impair the appetite, it is necessary only to expose groups of fish for 

 sufficiently long periods to different concentrations in small aquaria, to 

 supply them continuously or frequently with as much attractive, palatable, 

 and nutritious food (preferably live food) as they will consume, and to 

 measure the food consumed. The fish, held preferably in individual aquaria 

 or compartments, should be uniform in initial size and carefully weighed. 

 By weighing accurately the food offered and the uneaten food removed from 

 the aquaria at daily or other suitable intervals, the mean daily consump- 

 tion (in grams per gram of fish) at each tested concentration of toxicant 

 can be determined and compared with that of controls. The gross efficiency 

 of food conversion can be determined by dividing the gain in weight of the 

 fish during an experiment or a suitable time interval between weighings by 

 the weight of food consumed during that period. This efficiency can be 

 expected to be reduced, as compared with that of controls, when the food- 

 consumption rate is markedly reduced. If it is found to be reduced also at 

 a concentration of a poison at which the food consumption is not reduced, 

 impairment of metabolic processes is indicated, and lower concentrations 

 must be tested to determine the highest one at which no such effect is de- 

 monstrable. The duration of the food-consumption and growth tests need not 

 exceed a month and can be much shorter when growth is rapid, but it is ad- 

 visable to expose fish for a fairly long time before final measurements of 

 their growth and food intake are made, especially when substances known to 

 be accumulative poisons are tested. 



Any concentration of a poison at which the food intake is found de- 

 finitely to be reduced can be taken to be level at which the growth of the 

 fish probably would be impaired under natural conditions whenever the avail- 

 ability of food is not a limiting factor. This statement, or proposal, can 

 be reasonably countered, however, with the objection that the food consump- 

 tion and growth of fish in their natural environment generally are limited 

 by the availability of food and not be the appetite of the fish. One may 

 well argue that, for this reason, the concentration of a poison at which 

 food consumption begins to decline in aquaria where food is so abundant 

 that the fish can obtain as much as they can eat with little or no effort 

 is essentially meaningless. In addition, it is doubtless true that the 

 annual food consumption of fish in nature is, as a general rule, if not 

 always, far less than their annual assimilative capacity. At natural tem- 

 peratures growth rates of well-fed fish in laboratory can greatly exceed 

 those usually found in nature, where losses of weight during periods of 

 food shortage are not unusual. Perhaps few biologists who have studied the 

 growth of fish believe that the availability of food ever is not a limiting 

 factor for any considerable periods of time. But it seems to me not un- 

 reasonable to assume that in some very productive natural environments the 

 rates of food consumption are not limited by food availability during some 

 fairly extended periods of maximal or nearly maximal abundance of food in 

 the season or seasons in which most of the growth of fish takes place. An 

 inability of fish to take full advantage of a temporary abundance of food 

 in such a situation could have a considerable effect on their annual weight 

 gains. 



Whenever there is sufficient reason to reject the proposition that the 

 growth of fish in a given environment is not food-limited at certain times 



57 



