mount— found that guppies generally 



failed to have more than one litter while in 

 endrin. It was not possible in the short period 

 of my tests to determine if DDT had a similar 

 effect. Data on quail egg and chick survival 

 support the assumption that insecticides have 

 a harmful effect on fish and wildlife reproduc- 

 tion. 



For practical application, one must 

 compare the susceptibility of other fishes 

 relative to that of guppies before deducing con- 

 centrations safe for other species. The 14-day 

 bioassay with a local fish, the brown trout fry 

 and fingerlings, revealed a marked difference 

 in susceptibility to DDT. Once the young trout 

 fry were feeding, they became much more 

 sensitive to DDT than were the young guppies . 



While dosages above .0024 ppm were sublethal 

 to the young guppies, any concentration above 

 .00056 ppm proved lethal to 50 percent of the 

 trout fry at 10 weeks of age. It seems probable 

 that water is not brought into the intestine of 

 the young fry until they begin to feed, which is 

 normally at about the fourth or fifth week. Once 

 the treated water enters the intestine, the DDT 

 can enter the body quickly by way of the blood, 

 thus accounting for the sudden increased 

 sensitivity. However, after a period of growth 

 to the first fingerling stage, the trout became 

 much more resistant to DDT than when they 

 first began feeding. In a short bioassay on 

 number five fingerlings placed in .10, .0056, 

 and .0032 ppm that percent, 50 percent, and 

 50 percent, respectively, survived a 2 -week 

 exposure. The trout fingerlings thus appeared 

 to be only slightly more sensitive than the adult 

 guppies to DDT (.014 ppm for the 14-day TL/m 

 for trout compared with .018 ppm for guppies), 

 while the feeding trout fry were exceedingly 

 sensitive in comparison (.00056 ppm TL/m). 



While it was not possible to test the 

 sensitivity of adult trout under exacting labora- 

 tory conditions, the trend toward decreased 

 sensitivity displayed by the flnerlings makes it 

 seem likely that adult trout, like adult guppies, 

 are more resistant than the fry to DDT. 



It is the ultimate hope that effects of 

 field applications of insecticides on fish can be 

 predicted in advance, by the use of standard 

 laboratory tests. Accumulation of TL/m values 

 under standardized conditions, such as those 

 values determined in the present study, is the 

 first step in realizing this hope. Application 

 of laboratory findings to field conditions involve 

 major but not impossible difficulties. Close 

 approximations of safe insecticide dosages may 

 be obtained by short bioassays on the fish from 

 the water area involved, using the receiving 

 water for the test solutions to which the DDT 

 concentrations are added in the same formula- 

 tion as that to be used in the field. A comparison 

 of the short bioassay data with results obtained 

 after longer exposures under standarized con- 

 ditions, such as the values obtained in the survey 



6/ 



See footnote 4 

 See footnote 4 



11 



