By the various experimental methods that have been discussed in some de- 

 tail, a limit of water quality alteration not likely to have any consid- 

 erable effect on the feeding and growth of the experimental subjects when 

 the availability of food is constant can be determined. One can then pro- 

 ceed to the final step of the proposed scheme for the estimation of the 

 limit of alteration having no material effect of any kind on the growth of 

 the fish in their natural habitat. Possible effects on the food supply 

 (production and availability of food organisms) next can be investigated 

 experimentally with model environments, such as artificial or modified 

 natural streams or ponds, in which foods are produced naturally and fish 

 depend entirely on this natural production (except for some consumption of 

 terrestrial organisms that may unavoidably enter into their diet). Experi- 

 mental facilities and methods for such investigations cannot be adequately 

 described here. Suitable methods that have been used at Oregon State Uni- 

 versity to study the effects of enrichment of water with sucrose on the 

 foods and growths of trout in a modified natural trout stream have been des- 

 cribed by Warren et al . (1964). Those used more recently in a similar 

 study of effects of pulp and paper mill wastes in outdoor, artificial 

 streams have been described briefly by Warren et al . (1974). Only the 

 highest concentration of a toxic water pollutant that has been found not to 

 affect fish directly so as to impair their feeding and growth may need to 

 be tested in the difficult and costly experiments in which natural condi- 

 tions must be reproduced as nearly as possible. This concentration may be 

 found to have no demonstrable, adverse effect also on the food supply and 

 growth of the fish in the simulated natural environment. However, if a 

 considerable reduction of the growth of the fish is observed at this con- 

 centration, lower concentrations must be tested to determine the level at 

 which there is no such effect and, therefore, no material effect on the 

 availability and consumption of food. 



In the foregoing discussion emphasis has been placed on toxic water 

 pollutants, but effects of reduced concentrations of dissolved oxygen on 

 the growth of fish also have been considered. The literature on the latter 

 subject has been critically reviewed and the significance of the then 

 available data thoroughly discussed by Doudoroff and Shumway (1970). In 

 much the same way as oxygen deficiency, some toxic substances that inter- 

 fere with external respiration may very markedly impair the appetite of 

 fish but have little or no effect on their food-conversion efficiency when 

 the fish receive uniformly restricted food rations. Because of the well- 

 known influence of temperature on the metabolic rates of fish, thermal 

 pollution, now of great importance, presents some special bioenergetic 

 problems (Doudoroff, 1969) that can be only very briefly discussed here. 



The temperature optimum for the growth of fish is a function of the 

 food supply. At moderately elevated temperatures, fish may be able to grow 

 much faster than they do at lower, normal temperatures when the food supply 

 is unlimited, but more food is needed for mere maintenance of their body 

 weight, because of the elevated metabolic rate. When the supply of food is 

 limited so that the daily consumption cannot increase, growth is reduced as 

 the temperature rises, not because of any impairment of, or interference 

 with, metabolic processes, but only because of their acceleration and conse- 

 quent reduction of the fraction of the energy of food that remains to be 



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