In all such experiments it is most important to ensure that the 

 activity and behavior of the food organisms (prey) will be unaffected by 

 the pollutant at the concentration tested, o<" will be affected, at most, 

 far less than the foraging activity of the test subjects is affected, if 

 there is any effect. A debilitating effect on prey of the kind selected 

 more serious than the effect on the predator can result in improvement of 

 the predator's foraging efficiency under the experimental conditions. This 

 improvement would not necessarily occur in nature, where important food 

 organisms can be ones relatively resistant to the pollutant, and it 

 certainly would conceal an actual reduction of the predator's activity. 

 The food organisms selected should, therefore, be of a kind found through 

 preliminary experiments to be relatively insensitive to the pollutant that 

 is being tested, as compared with the fish that is the test subject. For- 

 tunately, the mosquitof ish, Gambusia aff inis , and other related species 

 with similar habits are handy fishes highly resistant to many poisons and 

 highly suitable for use as prey in the experiments and in other respects. 

 On the other hand, many of the predaceous species most valued by man as 

 food and game fishes, especially those of the family Salmonidae, are re- 

 latively sensitive. Suitable food organisms to be preyed upon by small 

 fishes usually should not be very difficult to find. Air-breathing aquatic 

 insects such as mosquito larvae and pupae, which are not sensitive to many 

 dissolved toxic substances, as well as to dissolved oxygen deficiency, and 

 young of the hardy brine shrimp, Artemia salina , hatched in the laboratory 

 from eggs that can be easily purchased, should not be overlooked in seeking 

 suitable forms . 



Since most poisons at low concentrations at which the appetite and food- 

 conversion efficiency of fish used as test subjects are not materially 

 affected can so affect the activity of resistant food organisms only after 

 fairly long exposures, the duration of exposure of the food organisms to a 

 poison usually should be minimal. They should be kept usually in clean 

 water while the test subjects are being exposed to a toxicant in prepara- 

 tion for a foraging activity test. Small fish to be used as prey can be 

 accustomed to the experimental environment and trained to avoid the preda- 

 tor by holding them for some time under the test conditions but in the ab- 

 sence of the poison until they are subjected to attack by fish previously 

 exposed to the poison. However, if adverse effects of relatively high con- 

 centrations of poison are found to be produced very rapidly and subse- 

 quently to become less pronounced because of acclimation of the organisms 

 to the poison, a different procedure may be advisable. All of the food 

 organisms, including those to be fed to control fish, then can be 

 acclimated before a test to the lower concentration to be tested. Various 

 other ways to minimize effects of the tested impairment of water quality on 

 the food organisms may be possible. For example, some clean water could be 

 continuously introduced into experimental aquaria over the productive 

 shelves described above that serve as cover for the prey (mosquitofish) . 

 The resulting, unavoidable dilution of the pollutant in the bulk of the 

 aquarium water could be compensated for by continuous introduction of a 

 sufficiently strong solution elsewhere in the tanks. 



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