Increases of food consumption by fish together with impairment of the 

 efficiency of their utilization of food in aquaria in which the food was 

 abundant have been observed in experiments on the effects of sublethal con- 

 centrations of sodium cyanide (Leduc, 1966) and of potassium pentachlor- 

 phenate (Chapman, 1965; Warren, 1971, p. 163-164). Cyanide levels not far 

 below lethal levels depressed the food conversion efficiency (gross) of ci- 

 chlids, Cichlasoma bimaculatum , by as much as 35% on the average, but 

 caused increases of the amount of food consumed during the 36-day tests 

 that averaged as much as 30%. Potassium pentachlorphenol (0.2 mg/liter) at 

 first depressed the food-consumption rate of these fish, but later in the 

 42-day tests the consumption rate of the poisoned fish was much greater 

 than that of controls. Growth rates were initially depressed by both the 

 cyanide at high concentrations and the pentachlorphenate, but before the 

 tests were concluded the fish exposed to the poisons were growing even 

 faster than the controls. Averaged weight gains of the experimental and 

 control fish during the entire experiments therefore differed little or not 

 at all. When food rations were uniformly restricted, however, the growth 

 of fish in the pentachlorphenate solutions was decidedly less than that of 

 controls because of the deranged metabolism and reduced efficiency of food 

 conversion. 



Whenever the food consumption of fish that are given all the food that 

 they can eat is found to increase in the presence of the toxicant studied, 

 the food supply should be uniformly restricted so that all of the fish, 

 including the controls, will consume all of the food offered. The re- 

 stricted daily ration in one series of tests should not be much less, 

 however, then the maximum ration found to be consumed by all the fish. The 

 highest concentration of the toxicant that does not demonstrably impair the 

 growth of the fish receiving this fixed ration should be determined. The 

 impairment of food-conversion efficiency by poisons can be much more pro- 

 nounced when the levels of food availability and consumption are high than 

 when they are low. Sometimes, however, the effect in question may be more 

 pronounced and readily demonstrable at low levels of food availability and 

 intake than at high levels. Therefore, in looking for possible inter- 

 ference with food-conversion efficiency, it is always advisable to perform 

 an experiment in which a small, uniformly restricted ration of food is pro- 

 vided to each fish, regardless of whether the appetite of the fish has or 

 has not been found to be stimulated by the poison tested. This ration 

 should not be much greater than the maintenance ration for the controls, 

 which is the ration that brings about neither growth nor loss in weight of 

 these fish. Should the food-conversion efficiency of fish receiving the 

 small, restricted ration be found to be impaired more markedly by a poison 

 than that of fish consuming much larger amounts of food, tests with this 

 small ration should be performed to determine the highest concentration of 

 poison that does not impair the conversion efficiency. Ideally, such 

 series of tests of different concentrations are performed at four or more 

 levels of food availability, ranging from unrestricted supplies to re- 

 stricted supplies barely sufficient or insufficient for maintenance of the 

 initial body weight of controls (Warren, 1971). Such more laborious experi- 

 ments certainly can be very instructive and can increase confidence in the 

 reality of observed small differences of food-conversion efficiency. They 



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