and for periods long enough to be of consequence, as when that growth is 

 known to be always much slower than that of controls in the proposed labora- 

 tory tests, a different alternative approach can be used. Each fish in 

 the aquaria then can receive daily a uniformly restricted food supply that 

 is believed or assumed to be not much less than the maximum amount of food 

 consumed per day by individuals of the same size in the natural environ- 

 ment, excepting rare occasions. The maximum concentration of a poison at 

 which this restricted food ration is fully consumed by the experimental 

 fish and their growth is not demonstrably impaired, as compared with that 

 of controls, than can be determined. In all such experiments the food 

 should be as much as possible like natural foods; for comparative purposes, 

 amounts of these foods should be expressed not in grams but in caloric 

 equivalents. Water temperatures normal for the season of maximum food con- 

 sumption in nature should be maintained during these tests. Various 

 methods for the estimation of rates of food consumption by fish in their 

 natural environments have been described (Da^is and Warren, 1968). Esti- 

 mates of amounts of food consumed during short intervals of time probably 

 cannot be very reliable. Average rates of growth of fish in nature during 

 longer periods (seasons of the year) can be evaluated through appropriate 

 observations, and their average daily food consumption during these periods 

 can be estimated through laboratory experiments by which rates of growth in 

 aquaria are related to food-consumption rates. In the absence of contrary 

 information, the assumption then can be made that a food-consumption rate 

 twice the average seasonal rate is a rate that should be attainable but 

 does not have to be exceeded at any time during corresponding season if 

 growth in the natural environment is not to be materially impaired. 

 Accordingly, a daily feeding rate twice the estimated, average, natural 

 food-consumption rate during the season of maximum food intake can be taken 

 to be appropriate for the proposed toxicity tests. Admittedly, this recom- 

 mendation is somewhat arbitrary, for frequent fluctuations of natural food 

 availability and consumption may be sometimes much smaller and sometimes 

 greater than those that it implies, but I believe that it cannot lead to 

 serious error in the estimation of maximum harmless levels of water pollu- 

 tants. 



When an unrestricted food supply is provided to the experimental and 

 control fish, as first suggested, it will sometimes be found that fish ex- 

 posed to toxic substances consume more food, and not less, than the con- 

 trols. In this way they may compensate partly or wholly for a reduction of 

 the efficiency with which the consumed food can be utilized, so that growth 

 in the aquaria may be reduced little or not at all. But such compensation 

 is possible only when food is extremely abundant and available. It is cer- 

 tainly true that the growth of most fish in nature is limited, at least 

 during a large part of each year, by the limited availability of food, 

 which renders the compensation impossible. In their natural habitat 

 poisoned fish are likely to find and capture less of their prey than normal 

 fish would when there is a shortage of food. Therefore, any reduction of 

 the efficiency of food conversion in fish affected by poisons must result 

 in impairment of their growth whenever the density of their prey is low 

 enough to be a limiting factor. 



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