Introduction 



Field Studies began in 194.5 indicate that tnere is a difference 

 in the susceptioility of fishes of different ages to DUT, as well as 

 interspecific differences. Hoffmann and Surber (1948) observed that 

 young of-the-year of smallmouth black bass (Llicropterus dolomieu), 

 conunon suckers ( Catostomus commersonnii ) . ancl stoneroiler suckers 

 ( Hypentelium nigricans ) vrere killed by a one-pound- per-acre aerial 

 application of a DDT suspension in a section of Back Creek, W. Va. 

 The adults of these species were not affected, Pielou (1946) found 

 young Kafu bream ( Tilapia kafuensis ) sensitive to DDT in paraffin 

 solution when the concentration of DDT reached one part in 25 million 

 parts of water in dirt-bottomed ponds. Fish were more sensitive to 

 DDT in the aquarium experiments. 



As a result of the field observations, it seemed advisable to 

 conduct experiments to determine differences in the toxicity of DDT 

 in relation to size and species of fish. 



The experiments were carried out at the U. S. Fish and V/ildlife 

 Service Experimental Station, Leetown, ?/. Va., with a variety of warm 

 and cold-water fishes . Various sizes of fish were exposed to different 

 formulations and concentrations of DDT so that differences between 

 age-group and species susceptibility could be determined. 



Ifethods 



The experiments were conducted in several types of ponds. A 

 number of experiments were performed in a series of dirt-bottomed 

 ponds (fig. l), while others were carried out in concrete pools and 

 raceways . 



Spraying was done vd-th a continuous-spray hand atomizer or a knap- 

 sack sprayer. The spray was applied to the larger ponds from a canoe 

 controlled by two men on the banks to insure a thorough coverage. It 

 Yfas necessary to use ttie same ponds several times during the season, 

 hence, a biological assay of each pond for DDT was made before its re- 

 use, by exposing larvae of Aedes egypti dinnaeais ) . a species of mos- 

 quito killed by 0.005 p. p.m. of DDT. 



Note; The writers are indebted to Edward F. Merkel and John Trach, 



Division of Forest Insect Investigations, Bureau of Entomology 

 and Plant (Quarantine, and to Dorothy D. Friddle, Branch of 

 Fishery Biology, U. S. Fish and T/ildlife Service, for techni- 

 cal assistance during the course of these studies . 



