Management Techniques 147 



seining and boat shocking and reduces die amount (and cost) of rotenone 

 needed to give a complete kill of undesirable fish. It may also pull the 

 water out of cattail marshes and shallow bays overgrown with aquatic 

 vegetation and pond weeds, and thereby eliminate the danger that a few 

 small fish might survive in these areas. 



Fish are more sensitive to rotenone than are most other aquatic organ- 

 isms except entamostraca.^^' "^^ The length of time that rotenone-treated 

 water will remain toxic to fishes depends largely upon water temperature; 

 at 70° to 80°F waters can be restocked with fish within 4 to 5 days after 

 treatment. Lakes treated during cold weather may remain toxic for much 

 longer periods— as much as 30 days. A color test has been developed for 

 measuring the amount of rotenone in water. "^^ Potassium permanganate 

 ( KMn04 ) or chlorine ( CI2 ) will quickly oxidize rotenone and disappear 

 from treated water.^^' ^^ 



Prevost, Lanouette, and Grenier ^^ have demonstrated that the toxicity 

 of some preparations decreases after an initial group of animals has been 

 in it for a time. Thus, die disappearance of toxicity in a derris-powder 

 suspension after 48 hours reported by Leonard '^- is not entirely an effect 

 of time but is certainly related to the fact that the first two sets of fishes 

 killed in the preparation had caused the toxicity to drop below the lethal 

 point. This may in part account for the fact that most preparations appear 

 to be less toxic out-of-doors than in laboratory aquaria. 



Selective Poisoning. Recently a search has begun for selective poisons 

 toxic to certain kinds of fish but not to others. The U. S. Fish and Wildlife 

 Service began this search while looking for a chemical to kill larval sea 

 lampreys, and after testing more than 4000 chemicals, they found an 

 effective lamprey poison, as well as other chemicals that were toxic to 

 certain other species.- As yet, selective poisoning of fishes is still in ex- 

 perimental stages. 



DDT and Other Insecticides. The release and widespread use of DDT 

 after World War II caused apprehension among conservationists and 

 numerous studies were made of the effects of small and large scale ap- 

 phcations of DDT on fish and wildhfe.-^' ^2, 53. 54, 74, los. 109 ^s with many 

 other chemicals, DDT was more toxic to fishes in laboratory tests than 

 in field tests. On land it was often applied in dosages calculated in pounds 

 or fractions of a pound per acre. These same dosages, based on surface 

 area, were tested on shallow ponds containing miscellaneous fishes. In one 

 series of experiments where DDT in the emulsifier Triton X-100 was fol- 

 lowed by a treatment of rotenone applied at 1 ppm to kill all remaining 

 live fish, a dosage of DDT at the rate of 1 pound per acre was found to kill 

 all fish. A similar application at the rate of one-half pound per acre killed 

 bass, crappies, bluegills, and some carp, but enough carp survived to 

 repopulate the pond had they been allowed to spawn. 



